Molybdenum and Molybdenum-Containing Enzymes

Molybdenum and Molybdenum-Containing Enzymes is a collection of papers that deals with the various concerns with molybdenum-containing enzymes. The text first covers the organometallic chemistry of molybdenum, and then proceeds to tackling molybdenum-containing enzymes, such as xanthine oxidase, aldehyde oxidase, and sulphite oxidase. The text also discusses the advancement in the understanding of molybdenum-containing enzymes. The remaining chapters deal with the genetics of molybdoenzymes and the nutritional aspects of molybdenum. The book will be of great use to students, researchers, and practitioners of biochemistry.

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 28, 2014
• ISBN: 9781483189123

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MOLYBDENUM AND MOLYBDENUM-CONTAINING ENZYMES

MICHAEL P. COUGHLAN

Department of Biochemistry, University College, Galway, Ireland

Table of Contents

Cover image

Title page

Copyright

Dedication

Foreword

Preface

Chapter 1: A Comparison between the Chemistry and Biochemistry of Molybdenum and Related Elements

Publisher Summary

I INTRODUCTION

II HYDROLYSIS

III AN OUTLINE OF MOLYBDENUM, TUNGSTEN AND VANADIUM CHEMISTRY

IV UPTAKE OF METALS INTO CELLS

V METALS FROM GROUP V TO GROUP VII

Acknowledgement

Chapter 2: The Structures and Spectra of Molybdoezyme Active Sites and Their Models

Publisher Summary

I INTRODUCTION

II X-RAY ABSORPTION SPECTROSCOPY (including EXAFS)

III THE CHEMISTRY OF IRON-MOLYBDENUM-SULFUR SYSTEMS

IV THE CHEMISTRY OF OXO MOLYBDENUM COMPLEXES

V MOLYBDENUM-SULFUR CHEMISTRY: FURTHER ASPECTS

VI FINAL REMARKS

Acknowledgements

Chapter 3: Model Reactions of Molybdenum Complexes

Publisher Summary

I INTRODUCTION

II REDOX REACTIONS

III INTRAMOLECULAR ELECTRON TRANSFER

IV LIGAND EXCHANGE REACTIONS

V SUMMARY AND CONCLUSIONS

Acknowledgement

Chapter 4: Aldehyde Oxidase, Xanthine Oxidase and Xanthine Dehydrogenase; Hydroxylases containing Molybdenum, Iron-Sulphur and Flavin

Publisher Summary

I INTRODUCTION

II BIOLOGICAL ASPECTS

III MOLECULAR PROPERTIES

IV REDUCTION AND REOXIDATION

ACKNOWLEGEMENT

Chapter 5: Concepts and Approaches to the Understanding of Electron Transfer Processes in Enzymes containing Multiple Redox Centers

Publisher Summary

I INTRODUCTION

II OXIDATION REDUCTION SYSTEMS CONTAINING MULTIPLE REDOX CENTERS

III GENERALIZED SCHEME FOR ELECTRON TRANSPORT PATHWAYS

IV GENERAL PROCEDURE FOR DESCRIBING THE EQUILIBRIUM DISTRIBUTION OF ELECTRONS IN A MULTICENTER ENZYME

V A GENERAL KINETIC PATHWAY FOR MULTICENTER ENZYMES

VI THE BEHAVIOR OF XANTHINE OXIDASE AND XANTHINE DEHYDROGENASE

VII MECHANISM OF REDUCTION OF XANTHINE OXIDASE

VIII MECHANISM OF OXIDATION OF XANTHINE OXIDASE

IX XANTHINE DEHYDROGENASE

Chapter 6: Studies by Electron Paramagnetic Resonance on the Nature and Reactions of the Molybdenum Centre of Xanthine Oxidase

Publisher Summary

I INTRODUCTION

II E.P.R. ANALYSIS OF THE MOLYBDENUM(V) SIGNALS

III INDIVIDUAL MOLYBDENUM(V) SPECIES OF XANTHINE OXIDASE

IV EXCHANGE RATES OF THE INTERACTING PROTONS

V DIRECT HYDROGEN TRANSFER

VI EVIDENCE FOR AN ANION-BINDING SITE

VII THE VERY RAPID SIGNAL

VIII THE CATALYTIC MECHANISM

IX THE INHIBITED SIGNALS

X THE NATURE OF THE SIGNAL-GIVING SPECIES

XI CONCLUDING REMARKS

Acknowledgements

Chapter 7: Sulfite Oxidase (Sulfite: Ferricytochrome C Oxidoreductase)

Publisher Summary

I INTRODUCTION

II PROPERTIES OF PURIFIED SULFITE OXIDASE

III TISSUE DISTRIBUTION AND SUBCELLULAR LOCALIZATION

IV REACTION MECHANISM

V TUNGSTEN-CONTAINING SULFITE OXIDASE

VI DOMAIN STRUCTURE OF SULFITE OXIDASE

VII AMINO ACID SEQUENCE OF THE HEME DOMAIN

VIII BIOLOGY OF SULFITE OXIDASE

IX CONCLUSION

Chapter 8: Nitrate Reductase Systems in Eukaryotic and Prokaryotic Organisms

Publisher Summary

I INTRODUCTION

II MOLECULAR WEIGHT AND SUBUNIT COMPOSITION

III OXIDIZING SUBSTRATES

IV ELECTRON DONORS AND ELECTRON TRANSPORT SEQUENCE

V DIFFERENTIAL INHIBITION AND DENATURATION OF NITRATE REDUCTASE ACTIVITIES

VI THE MOLYBDENUM COFACTOR AND ITS TUNGSTEN ANALOGUE IN NITRATE REDUCTASE

VII THE SUBSTITUTION OF MOLYBDENUM BY TUNGSTEN IN VIVO

VIII ELECTRON PARAMAGNETIC RESONANCE SPECTRA

IX KINETICS AND MECHANISM OF NITRATE REDUCTASE

Chapter 9: Prosthetic Groups and Mechanism of Action of Nitrate Reductase from Neurospora Crassa

Publisher Summary

I INTRODUCTION

II EXTRACTION AND STABILIZATION OF NITRATE REDUCTASE

III PURIFICATION AND CHARACTERIZATION OF NITRATE REDUCTASE

IV EPR STUDIES OF NITRATE REDUCTASE

V DISCUSSION

Acknowledgment

Chapter 10: The Molybdenum Cofactor Common to Nitrate Reductase, Xanthine Dehydrogenase and Sulfite Oxidase

Publisher Summary

I EVIDENCE FOR THE EXISTENCE OF A MOLYBDENUM COFACTOR

II ASSAY SYSTEMS

III METAL-FREE COFACTOR AND THE PRESENCE OF METALS OTHER THAN MOLYBDENUM

IV ISOLATION AND CHARACTERIZATION OF THE MOLYBDENUM COFACTOR

V ROLE OF THE COFACTOR IN MAINTAINING THE STRUCTURAL STABILITY OF MOLYBDOENZYMES

VI CARRIER MOLECULES FOR THE MOLYBDENUM COFACTOR

VII CONTROL OF MOLYBDENUM COFACTOR SYNTHESIS AND COFACTOR REGULATION OF MOLYBDOENZYMES

VIII FINAL COMMENT

Chapter 11: Molybdenum in Nitrogenase

Publisher Summary

I INTRODUCTION

II BIOCHEMISTRY OF NITROGENASE

III MOLYBDENUM COFACTORS

IV MOLYBDENUM UPTAKE AND STORAGE

V GENETICS OF NITROGEN FIXATION

VI REGULATION OF NITROGEN FIXATION

VII CONCLUDING REMARKS

Chapter 12: Nitrogenase: Electron Transfer and Allocation and the Role of ATP

Publisher Summary

I INTRODUCTION

II ELECTRON TRANSFER BETWEEN THE NITROGENASE PROTEINS

III VARIATIONS IN ELECTRON FLUX

IV ATP HYDROLYSIS

V CONCLUSIONS

Chapter 13: On the Prosthetic Groups of Nitrogenase

Publisher Summary

I INTRODUCTION

II CHEMICAL EVIDENCE

III ELECTRONIC SPECTRA

IV EPR EVIDENCE

V MöSSBAUER EVIDENCE

VI FUNCTIONAL CHARACTERISTICS OF M AND P CLUSTERS

ACKNOWLEDGEMENTS

Chapter 14: Chemical Aspects of Nitrogenase

Publisher Summary

I INTRODUCTION

II EXTRINSIC CHEMISTRY

III INTRINSIC CHEMISTRY

IV CONCLUSION

ACKNOWLEDGEMENT

Chapter 15: Formate Dehydrogenases: Role of Molybdenum, Tungsten and Selenium

Publisher Summary

I INTRODUCTION

II ESCHERICHIA COLI FORMATE DEHYDROGENASE

III FORMATE DEHYDROGENASES IN CLOSTRIDIUM SPECIES

IV FORMATE DEHYDROGENASE IN METHANOGENIC BACTERIA

Acknowledgement

Chapter 16: The Genetics of the Molybdenum-containing Enzymes

Publisher Summary

I GENERAL CONSIDERATIONS: GENETICS AND MOLECULAR ENZYMOLOGY

II MUTATIONS RESULTING IN LOSS OF ENZYME ACTIVITY: TAXONOMIC CRITERIA

III STRUCTURAL GENE MUTATIONS

IV GENETIC EVIDENCE FOR A COMMON COFACTOR AND POST-TRANSLATIONAL MODIFICATIONS

V THE SPECIFICITY OF THE MOLYBDENUM-CONTAINING HYDROXYLASES AND THE EVOLUTION OF SUBSTRATE BINDING SITES

VI CONTROL MECHANISMS: AN OVERVIEW

VII CONCLUSIONS

Acknowledgment

Chapter 17: Nutritional Aspects of Molybdenum in Animals

Publisher Summary

I INTRODUCTION

II MOLYBDENUM AS AN ESSENTIAL NUTRIENT

III MOLYBDENUM AS A METABOLIC ANTAGONIST OF COPPER

IV CONCLUSION

Chapter 18: Theoretical and Practical Aspects of Studying Molybdenum-containing Enzymes

Publisher Summary

I INTRODUCTION

II ELECTRON PARAMAGNETIC RESONANCE

III ELECTRON NUCLEAR AND ELECTRON ELECTRON DOUBLE RESONANCE

IV RAPID-FREEZING TECHNIQUE

V OXIDATION-REDUCTION POTENTIOMETRY

Acknowledgement

Index

Copyright

Copyright © 1980 Pergamon Press Ltd.

All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying. recording or otherwise, without permission in writing from the publishers.

First edition 1980

British Library Cataloguing in Publication Data

Molybdenum and molybdenum-containing enzymes.

1. Enzymes 2. Molybdenum compounds

I. Coughlan, Michael P

547′.758 QP601 79-42846

ISBN 0-08-024398-3

In order to make this volume available as economically and as rapidly as possible the authors’ typescripts have been reproduced in their original forms. This method has its typographical limitations but it is hoped that they in no way distract the reader.

Printed and bound in Great Britain by

William Clowes (Beccles) Limited, Beccles and London

Dedication

To

ANNE, TARA, SORCHA, CILLIAN and NEAL

and

To whomsoever my fellow authors

should like to dedicate their contributions

Foreword

Review articles on molybdenum-containing enzymes are scattered throughout the literature. Some are found in volumes dealing with nitrogen fixation, sulphate metabolism etc., subjects that reflect the functions of these enzymes. Others grace the pages of books devoted to topics such as iron-sulphur proteins, flavoproteins etc. This is attributable to the fact that all molybdoenzymes isolated to date contain additional prosthetic groups. My intention in planning this book was to redress the balance–to place the emphasis on the molybdenum moiety-bringing together in one volume the salient information on all molybdenum-containing enzymes and on those aspects of the chemistry of the metal and its simple complexes that help to further our understanding of its biological role.

The organometallic chemistry of molybdenum is the subject matter of the first three chapters, although Chapter 2 includes a description of EXAFS, an exciting new technique that promises to do much to unravel the mysteries of the biological coordination chemistry of the metal. Other chapters deal with xanthine oxidase, xanthine dehydrogenases, aldehyde oxidase, sulphite oxidase, nitrate reductase, nitrogenase and formate dehydrogenase–the molybdenum-containing enzymes about which sufficient information exists and whose functions are known. Such chapters describe each enzyme, what it does and, insofar as is known, how it does it. Much of the progress in recent years in our knowledge of molybdoproteins has come from an increased understanding of the molybdenum cofactor common to many of these enzymes and of the iron-molybdenum cofactor of nitrogenase. These are discussed in Chapters 10 and 11, while the genetics of molybdoenzymes and the nutritional aspects of molybdenum are dealt with in Chapters 16 and 17.

Xanthine oxidase is, without doubt, the most studied of the enzymes in question. Much significant information on this enzyme has been obtained from physicochemical studies such as EPR, rapid-freezing, potentiometry and, more recently, EXAFS. The application of these techniques to other molybdoenzymes should prove equally informative. Accordingly, brief descriptions of the theory and practice of some of the techniques used are given in Chapters 2, 5 and 18. Apart from encouraging their application I should like to think that these articles will benefit those workers who, like myself, may have difficulty in keeping up with advances on the physicochemical front.

In a book such as this some overlap in the subject matter of the various chapters is inevitable. Such overlap, when not extensive, can be useful if it presents alternative interpretations of experimental data or differing viewpoints reflecting the training of the authors. I trust the reader will find this to be the case here.

Since the inception of this book, two different molybdenum-iron-sulphur proteins from Desulphovibrio species have been described though their biological function has yet to be determined. Proteins involved in storing and transporting molybdenum and in complexing with molybdenum cofactor are becoming better understood. These subjects and descriptions of still-to-be discovered molybdoproteins, if any, may provide grist for the mill of some future volume.

The right-hand margin of the pages in this book may look somewhat untidy because of the use of camera-ready copy. However, I believe that speed of publication is a more important consideration–authors and readers prefer articles to be as up-to-date as possible. I have been most fortunate in having many skilled and generous individuals help me put this book together. It is a pleasure to record my gratitude to the many members of the Biochemistry Department at Duke University Medical Center where the final stages were completed: Professors Robert L. Hill and K.V. Rajagopalan for placing every facility at my disposal; Mrs. Pamela Gunter, Mrs. Brenda Allen, Mrs. Margaret Bock and Mrs. Pattie Lewis all of whom shared in preparing the typescript; and Dr. Nancy Amy who proofread many of the manuscripts. Finally, for the help and encouragement they gave a novice to this business, I should like to thank Robert Maxwell, Martin Richardson and John Lavender of Pergamon Press, Oxford.

June, 1979

Michael Coughlan

P.S. Inclusion of photographs of the authors of each chapter was my idea. As a reader I have often tried to visualize the sort of person the writer might be. At scientific meetings I have often wished I could recognize in person those people with whose work I am familiar. Besides, who knows what show-business agents may be interested in the ways of molybdenum.

Preface

The finding, by Westerfeld, that molybdenum is an essential component of xanthine oxidase was an unexpected historic event. At that time, it was generally appreciated that most organic compounds required in minute quantities in the diets of animals–vitamins–would prove to be cofactors that participate in the catalytic cycles of specific enzymes. It was suspected that trace elements required in the diet would also prove essential to specific catalyses but there were only a few available demonstrations of that thesis.

Although knowledge of the presence of molybdenum in xanthine oxidase was widely held, two decades elapsed before its relationship to the actual catalytic process was demonstrated. During that period, it became evident that, in addition to hemeiron, the copper in several different enzymes is also reversibly reduced and reoxidized during the catalytic cycle of those enzymes. The fact that zinc-deficient Neurospora fail to make alcohol dehydrogenase was overlooked as a clue to the presence of zinc in that enzyme; current views of its role suggest that it is necessary both for structural reasons and as a substrate ligand in the catalytic cycle, but does not change valence. Demonstration that a trace metal in an enzyme participates in the redox catalytic cycle–rather than as a structural element necessary for the three-dimensional conformation of the protein or as a substrate-binding ligand–requires evidence of a change in valence state of that metal during the catalytic cycle. For molybdenum in xanthine oxidase, such evidence was obtained by observation of changes in magnetic susceptibility and by electron paramagnetic resonance spectrometry. Utilizing stopped-flow techniques it was shown that these events occur at a rate compatible with actual participation in the catalytic cycle. This field was then free to advance as diverse other molybdenum-containing enzymes were isolated and increasingly sophisticated techniques became available for the dissection of proteins, examining the physical state of molybdenum and ascertaining the nature of the atoms in its surround, thereby enabling insight into details of molybdenum binding to these enzymes, the manner of its interaction with substrate, and its precise role in the catalytic cycle.

Our laboratory, at Duke University, has participated in this saga at various stages, in consequence of some fortuitous observations which have proved critical to the development of this history.

It all began with a search for a hepatic sulfite oxidase, clearly necessary to mammalian metabolism in view of the known overall fate of sulfur-containing amino acids. The early finding that hypoxanthine appeared to serve as a coenzyme for the oxidation of bisulfite was, at first, incomprehensible. Ultimately, it led to the demonstration that what we had observed was a chain reaction involving molecular oxygen and sulfite, in which each chain is touched off by superoxide ions released when hypoxanthine is oxidized by xanthine oxidase. This finding was pursued in diverse directions. Fridovich went on to discover the superoxide dismutases. Rajagopalan turned to the catalytic mechanism of xanthine oxidase, spurred on by my curiosity concerning an enzyme with a dual specificity such that it could oxidize both any aldehyde and several purines. That analogous enzymes may exist became evident from the work of Knox showing that liver possesses an enzyme which can oxidize any aldehyde as well as quinine and N-methylnicotinamide. The latter is transformed to its 6-pyridone, in keeping with earlier observations at Duke that ingested nicotinamide is excreted both as N-methylnicotinamide and its pyridone. It is this enzyme purified from rabbit liver that we have called aldehyde oxidase. In a series of studies, the analogous structures and catalytic properties of xanthine and aldehyde oxidases were demonstrated. Incidentally, perplexing mysteries concerning aldehyde oxidase remain, such as its exquisite sensitivity to the non-ionic detergent Triton X-100, and the fact that it grinds to a halt in the presence of menadione. Nor do we yet understand the unexpected inhibitory property of methanol for this enzyme.

Meanwhile we returned to the hunt for a real sulfite oxidase. A relatively crude preparation with rather strange properties was obtained and described by Farkas in our laboratory. More than a decade went by before Rajagopalan returned to this subject and, by electron paramagnetic resonance spectrometry, discovered the presence of molybdenum in this enzyme as well.

Since then the field has proliferated magnificently. The pace of recent developments concerning the nature of the molybdenum-containing moiety of these enzymes, its detailed structure and its functional role has been most extraordinary. It is, therefore, deeply gratifying to have available this volume which summarizes the current status of what is now an extremely exciting aspect of our understanding of enzymatic catalysis.

Philip Handler,     National Academy of Sciences, Washington, D.C.

CHAPTER 1

A Comparison between the Chemistry and Biochemistry of Molybdenum and Related Elements

Michael T. Pope,     Georgetown University, Washington, D.C., U.S.A.

Ebbe R. Still*,     *Abo Akademi, Abo, Finland

Robert J.P. Williams**,     **Inorganic Chemistry Laboratory, University of Oxford, Oxford, U.K.

Publisher Summary

This chapter presents a comparison of the chemistry and biochemistry of molybdenum and related elements. The interest in molybdenum as found in biological systems is because of the importance of this special metal ion as an essential trace element participating in a number of enzymatic reactions. The feature common to different types of reactions involving molybdenum is the transfer of an even number of electrons to or from the substrate. The reactions can be interpreted either as formal electron transfer steps or as atom transfer steps accompanied by the release or uptake of protons. Thus, the catalytic site, that is, the molybdenum atom, is required to have the ability to form metal complexes in several oxidation states and to form oxo- or sulphido-ions. The reason why in biology molybdenum has gained importance as a more useful catalyst in particular chemical transformations is that it iss the only metal in the second and third transition series that is essential for life.

I INTRODUCTION

Natural Abundance

II HYDROLYSIS

The Stability of Mononuclear Complexes

Hydrolysis to Oxocations

Polynuclear Complexes

The Hydrosulphylation (Ammonolysis) Reactions

III AN OUTLINE OF MOLYBDENUM, TUNGSTEN AND VANADIUM CHEMISTRY

The d⁰ Configuration: Mo(VI), W(VI), V(V)

The d¹ Configuration: Mo(V), W(V), V(IV)

The d² Configuration: Mo(IV), W(IV), V(III)

The d³ Configuration: Mo(III), W(III), V(II)

Summary of Sections II and III

IV UPTAKE OF METALS INTO CELLS

A Cation Uptake

B Anion Uptake

Vanadate and Phosphate

The Nature of ATP-ases

V METALS FROM GROUP V TO GROUP VII

Functional Significance

Likely Functional Oxidation States

Vanadium

Molybdenum

Manganese

The Significance of Oxo-Group Chemistry: Mo, Mn and Fe

Substitution Probes

VI REFERENCES

Michael Pope

Ebbe Still

Robert Williams

I INTRODUCTION

The current interest in molybdenum as found in biological systems is due to the importance of this special metal ion as an essential trace element participating in a number of enzymatic reactions (Stiefel, 1977, Moura and Xavier, 1978). Examples of the types of reactions involving molybdenum are:

(1)

(2)

(3)

(4)

The last two reactions refer to the vital role the metal ion plays in the earth’s nitrogen cycle, being involved both in the reduction of nitrate and in the fixation of nitrogen.

The feature common to the reactions above is the transfer of an even number of electrons to or from the substrate. The reactions can be interpreted either as formal electron transfer steps or as atom transfer steps accompanied by the release or uptake of protons. The first three reactions can be visualized as oxygen transfer reactions requiring an even number of electrons. It follows from these considerations, that the catalytic site, i.e. the molybdenum atom, is required to have the ability to form metal complexes in several oxidation states and to form oxo- or sulphido-ions.*

What we should like to be able to explain, in the light of these simple observations, is the reason why biology has chosen molybdenum since, at first glance, other metals in the Periodic Table might well be able to partake in the same reactions. Yet molybdenum is the only metal in the second and third transition series which is essential for life. Although we cannot directly ask why molybdenum should be used so extensively in biology, and in such a critical role as the fixation of nitrogen, we can look for those differences between the chemistry of molybdenum and other elements which might make it, rather than the others, a more useful catalyst in particular chemical transformations. In this chapter we shall examine therefore some general features of molybdenum chemistry in a comparative description with that of the elements V, Cr, W, and Mn. A more extensive comparison with other metals is not warranted since before Group VA of the Periodic Table there are no metals which give oxyanions, a characteristic feature of molybdenum chemistry, and oxyanion chemistry dies out after Group VIIA. Again a wide range of oxidation states of relatively similar stability is found among metals only in Groups VA to VIIA. In fact some of molybdenum chemistry is more like that of phosphorus and sulphur than that of later transition metals such as copper.

In order to simplify the description of the chemistry we shall limit discussions to certain oxidation states of the five chosen metals, Cr, Mo, W, V and Mn. Our reasons for this are that biological systems work within the range of the hydrogen and oxygen electrodes, a span of about 1.4 volts. Oxidation states which are not stable in this range can be short-lived intermediates in catalyzed reactions, but will not be a stable form in which the element occurs within biology. The oxidation state diagram in Fig. 1 (Phillips and Williams, 1965) shows that the important oxidation states are likely to be:

Fig. 1 Oxidation state diagram for some metal ions at pH = 7.

Complex formation is known to alter redox potentials dramatically, but it does not alter the above picture significantly.

A rapid inspection of the chemistry of these oxidations states suggest that any biological function of chromium will be very different from that of the other elments as redox reactions are excluded for it. We shall therefore treat chromium (III) chemistry on its own. Again the chemistry of manganese is closely associated with Mn(II) to Mn(IV) i.e., lower oxidation states relatively, while that of V, Mo and W is associated with higher oxidation states. We conclude that there is a closer chemical relationship between V, Mo and W than between any one of these elements and Mn. We therefore treat manganese separately.

The chemistry which is likely to be important in the case of the elements early in the transition metal series as compared with that of the elements later in the series has one immediate striking difference: the hydrolytic equilibria of the early elements. We shall, therefore, start the description of the chemistry of these five elements with a discussion of the principles of hydrolysis. Before engaging ourselves in this task we must note the concentrations and the media with which we shall be concerned.

Natural Abundance

It is generally accepted that the evolution of life began in the sea. Organic molecules of biological importance were synthesized from simple starting materials in the course of chemical evolution. These molecules aggregated and life evolved. Apparently these processes required metal ions, for structure formation, for electron transfer as well as for catalysis.

With these premises, a good starting point is to assume that the elements utilized in the evolutionary process were those that had suitable properties and which were present in the sea at sufficiently high concentrations. Let us assume that there are only minor differences between the composition of today’s ocean and that of the primeval sea at the time of the early evolution of life. Fig. 2 shows the abundance of all transition metal ions in the sea (Riley and Chester, 1971). The diagram clearly shows the close correlation between the concentration of the elements in the ocean and their biological activity. We note that all but one of the essential transition metal ions are present in concentrations higher than 10−8mol/l, the only exception being cobalt with a value of 1.4 10−9mol/l (higher values have been reported).

Fig. 2 Natural abundance of the most common transition metal ions in the sea.

If one were to consider the abundance of the transition metal ions in the earth’s crust, one would get a rather similar picture, the exceptions in this case being iron with a high, and molybdenum with a rather low abundance. For information the following list of concentrations in the earth’s crust are given (in millimoles/kg): V, 2.7; Cr, 2.0; Zn, 1.1; Cu, 0.87; Co, 0.43; Mo, 0.015.

Combining the abundance/availability concept with the requirement for redox activity we end up with the following list of metal ions for consideration: V, Cr, Mn, Fe, Cu from the first transition series, and Mo from the second. Tungsten is relatively abundant in the sea, even though its concentration is almost two orders of magnitude lower than that of Mo, and so will be included in our discussion.

Now the sea and land can be thought of as providing aqueous solutions, in which these metals are dissolved, and are therefore the sources of materials for biology. Our next task is to look at the chemical state of the elements of interest which takes us immediately to their hydrolytic equilibria. Later we shall ask: do these equilibria provide a ready means by which a biological system can differentiate between the different metals, and, can they be used so as to differentiate between metals, such as V and Mo, and non-metals such as P and S?

II HYDROLYSIS

Most of the oxidation states under discussion undergo hydrolysis reactions in aqueous solutions (Baes and Mesmer, 1976) and the chemistry of the elements is closely connected to their hydrolytic behaviour. For example, hydrolysis will affect the redox behaviour of the metal ions, the extent of complexation of metal ions by various reagents, the solubility of the metal hydroxides and oxides, the adsorption of metal ions on the surface of soil particles, and so on. A knowledge of the hydrolysis products is essential in recognizing the likely species, their charges, and their stabilities under certain given conditions.

The formation of mononuclear hydroxo species of the metal ions can be illustrated as the stepwise removal of protons from the metal hydration sphere:

(5)

where

(6)

(7)

(M denotes the aquated metal ion and all signs of charges are omitted for convenience).

If base (OH−) is added progressively to an acid metal ion solution, reaction (5) will be driven to the right eventually forming the neutral M(OH)Z species, which will precipitate as a hydrated hydroxide or oxide. At very low concentrations the neutral hydroxide may stay in solution as an aquated species.

The Stability of Mononuclear Complexes

Values for the hydrolysis constants of many of the ions of interest are available in the literature (Baes and Mesmer, 1976). In some cases (V²+, Mo³+, etc.) reliable stability constants are not available, and the hydrolysis behaviour has to be estimated. The model we have chosen is the electrostatic model. If this applies to hydrolysis, the stability constant for the first hydrolysis step can be expected to follow the equation

(8)

where z and d denote the cation charge and the interatomic M-O distance, respectively. A and B are adjustable parameters with B estimated to have a value of about 1.2 nm. The equation provides a fairly good correlation if we confine ourselves to cations of similar hard vs. soft behavior. Fig. 3, in which log KMOH is plotted as a function of the ratio of cation charge to interatomic distance, illustrates the fact that the cation charge is the most important parameter for the hydrolysis of a certain cation.

Fig. 3 The dependence of log KMOH on the ratio of the charge, z, to the M-O distance, d, for transition metal ions and some highly charged pre-transition ions. The regression analysis (excluding Mn³+ and Cu²+, see text) gave A = −21.3, B = 1.20 nm (r² = 0.96).

The cations included in Fig. 3 are transition metal ions with partly filled d-orbitals, and some highly charged pre-transition ions. These ions will most likely form ionic M–0 bonds and the expected effect is shown in the figure. Equation (8) does not take into account the electron configuration of the cation. The figure shows two cations, Mn³+ and Cu²+, that deviate markedly from the general trend. It will be remembered that these cations exhibit Jahn-Teller distortions of their octahedral coordination.

From eq. (3) we can now estimate the stability of the monohydroxy species of V²+ and Mo³+:

For subsequent hydrolysis steps a fairly uniform decrease in the log K-values of successive hydroxy complexes can be assumed. A brief survey of the hydrolytic behaviour of vanadium and molybdenum in their different oxidation states is given in Fig. 4. In general the stabilities of the mononuclear hydrolysis products are given, but as not much is known about the hydrolysis of Mo (IV) and Mo (V) the predominating dimeric species are shown. The hydrolysis species dominating at different pH-regions are given. Distribution diagrams including polynuclear hydrolysis species will be given later in the discussion of the chemistry of the various elements. We see that many species do not remain as hydroxides but go over into oxo-species.

Fig. 4 Predominance diagram for mononuclear hydrolysis products of vanadium and molybdenum in different oxidation states.

Hydrolysis to Oxocations

There are two possible steps in the hydrolysis of the monohydroxy-complexes:

(9)

or

(10)

The ratio of the equilibrium constants is independent of pH. It is observed that the oxo-cations are common only in high oxidation state compounds, e.g. VO²+, UO2²+ and we can now ask why the equilibrium

(11)

is balanced to one side or the other. This is also a common problem in non-metal chemistry, where we have, for example, the equilibrium

(12)

Equilibrium constants for equation (11) have not been given. The qualitative discussion given above indicates that the important parameters influencing equilibrium (11) are: i) the size of the central atom, ii) the charge of the central atom, and iii) the availability of orbitals for double bond formation. These factors control the coordination number of the central atom, or the capability of the central atom to change the coordination number to a higher value.

, with change of coordination number, is evenly balanced. In ionic states M(6+) only oxocations occur.

The hydrolysis of the oxo- or hydroxy-species can be very different. In non-metal chemistry we have the reactions

(13)

(14)

Thus the change in ionization state may alter the number of oxo-groups or merely effect a change in coordination number. We shall see examples of both types of behaviour in the metal chemistry.

Of special significance in this section is that change of oxidation state by the oxocations of V, Mo, W. Mn (Fe is also important) will mean change in the degree of protonation of the oxo-group. Thus redox reactions are coupled in a compulsory manner with proton transfer (see Section III).

A second problem will concern the aqueous solution chemistry of the oxocations themselves as compared with the chemistry of the bare ions on addition of ligands. It is obvious enough that VO²+ and UO2²+ do not behave like typical divalent cations. Rather they bind anions such as F−, OH−, SO4²− too strongly and are thus more like trivalent ions. The linear relationship shown in Fig. 3 can be used to estimate the effective charge of the oxocations using known hydrolysis constants:

We note that the shielding effect from the oxygen atom is less than for an ion of a charge of 2-.

The nature of the ligand which will assist or resist reaction of oxo-cations raises another question. In particular we have the influence of the cis and trans effects on leaving groups and protonation. A particularly important consideration to which we return when we discuss the individual elements is whether or not two oxo-groups are cis or trans to one another.

Before leaving this section the reader should note again Fig. 4 and the pH regions in which the different oxo-species exist, as the pH restricts grossly the interest in many of these species, e.g. VO+2 (see later).

Polynuclear Complexes

The simplest polymers to consider are the hydroxo-bridged dimers formed from the MOH species of reaction (5):

(15)

where the stability of the dimer is given by

(16)

Dimerization and polymerization processes occur frequently for multicharged metalions and almost certainly for metal ions forming hydroxide precipitates. In fact, the formation of a precipitate can be considered the final stage in the polymerization reaction, with the polynuclear ions as building stones in the crystal.

The law of mass action gives the important rule, i.e., the fraction of polynuclear species in a solution will decrease on dilution. This means that for living organisms, where the cation concentrations (except for Na+, K+, Mg++, and Ca++) are very low, the formation of free polynuclear species can in many cases, be neglected. For systems, in which trace concentrations of metal ions are present, the important species will be the soluble mononuclear hydroxo complexes of eq. (5).

In some cases polynuclear species may be formed even at very low concentrations, and may therefore be biologically important. In addition, there is the possibility that the formation of these aggregates can be favoured by complex formation through charge stabilization. As an example, there is a build-up of negative charges in the polymerization of oxo-anions, which process may be favoured by complex formation with highly charged cations.

An examination of the literature (Baes and Mesmer, 1976; Borgen et al., 1977) suggests that polymer formation can be important when the total concentration of metal ion exceeds the following values (calculated from the condition that half of the total metal ion concentration is in the form of the polymer).

We observe the strong pH-dependence of the polymer formation of Mo(VI) and W(VI), whereas that of V(V) is almost pH-independent in this pH-region.

The Hydrosulphylation (Ammonolysis) Reactions

Just as metal ions are hydrolyzed so also do they undergo reactions with other non-metal hydrides. For the complexes with H2S (or NH3) one might expect a similar complicated behaviour as in the metal hydrolysis case:

(17)

(The very low solubility of the sulphides is a restriction on the study of soluble sulphide complexes).

The equilibria between soluble M(SH)n and MSn/2 complexes, the corresponding polymeric species, and the precipitates depend on the oxidation state and the ionic radius of the metal ion as do the water hydrolyses. Now, however, H2S is a good reducing agent and a stronger Brønsted acid than is H2O, dissociating more easily to HS− and S²−. From qualitative analysis tables we know that the sulphides of Sb(III), As(III), and Sn(IV) are soluble in excess of alkaline sulphide solutions.

Thus we can expect that all transition metals after Group IVA, which have oxidation states that are higher than three and which are stable to reduction by H2S, will remain in solution in alkaline H2S solutions, the more so if they have b class character.

The ability to form soluble sulphide complexes can be used for separation purposes. The transition metals which go into this analytical group are Mo(VI), W(VI), and Re(VII). It is possible that this is one characteristic that has allowed molybdenum to be selected in biology.

As is true of oxocomplexes so it is true that protonation of sulphocomplexes occurs on lowering the oxidation state or on acidifying the solution.

Most of the corresponding ammonolysis reactions have been carried out in liquid NH3. The considerations regarding oxidation number and ionic radius can be repeated here for complexes with NH3, NH−2, NH²−, and N³−. An example of this type of complex is the anion, OsO3N−, which is stable even in acid aqueous solution. Similar anions are known for rhenium and molybdenum, ReO3N²− and MoO3N³−, but these are less stable. A stepwise ammonolysis reaction for Mo(V) chloride is

(18)

The reaction has to be carried out in the presence of varying amounts of KNH2 and can be reversed by the addition of acid.

In a formal way, the nitrogenase reactions are but another example of this set of reactions:

(19)

(20)

(21)

It is the higher oxidation states of metals which are involved in these reactions and it is to the higher oxidation states we must look for the catalysis of interest here. At the same time the redox potentials of the metals must match those of the non-metals involved. Thus molybdenum oxidation states can be compared with nitrogen/hydrogen; manganese and iron, can be compared with oxygen/hydrogen; but many metals can be compared with sulphur/hydrogen reactions.

We shall see that these simple protonation reactions of O, S, and N in the coordination sphere of the metals Mn, V, and Mo may be of critical consequence in the catalysis of reactions by these cations as well as in their mode of incorporation into enzymes (proteins). Once again, protonation is often linked to the lowering of the oxidation state.

Note that there are also polymeric forms of the sulphido-complexes which can be discussed in much the same way as oxo- or hydroxy-bridged species.

Now that the principles involved in the stabilities of different hydrolysis products have been outlined, we can turn to the chemistry of the individual elements. In each case we shall treat elements in separate oxidation states describing first the hydrolytic equilibria and then the chemistry. In a rather vague general way we might suppose that the extra-cellular chemistry of these elements is represented by the hydrolytic equilibria, while the intracellular chemistry is represented by complex ion coordination chemistry with organic ligands. On entering the cell membrane the ion will react in the higher oxidation state but while passing through it, or on entering the cell itself, lower oxidation states will be generated. In cell species, oxidation states and polymeric forms, can be preferentially stabilized by ligand binding.

III AN OUTLINE OF MOLYBDENUM, TUNGSTEN AND VANADIUM CHEMISTRY

The d⁰ Configuration: Mo(VI), W(VI), V(V)

We shall discuss the different oxidation states of molybdenum, concentrating first upon hydrolytic equilibria (Baes and Mesmer, 1976), and compare this chemistry with that of the corresponding oxidation states of tungsten and vanadium. In their highest oxidation states these elements form tetrahedral oxoanions in alkaline solutions. As the pH of these solutions is lowered, two processes occur, protonation, and expansion of coordination number from four to six. Thus,

(22)

We write the neutral and acidic species with MoO2²+ units for the following reasons.

The ionic radius of Mo⁶+ is small (73 pm) for octahedral coordination by oxygen, and, displacement of the metal atom from the centre of an MoO6 octahedron will always occur. These displacements most frequently result in two short Mo-O bonds in a cis configuration, but occasionally one or three bonds are found (see below).

Expansion of coordination number is also effected by polyoxoanion formation which becomes significant for Mo and W only at pH<6 and total metal concentration of 10−3 moles/l. The most important aquo species are

(23)

(24)

These are formed rapidly, although in the case of tungstate solutions, subsequent much slower reactions produce polyanions, such as H2W12O40⁶−, that are stable at a tungsten concentration of 10−5 mol/l and pH<6. In contrast to molybdenum, there is no convincing evidence for aquo cations of W(VI), although an octahedral cis WO2²+ unit is prevalent in the relatively few oxocomplexes that have been studied, e.g. WO2F4²−, WO2(acac)Cl−2.

We see, however, that at pH 6 to 7 hydrolysis keeps Mo⁶+ as MoO4²−. We are then interested in two types of reaction:

(1) of the anion itself binding by ion-exchange to positive centres

(2) of the cations with donors which stabilize oxo-compounds, e.g.

(25)

We now can write the reactions (valid at pH 7) and where Pr is a protein

(26)

Thus incorporation of MoO3 or MoO²+2 requires a very large constant in order for the chelation reaction to take place. The complex ion chemistry of MoO3 and MoO2²+ shows that complexes containing these units can be formed especialy with anionic ligands. We therefore describe some of their structures.

In the majority of cases, the Mo(VI) complexes are based on an octahedral coordination containing the MoO2²− unit with the terminal oxygen atoms occupying cis-positions. The Mo-Ot distances in the MoO2²+ unit are around 167 pm showing the π-back donating ability of the oxygen atoms and the multiple bond character of the linkage. The short Mo-Ot linkage weakens the ligand in the trans-position causing the trans-ligand to have a longer bond length. The trans-effect is a general propertiy of the Mo-Ot bond and will be noted in the chemistry of Mo(VI), Mo(V), and Mo(IV).

The trans-effect is shown in the first two structures given above. In the crystal structure of MoO2(cat)2²+ the bond length of the Mo-O bond trans to the terminal oxygens is on the average 215 pm, whereas the bond length for the atom in the cis position is 205 pm. The corresponding Mo-S distances in the complex MoO2(Et2dtc)2 are · 263 pm (trans) and 244 pm (cis), respectively. In the complex MoO3(dien) all nitrogen atoms show the trans effect. The complex has a low stability and is almost completely dissociated in aqueous solution.

Molybdenum (VI) and molybdenum (V) form rather similar dimeric complexes, where the bridges are formed by oxygen and sulphur atoms, or by chelating organic ligands. Two examples of Mo(VI) complexes are shown above.

When discussing the hydrolysis behaviour we pointed out that the addition of protons to molybdate (MoO4²−) solutions changes the geometry around the metal ion from tetrahedral to octahedral. The shift from 4- to 6-coordination also seems to be a dominant feature of Mo(VI) chelate formation reactions. From the few relaxation studies of molybdate kinetics it is not clear whether the reaction mechanism implies an expansion of the coordination as a precursor rather than as a result of complex formation (Kustin and Liu, 1973). It is however, clear that the ability of Mo(VI) to expand the coordination number from 4 to 6, from tetrahedral to ocatahedral coordination, is an extremely important feature of this anion, in marked contrast with SO4²−.

Tunsten(VI) has an almost identical ionic radius to that of Mo(VI) and there are consequently many similarities in the chemistry of these two elements. Thus, the octahedral cis WO2²+ unit is prevalent in the relatively few oxo-complexes that have been studied. There are no fundamental differences between Mo and W in their coordination chemistry and, as far as they are known, the stabilities of Mo(VI) and W(VI) complexes are rather similar.

Vanadium(V) hydrolysis follows a pattern similar to that of Mo(VI) below pH 5, i.e. solute species based on six-coordinate vanadium, e.g. V10O28⁶−, cis VO2²+.

However in neutral or basic solutions analogies to phosphorus chemistry appear, V2O7⁴−, V3O9³−. The formation constants (Borgen et al., 1977) are such that appreciable concentrations of these polyanions could exist under biological conditions. This is not the case for molybdenum as comparison of Figs. 5 and 6 illustrates.

Fig. 5 The distribution of molybdate species in 10−4 mol/l Mo(VI). Mo7O24 denotes the Mo7O24⁶− ion and its protonated forms. The calculations were based on constants from Baes and Mesmer (1976).

Fig. 6 The distributions of vanadate species in 10−5 mol/l V(V). Constants are from Baes and Mesmer (1976) and Borgen et al. (1977).

In biology we are concerned with the same type of binding equilibria as mentioned above for molybdenum:

(27)

We note that the effect of hydrolysis is less for VO2+ than for MoO2²+, but the same trend holds for the chelate stabilities. The net result will Be slightly in favour of VO2+, as shown below for EDTA-chelates. The vanadium chemistry in biology will not necessarily be concerned just with vanadate (VO4³−) and its polymers but also with the complexes of the oxocation. Note however that V(V) is more readily reduced than is Mo(VI).

V(IV) reduction can be observed. This kinetic limitation on the redox reactions of the elements under consideration must be kept in mind when we look at biological systems.

Examples are of the kind:

(28)

(29)

implying that reduction occurs with a lowering of the number of oxo-groups and uptake of protons. This is a common feature of non-metal chemistry but not of Fe(III)/Fe(II) or of Cu(II)/Cu(I) chemistry.

The best examples of the above redox equilibria are given by the heteropoly anions (Altenau et al., 1975). Data are summarized in Fig. 7. When allowance has been made for the effects of anion charge, the mean difference in the reduction potentials for V-Mo and Mo-W are ca. 0.2 and 0.4 V, respectively. Vanadium (V) is therefore a slightly better oxidizing agent than Mo(VI) under comparable conditions, but tungsten (VI) is significantly poorer. In subsequent reduction steps the difference between Mo and W may be smaller, however. The reduction potentials of Mo(CN)8³− and W(CN)8³−, M(V)→M(IV), differ by 0.16 volt only.

Fig. 7 pH-independent reduction potentials for a series of isostructural heteropolyanions XM12O40n−, corresponding to M(d⁰, oct)→M(d¹, oct). x, M = V; o, M = Mo; •, M = W.

The d¹ Configuration: Mo(V), W(V), V(IV)

0, due to precipitation. The same may be said for other lower oxidation states. The cation, Mo2O4²+, may be prepared by electrolytic reduction of Mo(VI) at −0.5 V(Chalilpoyil and Anson, 1978). Cryoscopy, spectroscopy and kinetic measurements confirm that Mo2O4²+ remains dimeric at concentrations of 10−3 mol/l. No evidence for significant concentrations of monomeric species has been put forward. As the pH of a solution of Mo(V) is raised a soluble brown (polymeric?) species is formed (1

Oxocomplexes of Mo(V) may be divided into mononuclear, paramagnetic MoOL4 and MoOL5 species, most of which are stable only in acidic solution and incorporate halide or pseusohalide ligands, and a considerable number of binuclear, generally diamagnetic, species with oxo or sulphido bridges (Stiefel, 1977) e.g.,

Without exception oxomolybdenum (V) complexes incorporate the MoO³+ unit with a strong Mo-Ot bond. There seems little doubt that the oxo cation dimer involves a di-μ-oxo group as found in the cysteine and EDTA complexes. Bond distances in these structures show trends similar to those of the Mo(VI) oxidation state. The strong interaction between molybdenum and the terminal oxygen will again be displayed in the trans-effect. An interesting example is Mo2O3(Etxan)4, where the order of the Mo-S distances is Mo-S (trans to S) < Mo-S(trans to Ob)⁴ < Mo-S (trans to 0).

The bridging oxygen can be changed for sulphur atoms, and often the disulphido bridge will be formed merely by bubbling H2S into an aqueous solution of the di-μ-oxo analogue. The molecular structures of the Mo2O4²+ and Mo2O2S2²+ compounds are very similar.

The electrochemistry of the binuclear Mo(V) complexes will, however, be significantly altered in the presence of sulphur bridging atoms (Ott et al., 1977). In aqueous solution the Mo2O2X2(edta)²− compounds (X = 0 or S) are reduced in a single 4-electron step to Mo(III) complexes:

(30)

the substitution of one or two sulphur atoms into the bridge of Mo2O4(edta)²− causes the electron transfer process to change its nature from a highly irreversible to a reversible one. In recent years, some triply-bridged complexes have been structurally resolved (Huneke et al., 1978). The two examples shown above with triply (O, S, O) and (S, O, S) bridged structures may be indication that supplemental bridging in molybdenum dimers is a common feature.

The aqueous chemistry of W(V) has received less attention than that of Mo(V) and fewer complexes have been described. Mononuclear species such as WOCl5²− and W(CN)8³− have corresponding molybdenum complexes, and the dimeric EDTA complex has been shown to be isostructural with the molybdate analogue shown above. Simple oxo cations of W(V) have so far not been detected. In contrast, the chemistry of vanadium (IV) is particularly extensive and is dominated by the VO²+ group. A dimeric species (VO)2(OH)2²+ appears prior to precipitation of the hydroxide, VO(OH)2, at pH 4. The latter redissolves in alkaline solution (pH > 12) to give VO(OH)3− (Iannuzzi and Rieger, 1975) and, in more concentrated solutions, a polyanion, V18O42¹²− (Johnson and Schlemper, 1978). Under extremely alkaline conditions (5 mol/l NaOH) V(IV) disproportionates into V(III) and V(V). Analogous behaviour is observed for Mo(V) and W(V) which yield the +4 and +6 oxidation states under slightly less basic conditions.

In biology we are unlikely to be concerned with the reaction of Mo(V)

(31)

since MoO4− is not a stable species. We are then interested in the reaction

(32)

We note that KMoOY is particularly large for this oxo-cation as the effective charge on the cation as seen by an anionic ligand is again considerably higher than the formal ionic charge as written (see Sections II and IV).

The reduction of Mo(V) species to Mo(IV) may also involve the uptake of protons to which we refer later.

The d² Configuration: Mo(IV), W(IV), V(III)

The coordination chemistry of Mo(IV) is extensive and in many respects parallels that of vanadium(IV). Thus, numerous monomeric oxocomplexes, MoOL4, MoOL5, are known, and non-oxocomplexes sometimes have V(IV) analogues, e.g. MoCl6²−, Mo(Etdtc)4. Binuclear complexes, a predominant feature of Mo(VI) and Mo(V) chemistry, are less common, although the aquo cation, Mo2O2⁴+, has been shown to be dimeric. Solutions containing this cation are most conveniently prepared by reaction of Mo(V) and Mo(III) in p-toluene- or trifluoromethylsulphonic (TFMS) acid at 90°C (Chalilpoyil and Anson, 1978), and are stable against air oxidation and dissociation. The cation is reducible at ca. −0.7 V in HCl or TFMS to a Mo(III) species. In less acid solutions, colour changes interpreted as the formation of an oligomer of MoO(OH)+ culminate in the precipitation of MoO(OH)2 at pH ≈ 1.5. The precipitation is complete at pH = 3 and the hydroxide ultimately redissolves in 7–8 mol/l NaOH to give a blue-green Mo(IV) anion.

The coordination compounds of Mo(IV) show a great range of coordination numbers. The oxocomplexes contain the MoO²+ unit with the ligand trans to the terminal oxygen either weakly bonded or absent (the trans-effect). An example of 5-coordination is the dithiocarbamate complex:

The complex is coordinatively unsaturated and can undergo an oxygen transfer reaction:

(33)

The MoO²+-complex can be prepared from the MoO2²+-complex by a reverse reaction using triphenylphosphine as an oxygen abstracting agent.

Protonation equilibira are also important and here we quote as an example trans-MoO2(CN)4⁴−, which can be isolated from strongly alkaline solutions. The complex is readily protonated at normal pH-values and the successive protonation constants have been determined as 12.6 and 9.96 at 25°C. We see now how oxo and hydroxo chemistry have come closely in balance at this oxidation state.

Apart from a few well-known complexes, W(CN)8⁴−, WO2(CN)4²−, WCl6²− etc., all of which have Mo analogues, the coordination chemistry of W(IV) has not been well studied. There is no reason to anticipate any major differences between the two elements in this oxidation state.

The formation of metal-metal bonded units in complexes of the second and third transition metals in their lower oxidation states is well-established. The oxo dimer of Mo(IV) almost certainly contains such a metal-metal bond in contrast to the much more weakly associated dimers of VOOH+ and FeOH²+. Other metal-metal bonded complexes of Mo(IV) and W(IV) stable in aqueous solution are the trimeric oxocomplexes (Wendling and Rohmer, 1964; Mennemann and Mattes, 1976; Bino et al., 1978).

An identical W3O4⁴+ unit is found in reduced polyoxotungstates (Kazansky and Launay, 1977)

The chemistry of vanadium(III) more closely resembles that of other trivalent first row transition elements (Cr, Fe, etc.) than that of Mo(IV). The majority of the complexes formed in aqueous solution are monomeric octahedral species with O- or N-donor ligands. Like Fe(III), ligand substitution reactions are generally rapid (k = 1s−1). In the hydrolytic behaviour the dimerization is not as pronounced as for V(IV).

For Mo(IV) in biology we look at the equilibria

(34)

At pH 7 cationic species will be maintained in solution but it is no longer clear that the complex will be an oxo-species. Thus, once again proton uptake may occur with reduction:

(35)

Reduction of VO²+ to V³+ also eliminates the oxo-group.

The d³ Configuration: Mo(III), W(III), V(II)

Monomeric and dimeric Mo(III) complexes are well established. The aquo cation, Mo(H2O)6³+ prepared by aquation of MoCl6³−, undergoes substitution reactions at rates which are faster than those of the corresponding isoelectronic octahedral Cr(III) complexes. This may be important in nitrogen fixation. The kinetic inertness associated with Cr(III) does not therefore seem to be a feature of Mo(III). Two other aquo cations of Mo(III) are known (Chalilpoyil and Anson, 1978), the green dimer Mo2(OH)2⁴+, which is produced by electrolytic reduction of Mo2O4²+ at −1.0 V or by reduction of Mo(VI) with amalgamated zinc, and a yellow, probably dimeric species, produced by electrolytic reduction of Mo2O2⁴+ at −0.7 V. The yellow and green species do not seem to be readily interconvertible: oxidation of the former (−0.15 V) yields Mo2O2⁴+, and oxidation of the latter (+0.05 V) yields Mo2O4²+.

Electrolytic reduction of the dimeric Mo(V)-EDTA complex (see above) yields dimeric Mo(III) complexes, one of which has been structurally characterized,

The chemistry of W(III) complexes consists at present of a relatively small number of metal-metal bonded halide complexes such as W2Cl9³− (cf. Mo2Cl9³−, V2Cl9³−) and W2Cl6(py)4. Vanadium(II) is generally obtained by electrolytic or zinc amalgam reduction of V(V), V(IV), or V(III). The reduction potential for V³+/V²+ in acid solution is −0.255 V (cf. Cr³+/Cr²+, −0.41 V; Fe³+/Fe²+, +0.77 V). The majority of V(II) complexes are monomeric octahedral species that undergo relatively slow ligand substitution reactions (k = 90 s−1). Little is known about the hydrolytic behaviour of V(H2O)6²+.

The hydrolytic behaviour of chromium(III) is well known. Fig. 8 shows the distribution of mononuclear and polynuclear hydrolysis products. It is remarkable that a trinuclear species will persist in a 10−6 mol/l solution. (When comparing the distribution diagrams above it should be noted that these diagrams have been calculated for different total concentrations of metal ion.)

Fig. 8 The distribution of chromium species in 10−6 mol/l Cr(III). Constants from Baes and Mesmer (1976).

The complex ion chemistry of M³+ ions is well-known and it is well-recognized that hydrolysis is readily prevented by chelation, e.g. in Fe³+ chemistry. This point is made in more detail below. We stress, however, that terminal oxo-groups are not found amongst the d³-species.

Summary of Sections II and III

MS + H2O, is well on the side of MS at low H2S concentrations.

The stereochemistry of all of these oxidation states leaves us with complexes with one open side, in that the oxo-species are cis not trans. The large majority of the complexes are octahedral but lower coordination numbers are known.

The kinetic constraints are as follows: (i) rapid reduction of a dioxo Mo(VI) is difficult since it requires the replacement of one oxygen atom by hydroxide and change from tetrahedral geometry. The reduction is helped by octahedral coordination or by replacement of the oxo-groups by sulphur; (ii) the Mo(III) state is not very inert in a kinetic sense, and, given some strong donor ligands, Mo(III) may be labile (compare cobalt(III) in vitamin B12); (iii) there will be changes in bond lengths and bond angles due to the strong effect of oxo-groups and the stereochemical changes on going from d⁰ to d¹, d², and d³. These can be overcome if a protein restricts the geometry of a particular oxidation state; (iv) reduction of molybdenum from VI to III requires a smaller potential than reduction of vanadium from V to II.

IV UPTAKE OF METALS INTO CELLS

Since it is known that the uptake of metal ions into cells is selective and must work at very low metal ion concentration so that the metal ion can cross the membrane, complex formation is essential (da Silva and Williams, 1976). For example we know that bacteria scavenge for iron by using reagents, R, which will bind to Fe³+ in aqueous media with a binding constant of log K>25, and at the same time they have a partition coefficient as a chelate Fe³+-R that favours an organic medium. Inside the cell the chelate can be metabolized or the chelate reduced:

(36)

In the circulating fluids of higher animals Fe³+ is transported by a protein, conalbumin or transferrin, which also