29 Bioinorganic chemistry J. D. Crane Department of Chemistry, University of Hull, Cottingham Road, Kingston-upon-Hull, UK HU6 7RX 1 Introduction A new reference text on spectroscopic methods in bioinorganic chemistry has been published.1 Specialist review articles have covered enzyme bioelectrochemistry in biomembrane-like films,2 organometallic derivatives of amino acids and peptides,3 and the biomedical chemistry of technetium and rhenium.4 The bond valence sum analysis of copper and iron metalloproteins, and of biologically relevant iron coordination compounds has been reported.5 The various roles of protein radicals in enzyme catalysis have been reviewed,6 as has the chemistry of the three classes of ribonucleotide reductase.7 An ab initio study of the oxidative damage to cysteine residues in proteins and the oxidative damage caused as a result of H· abstraction by the resulting thiyl radicals has been reported.8 The incorporation of a cysteine residue in a Thr-89-Cys mutant of bacteriorhodopsin in order to use the S–H group as an IR probe for hydrogen-bonding has also been described.9 The chemistry of RNA/DNA cleavage has been extensively reviewed.10 The DNA binding properties of several ruthenium(II) and rhodium(III) polypyridine luminescent probes have been investigated.11 A CD and NMR spectroscopy study has shown that the observed, fast, long-range electron transfer mediated byDNAis not simply a result of the metallointercalator probes binding cooperatively within close proximity of one another.12 The DNA binding of tetracationic porphyrin complexes of vanadyl(IV) and cobalt(III),13 dicationic polypyridyl platinum(II) complexes,14 and the nuclease properties of phenanthroline complexes of copper(I/II) have also been reported.15 2 Magnesium and calcium Three crystal structures of adenylate kinase from Bacillus stearothermophilus with bound substrate and magnesium(II) or manganese(II) have been reported.16 The role of the nucleophilic Ser-102 residue in the alkaline phosphatase from Escherichia coli has been investigated through crystal structure determinations of the Ser-102-Gly, Ser- Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 611102-Ala and Ser-102-Cys mutants with phosphate bound at the active site.17 The functional roles of the two magnesium(II) ions at the active site of DNA polymerase b have been investigated using inert chromium(III) nucleotide complexes.18 The stability constants for the binding of magnesium(II), zinc(II) and copper(II) to the nucleotide 2-deoxyguanosine-5-monophosphate (dGMP2~) and its platinum(II) complex cis- [Pt(NH 3 ) 2 (dGMP) 2 ]2~ have been determined; the platinum(II) centre had little influence upon metal ion binding to the phosphate group.19 Recent progress in the development of synthetic nucleases and related metalloenzymes has been reviewed.20 The structure and function of calcium binding proteins have been reviewed,21 as have the EF-hand proteins in particular.22 The thermodynamics of the binding of magnesium(II) and calcium(II) to calmodulin have also been described.23 The crystal structure of calsequesterin from rabbit muscle has been reported and a mechanism proposed for the binding and release of 40–50 calcium(II) ions for each contraction/ relaxation cycle.24 The role of the calcium(II) ion in the reoxidation of PQQH 2 to the coenzyme PQQ by dioxygen has been investigated using semi-empirical MO calculations.25 Biomineralisation processes have been investigated with a variety of artificial systems.The crystallisation of spherical calcite particles on colloidal gold particles coated with a monolayer of p-sulfanylphenol has been described,26 as has the deposition of aragonite and strontianite using similar monolayers on a gold(III) surface.27 The nucleation of calcium phosphate and calcite/aragonite with polymers,28 and the crystallisation of apatite and vaterite in polymer matrices,29 have also been described. 3 Vanadium The full speciation for the aqueous vanadate(V)–uridine (UrH 2 )–imidazole system has been determined.30 In the absence of imidazole the major species are dimeric complexes of general composition [V 2 Ur 2 ]2~,3~, whereas in the presence of imidazole monomeric mixed ligand species are formed.Model compounds for the binding of vanadium(III) and vanadyl(IV) to peptides,31 and vanadate(V) esters of chelating alcohols as models of phosphate esters,32 have been reported. The catalytic oxidation of thioethers using several vanadium halogenoperoxidases has been described;33 the bromoperoxidase from Ascophyllum nodosum oxidises phenylmethylsulfide to the (R)-sulfoxide with 85% ee.Several model complexes for vanadium halogenoperoxidases have also been reported.34 Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 6124 Manganese The structure and reactivity of manganese redox enzymes and relevant model compounds have been reviewed.35 The di§erences in reactivity for iron and manganese haems in peroxidases have been studied through the preparation of the manganese(III) complex of microperoxidase-8 via demetalation of the well-known iron(III) analogue.36 The active site of human manganese superoxide dismutase has been investigated with the Gln-143-Asn and Tyr-34-Phe mutants, both of which have been structurally characterised;37 the Gln-143 residue appears to control the redox balance of the manganese ion and the Tyr-34 residue is proposed as the proton source for the production of hydrogen peroxide.Inactivation of the wild type enzyme with peroxynitrite proceeds by exclusive nitration of Tyr-34 at the 3-position.38 Despite the fact that manganese and iron superoxide dismutase enzymes often have similar site structures the presence of the wrong metal ion usually produces little or no catalytic activity.The determination of the crystal structure of the iron-substituted manganese superoxidase dismutase from Escherichia coli has shown that the stereochemistry of the resting state di§ers significantly from the active manganese form.39 The inactivity of these metal-exchanged superoxide dismutases has been explained by the incorrect redox balance of the metal ion rather than competitive inhibition of superoxide binding by hydroxide.40 Simple model complexes for manganese superoxide dismutases have been described,41 as has a stable alkylperoxo complex of manganese( II).42 Spin-coupled binuclear manganese(II) sites have been identified by EPR spectroscopy in the aminopeptidase P from Escherichia coli and the dinitrogenase reductase activating glycohydrolase from Rhodospirillum rubrum.43 A dicobalt(II) functional model complex for the dimanganese(II) site of arginase has also been described.44 The oxidised dimanganese(III) form of manganese catalase from Thermus thermophilus exists as an equilibrium mixture of two forms, dependent upon pH;45 one displays weak antiferromagnetic coupling (low pH, J[[2 cm~1) whereas the other is strongly coupled (high pH, J[[100cm~1).Several model complexes for manganese catalase have also been studied.46 Dioxygen production by the oxygen evolving complex of photosystem II has been reviewed.47 A multiline EPR signal has been reported for the S 1 state of photosystem II.48 In calcium depleted photosystem II advancement past the S 2 state is inhibited and illumination produces a new state formally equivalent to S 3 .ESE-ENDOR spectroscopy of this state indicates that a tyrosyl radical (YZ ·) has been generated and is magnetically coupled to the manganese cluster.49 This redox-active tyrosine residue has been proposed as facilitating hydrogen abstraction during water oxidation.50 The reaction of the S 2 state of photosystem II with nitric oxide at low temperature generates a state with an EPR spectrum characteristic of a magnetically isolated, mixed-valence dimanganese(II/III) unit.51 The nature of the species responsible for the g\4.1 EPR signal of the S 2 state has been investigated by EPR spectroscopy and a SQUID magnetisation study, and an S\5 2 spin state is proposed.52ESEEM spectroscopy has shown that the manganese cluster of photosystem II is accessible to small alcohols, specifically methanol and ethanol, with limited access for propan-1-ol and none for propan-2-ol.53 Several multinuclear manganese model complexes for the oxygen-evolving complex of photosystem II have been decribed.54 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 6135 Iron haem biosites The autoxidation chemistry of oxygenated myoglobin and haemoglobin,55 the spectroscopy of metal ion reconstituted haemoglobins,56 and the chemistry of leghaemoglobin, 57 have all been reviewed.The majority of the protons of the haem region for several deoxy-myoglobins have been assigned by 1H NMR spectroscopy.58 The photodissociation and rebinding of the axial water molecule of the low temperature (20 K), low-spin form of sperm whale deoxy-myoglobin has been studied.59 The iron–ligand bond lengths in horse heart deoxy- and aquamet-myoglobin have been determined by XAFS spectroscopy.60 ResonanceRaman studies of the carbon monoxide adducts of haemoglobin with selectively deuteriated haems have helped to clarify the debate concerning the assignment of the d(Fe–CO) fundamental, favouring the weak band at[580 cm~1.61 Resonance Raman spectroscopy of the carbon monoxide adduct of the His-64-Leu mutant of sperm whale myoglobin has shown that the Fe–CO group is hydrogen-bonded to the distal His-64 residue in the wild type protein.62 Density functional theory has also been used to investigate the geometry and the ease of deformability of the Fe–COgroup in haem proteins.63 The reactivity of haemoglobin towards nitric oxide under physiological conditions has been described. 64 The iron–ligand bond lengths in the nitric oxide adducts of iron(II) and iron(III) horse heart myoglobin have been determined by XAFS spectroscopy and are in good agreement with model compounds.65 Haemoglobin I from Lucina pectinata is known to bind reversibly hydrogen sulfide rather than dioxygen; resonance Raman and 1H NMRspectroscopy have indicated that the haem is not firmly anchored in the haem pocket and that the resulting rocking freedom may facilitate substrate binding.66 A 13C NMR spectroscopy study of six-co-ordinate haem model complexes has shown that the chemical shift of the meso carbon atoms is a good indicator of the degree of porphyrin rußing.67 The chemistry of haem oxygenase has been reviewed.68 The role of the distal His residue in haem oxygenase has been studied by 1H NMR and EPR spectroscopy,69 and by the e§ect of mutation of this residue upon the products of haem degradation.70 The co-ordination chemistry and reactivity of verdohaems, which are observed as intermediates in haem degradation, have been investigated with model compounds.71 Barley grain peroxidase is reversibly deactivated at pH[5 and crystal structure determinations at several pH values have allowed the identification of the structural changes responsible for the change in activity.72 Resonance Raman and infrared spectroscopy have been used to investigate the role of the proximal His residues in the chloroperoxidase from Notomastus lobatus and the dehalogenoperoxidase from Amphitrite ornata.73 Resonance Raman spectroscopy has also been used to investigate the hydroxylation of benzene by horseradish peroxidase.74 An agostic mechanism for the oxidation of C–H bonds by cytochrome P450 has been suggested as an alternative to the rebound mechanism.75 This is not supported by subsequent studies with substrate probes, which indicate that there are two di§erent oxidants present in the P450 system, both of which can e§ect hydroxylation.76 Cytochrome P450 has been the subject of several theoretical investigations, one conclusion of which is that simultaneous delivery of two protons to the bound dioxygen is necessary to generate the ferryl Fe––O intermediate e¶ciently.77 The role of Asp-251 as a proton delivery group at the active site of cytochrome P450 has been investigated through determination of the Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 614crystal structure of the Asp-251-Asn mutant.78 Several model compounds for cytochrome P450 have also been described.79 Resonance Raman spectroscopy has been used to study the reduction of nitric oxide to nitrous oxide by cytochrome P450nor (nitric oxide reductase) from Fusarium oxysporum.80 A Cys-ligated, low-spin, iron(III) haem similar to cytochrome P450 has been found in the carbon monoxide sensing protein CooA from Rhodospirillum rubrum.81 The structure, spectroscopy and function of cytochrome c oxidase have been reviewed. 82 The binding of carbon monoxide at the haem a 3 /Cu B site of cytochrome bo 3 from Escherichia coli has been investigated with photoacoustic calorimetry.83 The magnetic coupling between haem a 3 and Cu B of cytochrome bo 3 has been quantified through the successful simulation of the EPR spectra of this site.84 This work indicates very weak coupling (J[1 cm~1) in contrast to the currently accepted description for this site (J[100 cm~1).Several model complexes for the haem a 3 /Cu B unit have been described.85 The crystal structure of turnip cytochrome f has been determined at 1.96Å resolution.86 6 Iron non-haem biosites The structure of the outer membrane receptor for the iron-bound form of the ferrichrome siderophore of Escherichia coli has been crystallographically characterised both with and without bound siderophore.87 The solution structure of the gallium(III) complex of the peptidic siderophore pyoverdin G4R from Pseudomonas putida G4R has been determined by 1H NMR spectroscopy.88 The co-ordination chemistry of the bis(catecholate) siderophores (amonabactins) from Areomonas hydrophilia and the cyclic dihydroxamate siderophores alcaligin and bisucaberin has been studied.89 Models for the co-ordination of tris(catecholate) siderophores have also been described. 90 The crystal structures of several forms of human transferrin have been determined which have enabled the identification of structural changes upon iron binding/release. 91 The role of second co-ordination sphere hydrogen-bonding on the ironbinding properties of human transferrin has been investigated with a series of mutant proteins.92 The reduction potentials for the iron centre in several forms of human transferrin at pH 5.8 have been determined.93 In all cases the potentials were more negative than [500mV (vs.NHE) and thus too negative for physiological reducing agents, suggesting that there is an additional process involved in iron release which significantly raises this potential. Various derivatives of the iron(II) and iron(III) forms Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 615of bleomycin and the interaction of its substituted hydroperoxocobalt(III) form with oligonucleotides have been studied with a range of spectroscopic techniques.94 Mononuclear iron(III) complexes as functional models for iron superoxide dismutase,95 and as spectroscopic models for the binding of peroxide at an iron(III) centre,96 have also been reported.A combined NIR and MCD spectroscopy study of a selection of structurally characterised, high-spin, iron(II) model complexes has allowed the correlation of spectroscopic properties and structural features, which may then be used to elucidate the structures of iron(II) sites in metalloproteins.97 Specifically, this approach has been used to investigate the binding of the a-ketoglutarate cosubstrate at the iron(II) centre in clavaminate synthase.98 The structure of the mononuclear iron(II) site of peptide deformylase from Escherichia coli and those of the nickel(II) and zinc(II) substituted forms, one with bound peptide reaction product, have been determined by X-ray crystallography.99 The axial Tyr-447 iron ligand in protocatechuate 3,4-dioxygenase dissociates from the metal centre upon binding of the substrate. In order to investigate this process the crystal structure and reactivity of the Tyr-447-His mutant has been determined; both substrate binding and product release were substantially slower than for the wild type enzyme.100 This ligand dissociation process has also been established for several other dioxygenases by XAFS spectroscopy.101 Several functional model complexes for catechol dioxygenases have been described.102 The crystal structure of rat tyrosine hydroxylase with bound cofactor analogue 7,8-dihydrobiopterin has been determined at 2.3Å resolution.103 The structure shows that the Phe-300 residue is autocatalytically hydroxylated at the 3-position, hydrogen-bonds through this group to the active site and p-stacks with the pterin cofactor.The crystal structure of the similar human phenylalanine hydroxylase has also been reported.104 The relationship between iron co-ordination environment and activity for soybean lipoxygenase-1 and human 15-lipoxygenase has been investigated by EPR and MCD spectroscopy on the wild type enzymes and the Asn-694-His mutant of the soybean enzyme, a mutation designed to produce a site structure more similar to that of the human enzyme.105 A structural and spectroscopic model for the hydroxyiron(III) form of soybean lipoxygenase-1 has also been described.106 The crystal structure of dark-inactivated nitrile hydratase with nitric oxide bound at the iron(III) centre has been determined.107 Two of the Cys ligands are modified to sulfenic and sulfinic acids respectively, and together with the Ser residue these surround the nitric oxide ligand with oxygen atoms, an arrangement that is postulated to assist in the photodissociation of the Fe–NO group required to generate the active enzyme.Several model compounds for nitrile hydratase, both with and without bound nitric oxide, have been described.108 Low temperature radiolytic reduction of the diamagnetic di-l-oxo-diiron(IV) form of methane monooxygenase (intermediate Q) has been shown by 57Fe Mo� ssbauer spectroscopy to generate a mixed-valence diiron(III/IV) form.109 In addition, the 57Fe Mo� ssbauer parameters for this species are very similar to those of the diiron(III/IV) form of ribonucleotide reductase (intermediate X), suggesting a strong similarity in structure and reaction chemistry for the two biosites.ENDOR spectroscopy of the diiron(II/III) form of methane monooxygenase has shown that methanol binds directly to a vacant site at the diiron core, and that methanol, water and dimethyl sulfoxide are able to bind simultaneously to the metal centres.110 A resonance Raman spectroscopy signature Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 616for the postulated di-l-oxo-diiron(IV) diamond core of intermediate Q has been identified for model complexes containing this structural unit.111 The reaction of methane with a di-l-oxo-diiron(IV) diamond core has also been studied theoretically. 112 XAFS spectroscopy of ribonucleotide reductase intermediate X from Escherichia coli has identified a short Fe–Fe distance of 2.5Å that is not present in the diiron(III) form.113 ENDOR spectroscopy of intermediate X generated from the reaction of the diiron(II) form with dioxygen has shown that the latter is incorporated as a single oxo bridge and a terminal water ligand. If there are additional bridging ligands they are carboxylates rather than oxo or hydroxo.114 Comparison of the site structures of methane monooxygenase from Methylococcus capsulatus and ribonucleotide reductase from Escherichia coli reveals only one non-equivalent amino acid residue; the counterpart of Glu-114 in the former is Asp-84 in the latter.The consequence of this di§erence has been investigated through the preparation of the Asp-84-Glu mutant of the ribonucleotide reductase.115 Unlike the wild type protein the reaction of the diiron(II) form of this mutant with dioxygen forms an observable peroxodiiron(III) intermediate similar to that known for methane monooxygenase.For the successful generation of intermediate X a reducing equivalent must be transferred to the diiron core and site-directed mutagenesis studies have indicated that the reductant is probably H· and that it is transferred via Trp-103 and Asp-266.116 In other mutagenesis studies the absence of the Tyr-122 residue (the normal residue oxidised to a tyrosyl radical by the diiron centre) in the Tyr-122-Phe–Glu-238-Ala mutant results in 3- hydroxylation of the Phe-208 residue, which then binds to the diiron core.117 Furthermore, in the Phe-208-Tyr mutant the Tyr-208 residue is co-ordinated to iron even in the diiron(II) form and upon reaction with dioxygen is hydroxylated to dopa (3,4- dihydroxyphenylalanine) which remains co-ordinated to the diiron(III) core.EPR and ENDOR spectroscopy have shown that the tyrosyl radical in mouse ribonucleotide reductase is probably directly hydrogen-bonded to the water ligand formed at the diiron core; it is postulated that this interaction helps to stabilise the radical.118 Functional diiron model compounds for methane monooxygenase and ribonucleotide reductase have been reviewed.119 The structure, spectroscopy and reactivity towards dioxygen of several new model complexes for these metalloenzymes have also been described.120 The alkene monooxygenase from Xanthobacter Py2 contains a diiron core similar to that found in toluene 4-monooxygenase.121 XAFS and 57Fe Mo� ssbauer spectroscopy of the diiron(III) site of stearoyl-ACP D9-desaturase has identified a mixture of two site structures: [Fe 2 (l-O)(l-O 2 CR 2 ) 2 ]2` with Fe–Fe 3.12Å and [Fe 2 (l-OH)(l- O 2 CR 2 ) 2 ]3` with Fe–Fe 3.41Å.122 The peroxodiiron(III) intermediate form of this biosite has also been characterised by optical and resonance Raman spectroscopy.123 The mixed metal iron–zinc form of uteroferrin with bound molybdate and tungstate anions has been studied by XAFS spectroscopy.124 A dioxygen binding model complex for haemerythrin has also been described.125 The mineralisation of ferritin has been reviewed.126 The formation of a peroxodiiron( III) intermediate during the ferroxidase reaction of ferritin mineralisation has been observed by 57Fe Mo� ssbauer spectroscopy.127 The structure and electronic properties of iron–sulfur proteins,128 and recent developments towards understanding the many biological roles for these proteins,129 have been reviewed.The crystal structure of the rubredoxin from Pyrococcus furiosus has Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 617been determined at 0.95Å resolution.130 Isotope-dependent (2H/1H) Fermi-contact e§ects on the 15N NMR chemical shifts for the iron(II) and iron(III) forms of the rubredoxin from Clostridium pasteurianum have been used to identify the peptide hydrogen-bonds to the Cys ligands.131 In addition, the origins of the observed contact shifts for 1H, 2H, 13C and 15N nuclei for both oxidation states of this rubredoxin have been investigated with density functional calculations.132 The solution structure of the iron(II) form has also been determined by 1H NMR spectroscopy.133 Sulfur K-edge XAFS spectroscopy has been used to determine the degree of Fe–S covalency in a range of iron(III) rubredoxins and model compounds, although there was no apparent correlation between the degree of covalency and the site structure.134 Rubredoxin-like biosites have been rationally designed into iron–sulfur free proteins by site-directed mutagenesis.135 The crystal structure of the Cys-42-Ser mutant of the iron(III) rubredoxin from Clostridium pasteurianum reveals an Fe–Oc(Ser-42) distance of 1.83Å and significant structural changes around the Ser-42 residue.136 An attempt to reduce this biosite to the iron(II) form with dithionite resulted in loss of iron and SO 2 addition to the Cys-6 and Cys-9 residues of the iron binding site.Rubredoxin model compounds which incorporate amide NH· · · S hydrogen-bonding have been reported.137 The crystal structure of the [2Fe-2S] ferredoxin adrenodoxin has been determined at 1.85Å resolution.138 A method for incorporating protein environment e§ects into density functional calculations on [2Fe-2S] ferredoxins has been described.139 The electronic and magnetic properties of [4Fe-4S] ferredoxins and their model compounds have been reviewed.140 The all-iron(II) form of the [4Fe-4S] cluster of the iron protein of Azotobacter vinelandii nitrogenasen studied by MCDspectroscopy; it exhibits transitions in the 450–1500nm range, very di§erent from iron(II) rubredoxins.141 The solution structure of the oxidised ferredoxin I from Desulfovibrio africanus has been determined by 1H NMR spectroscopy and compared with the crystal structure.142 The minimal protein ligand requirements for the incorporation of a stable [4Fe-4S] cluster has been investigated using a series of models with peptides containing sixteen amino acids.143 The structure and properties of high potential iron proteins (HiPIPs) have been reviewed.144 The crystal structure of the HiPIP from Chromatium purpuratum has been determined,145 and the kinetics of iron cluster assembly for the HiPIP from Chromatium vinosum have been studied by time-resolved fluorescence spectroscopy.146 The crystal structure of the [3Fe-4S]-containing ferredoxin I from Azotobacter vinelandii has been determined at 1.35Åresolution.147 The [3Fe-4S] and [4Fe-4S] forms of the iron–sulfur cluster of Pyrococcus furiosus ferredoxin have been studied by 57Fe ENDOR spectroscopy.148 A new type of [3Fe- 4S]` cluster with S\5 2 has been spectroscopically identified in the Ala-33-Cys mutant of this ferredoxin.149 The proton-coupled electron transfer behaviour of several [3Fe- 4S] clusters has been investigated by protein film voltammetry.150 The solution structure of the [3Fe-4S]][4Fe-4S] ferredoxin from Bacillus schlegelii has been studied by 1H and 2H NMR spectroscopy.151 The crystal structure of the iron-only hydrogenase from Clostridium pasteurianum has been determined at 1.8Å resolution.152 The active site consists of a [4Fe-4S] cluster bridged through a Cys residue to an unusual triply bridged diiron site.A structural model for this diiron site has also been reported.153 Annu. Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 6187 Cobalt The crystal structures of two di§erent forms of methylmalonyl-CoA mutase from Propionibacterium shermanii have been determined.154 The structure of an isomer of coenzyme B 12 in which the configuration of theN-glycosidic bond is inverted has been reported, and its reactivity compared with that of the normal coenzyme.155 The chemical environment of the cobalt nucleus has been investigated by single crystal 59Co NMR spectroscopy.156 A resonance Raman study of the binding of coenzyme B 12 to methylmalonyl-CoA mutase has shown that although the conformation of the corrin ring is significantly altered upon binding, the Co–C bond appears una§ected.157 The kinetics of Co–C bond cleavage in enzyme bound coenzymeB 12 have been studied by stopped flow spectrophotometry.158 The role of coenzyme B 12 in the interconversion of methylmalonyl-CoA and succinyl-CoA has been investigated with ultrafast radical clocks.159 The results indicate that the isomerisation does not occur by a radical mechanism, but by either an anionic or organocobalt route.The photolysis of methylcobalamin and coenzyme B 12 has been studied by ultrafast transient absorption spectroscopy,160 and time-resolved photoacoustic calorimetry.161 The crystal structure of trifluoromethylcobalamin has been reported.162 The possible sites for ion binding to cobalamins have been identified in the LiCl-containing crystal structures of azido- and chloro-cobalamin.163 The synthesis of the pentafluorothiophenolate derivative of cobalamin has also been described.164 Some methanogenic bacteria are known to use methanol as a source of methyl for the synthesis of methane and acetyl-CoA.This chemistry has been successfully modelled by the synthesis of a Co–CH 3 derivative from the reaction of methanol with a reduced cobalt(I) corrin complex in the presence of a Lewis acid.165 Cobaloxime model compounds for coenzyme B 12 and their evaluation by molecular modelling have also been described. 166 The crystal structure of the methionine aminopeptidase from Pyrococcus furiosus containing a dicobalt(II) active site has been determined at 1.75Å resolution.167 8 Nickel Recent developments in elucidating the structure and function of nickel metalloproteins have been reviewed.168 The crystal structure of urease from Bacillus pasteurii with the inhibitor 2-mercaptoethanol bound at the active site has been determined at 1.65Å resolution.169 Binuclear nickel(II) model complexes for urease which contain bridging deprotonated urea or carbamate ligands have been prepared and structurally characterised.170 Several other model compounds for urease have also been reported. 171 XAFS spectroscopy has shown that the nickel-binding protein NikA from Escherichia coli contains a six-co-ordinate nickel(II) ion with O/N-donor ligands at an average distance of [2.06Å.172 The solution structure of nickel peptide deformylase has been solved by 1H NMR spectroscopy.173 The structure and properties of the nickel–iron hydrogenases have been reviewed. 174 The mechanism of dihydrogen activation by the mixed metal site of the nickel–iron hydrogenases has been investigated with density functional theory; the dihydrogen is proposed to bind to the iron centre and undergo heterolytic cleavage to Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 619generate a bound, protonated Cys residue and a hydride on the iron centre which is then transferred to nickel.175 A series of model compounds based on [Fe(CN) 2 (CO)Cp]~ have been used to interpret the structure and IR spectroscopy of the iron centre at this biosite.176 The [Ni-4Fe-4S] cluster of carbon monoxide dehydrogenase from Clostridium thermoaceticum in both its nickel labile and nonlabile forms has been studied by XAFS, EPR and UV/vis spectroscopy.177 In the labile form the nickel(II) centre can be reduced to nickel(I) and is then able to bind carbon monoxide.The nickel(II) centre in the nonlabile form is square planar with two N/O-donor ligands at 1.87Å and two S-donor ligands at 2.20Å, and is probably no longer directly bridged to the [4Fe-4S] cluster. ENDOR spectroscopy of the reduced form of this biosite has indicated that HxO and a His residue are probably co-ordinated to the [4Fe-4S] cluster.178 Model complexes for the reduced nickel(I) centre in this metalloprotein have also been reported.179 9 Copper Recent developments concerning general copper biosites,180 and plastocyanin in particular,181 have been reviewed.The crystal structures of spinach plastocyanin, the azurin from Pseudomonas putida and a mutant of the plastocyanin from Cyanobacterium synechocystis have been reported.182 The copper site of plastocyanin has been studied by resonance Raman spectroscopy of various isotopically labelled derivatives, 183 and copper L-edge XMCD spectroscopy.184 The rate of intramolecular electron transfer for the Ru(bipy) 2 L–His-83 derivative of azurin increases slightly below 220 K.185 This unusual behaviour is rationalised as a near activationless electron- transfer process combined with the contraction of hydrogen-bonding distances along the electron transfer pathway.The solution structure of the complex of plastocyanin and cytochrome f has been determined by 1H NMR spectroscopy, and reveals a short copper–iron electron transfer distance of 10.9Å.186 The designed construction of a blue copper protein site in a host peptide has been described.187 The structure and spectroscopy of blue copper centres have also been modelled with several theoretical studies.188 The spectroscopy of cobalt(II) and nickel(II) substituted azurins has been described,189 and in particular the site of cobalt(II) binding to unfolded azurin has been elucidated by 1H NMR spectroscopy.190 XAFS spectroscopy has shown that the Menkes disease protein binds copper(I) with two Cys residues in a two-co-ordinate geometry.191 A model mononuclear copper(I) bis(thiolate) complex for this site has also been described.192 The crystal structures of several forms of the nitrite reductase from Alcaligenes xylosoxidans have been determined.193 The crystal structure of the copper amine oxidase from Hansenula polymorpha in the active conformation has been determined at 2.4Åresolution, revealing the position of the topaquinone redox cofactor relative to the copper centre.194 XAFS spectroscopy has been used to investigate the active site structure of the resting copper(II) and reduced copper(I) forms of several amine oxidases.195 Simple model compounds for the resting copper(II) form have also been reported.196 Mechanistic studies on the copper–cysteinyl–tyrosyl radical site of galactose oxidase have been reported.197 The wild-type enzyme and several mutant forms Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 620have also been studied by CD spectroscopy.198 The observed diamagnetic ground state for the copper(II)–cysteinyl–tyrosyl radical form of galactose oxidase has been explained by the particular orientation of the tyrosyl radical relative to the copper(II) centre and confirmed through the study of a series of model compounds.199 A structural and spectroscopic model for the putative mononuclear copper(II)–hydroperoxo species formed in galactose oxidase and related copper enzymes has been reported.200 Several model compounds for general mononuclear copper biosites have also been described.201 Recent developments concerning multinuclear copper sites in metalloproteins have been reviewed.202 The crystal structure of a functionally active subunit of octopus haemocyanin has been determined at 2.3Å resolution.203 The nature of the species formed in the binding of dioxygen to haemocyanin has been discussed in the light of model compound studies;204 oxyhaemocyanin displays catechol oxidase activity if the substrate is able to access the active site.205 Two catechol oxidases with dicopper active sites have been isolated from sweet potato and characterised by EPR, XAFS and UV/vis spectroscopy.206 Resonance Raman spectroscopy has been used to identify the metal-bound His residues in copper superoxide dismutase.207 The formation of a non-symmetrical copper(II) complex of ochratoxin A has been reported, and the relevance of this for the understanding of its observed toxicity and DNA cleavage activity is discussed.208 1HNMRspectroscopy has been used to study the active site of the dicopper-substituted, functionally active form of aminopeptidase from Aeromonas proteolytica.209 The crystal structure of the type II copper depleted laccase from Coprinus cinereus has been determined at 2.2Å resolution; the absence of the type II copper is found to perturb substantially the structures of the remaining type I and type III sites.210 ESEEM spectroscopy has been used to identify His co-ordination at the multinuclear copper site of particulate methane monooxygenase.211 The copper(II) sites of the multicopper Fet3 iron uptake protein from Saccharomyces cerevisiae have also been spectroscopically characterised.212 The chemistry of oxidative copper metalloenzymes and the similarity to recently reported model compounds have been reviewed.213 The dioxygen binding to and activation by di- and multi-copper biosites has been theoretically investigated.214 A number of functionally active model compounds have also been reported.215 Nitrous oxide reductase contains two binuclear bis(Cys) bridged copper sites, an electron-transfer Cu A site and the presumed active site Cu Z .MCD spectroscopy has indicated that the Cu Z site is a variant of a Cu A site and that the true active site may be a copper centre without Cys co-ordination.216 The Cu A sites of several proteins, and an engineered Cu A site in azurin, have been investigated by EPR, 1H NMR, UV/vis, MCD and XAFS spectroscopy.217 Spectroscopic models for Cu A sites have also been described.218 Annu.Rep. Prog. Chem., Sect. A, 1999, 95, 611–630 62110 Zinc Recent developments in understanding the structure and reactivity of zinc metalloenzymes have been reviewed.219 The role of the Tyr residue as a detachable ligand in the zinc protein astacin has been studied in the cobalt(II) and copper(II) substituted forms.220 Orotidine 5@-monophosphate decarboxylase is known to increase the rate of reaction of its substrate by a factor of 1017 compared with the uncatalysed reaction.The protein needs two zinc ions for activity and Lys-93 may act as a bridging carbamylated Lys ligand.221 The proton-transfer reactions of the zinc-bound intermediates in carbonic anhydrase have been investigated by ab initio calculations.222 The crystal structures of several forms of the alcohol dehydrogenases from Clostridium beijerinckii and Thermoanaerobacter brockii have been determined.223 The structural basis for the enantioselectivity of the secondary alcohol dehydrogenase from Thermoanaerobacter ethanolicus has been studied with reference to its Ser-39-Thr mutant. 224 The crystal structure of the dizinc enzyme b-lacamase from Bacillus cereus has been determined at 1.9Å resolution.225 The dicobalt(II) and mixed zinc(II)–cobalt(II) substituted forms have also been studied by UV/vis and 1H NMR spectroscopy.226 The zinc(II) ion e§ectively inhibits the oxidation of the ligand Cys residues in zinc finger proteins whereas substitution with nickel(II) appears to enhance oxidation to disulfide and thiosulfinate groups.227 The di¶culties of determining the active site structures of zinc enzymes with a mixture of S-donor and N/O-donor ligands by XAFS spectroscopy have been investigated with a study of crystallographically characterised model compounds.228 The mom gene activator proteinCOMbinds a single zinc(II) ion at a tetrakis(Cys) site with an a¶nity 105 times greater than for cobalt(II).229 Zinc(II) binding is even maintained at pH 4, the pH at which zinc(II) dissociates from zinc finger proteins.The role of a zinc ion in the activation of a bound Cys residue towards methylation has been studied for the DNA repair protein Ada and methionine synthase.230 Model systems for zinc-promoted Cys methylation,231 and general mononuclear zinc biosites,232 have been described.The speciation of cadmium(II) with the simplest plant phytochelatin PC 2 has been investigated in detail to elucidate the co-ordination chemistry of these heavy metal stress relief peptides.233 11 Molybdenum, tungsten and the nitrogenases The structure and reactivity of molybdenum and tungsten metalloenzymes containing pterin cofactors have been reviewed.234 The crystal structure of dimethyl sulfoxide reductase with the substrate bound at the active site has been determined at 1.9Å Annu.Rep. Prog. Chem., Sect.A, 1999, 95, 611–630 622resolution.235 The crystal structure of the similar enzyme trimethylamine N-oxide reductase has also been determined at 2.5Å resolution.236 XAFS spectroscopy of the selenium-containing formate dehydrogenaseH from Escherichia coli has revealed four S-donor ligands at 2.35Å, one O-donor ligand at 2.1Å and a Se-donor ligand at 2.62Å. However, there also appears to be a close Se–S interaction of 2.19Å suggesting the presence of an unusual seleno–sulfide ligand.237 Isotopic labelling studies have also shown that this enzyme does not display typical oxotransferase activity as formate oxidation occurs without any oxygen transfer.238 The active site structure of chicken liver sulfite reductase has been investigated by ESEEM spectroscopy at pH 9.5 and 7.0,239 and the binding of arsenate to human sulfite reductase has been studied by EPR and XAFS spectroscopy.240 Substrate binding and oxidation in xanthine oxidase has been the subject of density functional calculations.241 Several model compounds for oxotransferase biosites have also been reported.242 The crystal structures of the nitrogenase iron proteins from Azotobacter vinelandii and Clostridium pasteurianum have been determined.243 The two oxidation states of the P-clusters in nitrogenase have been investigated by XAFS spectroscopy.244 The binding of carbon monoxide to molybdenum nitrogenase has been studied by EPR spectroscopy and the binding of thiols and selenols has been studied by XAFS spectroscopy.245 The role of the R-homocitrate ligand of the molybdenum centre of the FeMo-cofactor of nitrogenase has been investigated.It is proposed that Rhomocitrate is uniquely able to hydrogen bond to the His-a-442 ligand and that, in addition, the bound carboxylate group can dissociate from the molybdenum centre to allow substrate access.246 Model systems for the proton transfer from bridging SH groups of [M 2 L 2 (l-SH) 3 ]` complexes to dinitrogen bound at a tungsten centre have been reported.247 References 1 Spectroscopic Methods in Bioinorganic Chemistry, ed.E. 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