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This chapter is an introduction to the main concepts underlying the present unterstanding of the physical properties of actinide solids. In the first half of the actinide series the elements behave as 5 f transition metals; 5 f electrons are then described as itinerant in metallic solids. Between plutonium and americium a cross-over from itineracy to localization, i.e. a true Mott transition is observed. Heavier actinides have a lanthanide-like behaviour, the 5 f states exhibiting similar properties to the 4 f states in lanthanides. The characteristics of 5 f wavefunctions in the actinide atom as well as in actinide solids are reviewed. In the latter, overlapping of 5 f wavefunctions between neighbouring atoms as well as their hybridization with other orbitals (actinide 6 d, or orbitals of non-actinide elements in compounds) are pointed out as being the two phenomena determining physical properties and bonding characteristics, in particular magnetism. Theories describing them, in particular Stoner's and Mott-Hubbard's models, are discussed in the light of their application to actinide physics. A short summary is presented of the main experimental evidence which justifies the theoretical decription of actinide solids. [ABSTRACT FROM AUTHOR]

Substitution of a closed-shell metalloporphyrin (MP) within one partner of a protein-protein complex makes it possible to study long-range electron transfer (ET), including direct measurement of both photoinitiated and thermal ET between the MP and heme (FeP) groups. Mixed-metal [M, M] hemoglobin hybrids have a structure that is fixed and well-characterized, which permits us to explore the factors that control the ET process by judicious selection of both the heme ligands and the substituting metal ion. In contrast, complexes between MP-substituted cytochrome c peroxidase (MCcP) and cytochrome c (FeCc) are conformationally mobile and the photoinitiated redox cycle provides a tool with which to explore the interplay between conformational interconversion and ET. Through a combination of metal substitution and site-directed mutagenesis, we probe the interfacial regulation of these processes. [ABSTRACT FROM AUTHOR]

The field of heteropolymetallic systems with magnetic metal centers occupies a crossing point between biology and physics. For instance the Cu(II)-Fe(III) interaction in cytochrome oxidase is of the same nature as the Cu(II)-Mn(II) interaction in a novel system which could be the first molecular ferromagnet. The mechanism of the interaction is discussed, both from a phenomenological view point using a spin Hamiltonian, and from an orbital view point. An orbital model for the isotropic interaction is presented. It is based on the concept of natural magnetic orbitals. The mechanism of the anisotropic and antisymmetric interactions is more briefly treated. The role of the Zeeman perturbation is then considered in relation with the magnetic and EPR properties of the heterobimetallic complexes. Several examples are presented to emphasize that the nature, ferro- or antiferromagnetic of the isotropic interaction is controlled by the symmetry of the magnetic orbitals. The concept of overlap density is introduced. It permits an estimation of the magnitude of the ferromagnetic stabilization in the case of orthogonality of the magnetic orbitals. The Cu(II)-Fe(III) interaction, in relation to the situation encountered in cytochrome oxidase, the Cu(II)-Ni(II) interaction and a few additional selected examples are discussed. A section deals with the case where one of the interacting ions has an orbital degeneracy. Afterwards, the heterotrinuclear complexes are studied. The important concept of regular and irregular spin state structure is developped and the Mn(II)Cu(II)Mn(II) triad is presented as a spectacular example of irregular spin state structure. A section is devoted to the ordered bimetallic chains. The theory is presented, both at a qualitative and quantitative levels and the already reported compounds of this kind are discussed. One of them may be considered as one of the first molecular ferromagnets. The last but one section concerns the systems with even more subtle spin orders. In conclusion, the vast perspectives of this area are outlined. [ABSTRACT FROM AUTHOR]

The theoretical models which have been used to describe the bonding in cluster compounds of the main group and transition metal elements are reviewed. The historical development of these models is outlined and special emphasis is placed on those studies which have led to the elucidation of structure-electron count correlations. Theoretical treatments of cluster bonding are based on localised, delocalised (molecular orbital) or free electron methods derived from the solution of the Schrödinger equation for a particle on a sphere. A detailed analysis of the Tensor Surface Harmonic method, as an example of a free electron model, is presented. Group theoretical consequences of the model are also presented. [ABSTRACT FROM AUTHOR]

A novel method of treating point group symmetry is presented. It is based on finding common eigenfunctions to sets of non-commuting operators, the operators being suitably chosen generators for the point groups. The character tables of the conventional method are replaced by eigenvalue tables, that are no more complex than character tables but allow straight forward calculation of matrixelements (Wigner-Eckart theorem). Half-integer spin is treated as simply as integer spin, without introducing the socalled double groups. [ABSTRACT FROM AUTHOR]

The concept of bond valence and its correlation with bond length are reviewed. It is shown how to augment bond valence sums at an atom with other constraints so that bond lengths can be predicted for a given topology. These constraints can be read directly from the connectivity matrix for the structure. The physical reasons for the constraints are analysed, and this analysis leads in turn to a reformulation of Pauling's electrostatic valence sum rule. The apparent valences of atoms, calculated as a sum of bond valences derived from bond lengths, are often significantly different from the actual valences. It is demonstrated that such observations are often diagnostic of non-bonded repulsions. Limitations and possible extensions of the method are outlined. [ABSTRACT FROM AUTHOR]

An epikernel is an intermediate subgroup in the decomposition scheme of a given point group. The epikernel principle states that the preferred distortions of Jahn-Teller unstable molecules are directed towards the maximal allowed epikernels of the undistorted parent group. The group theoretical foundations of this principle are explained, and a wide variety of applications in different areas of chemistry is discussed. [ABSTRACT FROM AUTHOR]

A ligand field model which considers the dynamics of vibrating ligands of transition metal ions is formulated, maintaining fundamental assumptions and model parameters of common (static) ligand field theory. It allows a survey to be obtained on corresponding theoretical approaches available on various occasions in the literature which interpret vibrational fine structures of band progressions in electronic absorption and emission spectra including the entanglements caused by Jahn-Teller effects. In the unified model the electronic d-states are vibronically coupled to the nuclei moving in a harmonic force field applying first order perturbation terms in the Herzberg-Teller approximation. The linear vibronic coupling constants, stabilization energies and geometric distortions due to excitation can be calculated from appropriate ligand field parameters of the ground state which are obtained by comparison with the measured spectra and from metal-ligand atomic equilibrium distances. The model is applied to systems of octahedral symmetry representing d-electron coupling with α1, ɛ and/or τ2 vibrations for which well resolved vibronic spectra have been reported. [ABSTRACT FROM AUTHOR]

A survey of the theoretical foundations of vibronic interactions in molecular systems, with special attention to metal complexes, is given. A detailed analysis of the conditions leading to the adiabatic potential surface in various degrees of generality represents the central idea of the article. Use of the partitioning method enables consideration of the Jahn-Teller effect, the Renner-Teller effect and the pseudo Jahn-Teller effect on an equal footing. Analytic forms of the adiabatic potential surfaces are rederived for the most important cases by including the totally symmetric vibrational mode up to the second order of vibronic expansion. A complete third-order formula for the adiabatic potential surface of Eg − (a1g + eg) vibronic coupling is presented. The consequences for structural features of molecular systems are discussed and exemplified by various metal complexes. Recent progress in molecular-orbital calculations of vibronic constants is reviewed. This paper deals with pure (pseudo) Jahn-Teller effect only. On the other hand, some qualitatively new types of interaction may arise due to interaction of (pseudo) Jahn-Teller centers with the lattice. We do not consider the competitive spin-orbit-lattice coupling that may arise below a magnetic-ordering temperature at Jahn-Teller ions with a threefold orbital degeneracy. Inclusion of interactions like this is, however, outside the scope of the present work. [ABSTRACT FROM AUTHOR]

The origin of non-bonding orbitals in molecular compounds is reviewed and analysed using general quantum mechanical considerations. A combination of the pairing theorem and a group theoretical analysis leads to a definition of the number of non-bonding molecular orbitals in co-ordination, polyene and cluster compounds. The non-bonding molecular orbitals have been generated by defining the nodal characteristics of the relevant orbitals and evaluating the solutions under the appropriate boundary conditions. The stereochemical rôle of non-bonding molecular orbitals in co-ordination compounds is discussed with particular reference to main group and transition metal examples. [ABSTRACT FROM AUTHOR]

Solid hydrate research of the last fifteen years is critically evaluated with regard to bonding and structure of water molecules. This review focusses on new results of structure determination and infrared and Raman studies in terms of hydrogen bonding and other intermolecular bonding interactions, distortion and disorder of water molecules, intermolecular and intramolecular coupling and anharmonicity of water bands, isotopic effects, and phase transitions. The techniques used for structure determination and spectroscopic measurements of solid hydrates are discussed. [ABSTRACT FROM AUTHOR]

The first excited quartet state 4T2 of chromium(III) provides broad-band luminescence in the near infrared. If the first doublet state 2E is below the 4T2 potential minimum, a narrow band may be emitted with high yield, as known from ruby. Glass-ceramics containing Cr(III) may show high quantum efficiency of 4T2 at room temperature, contrary to most other compounds. When the heat treatment is such that micro-crystallites of spinel, gahnite, mullite, virgilite... are obtained having dimensions smaller than the wave-length of violet light, transparent glassy-looking materials are obtained which may be suitable for tunable lasers similar to alexandrite, and luminescent solar concentrators. The absorption and emission spectra of Cr(III) are treated here in the angular overlap model. The sub-shell energy difference Δ (= 10 Dq) of Cr(III) in glasses is around 0.86 and in glass-ceramics 0.96 to 1.05 times its value of the aqua ion. Evidence is presented that the band widths and Stokes shift cannot be fully explained by effects of totally symmetric stretching. The multi-dimensional potential surfaces, combined with the Tanabe-Kamimura theorem and the Jahn-Teller effect, provide a more complete explanation of the spectroscopic behaviour. Anti-ferromagnetic and spin-orbit coupling are shortly reviewed. Comparison between chromium(III) luminescence and the exceptional Stokes shift of hexacyanocobaltate(III) is made. [ABSTRACT FROM AUTHOR]

Titanium(IV) oxide is continuing to attract wide interest as the photosensitizer for a large variety of photocatalytic reactions. Yet, the mechanisms of the photoreactions occurring at this semiconducting material are still poorly understood. In an attempt to clarify the existing ambiguities, the experimental evidence concerning the nature and role of the intermediates involved in the light-induced charge transfer reactions at TiO2 is analyzed. Pathways for the photo-oxidation reactions, consistent with the available experimental data, are considered. [ABSTRACT FROM AUTHOR]

The question, in what sense matter (with positive rest-mass) has constituents, is of great concern to the chemist. A decade before the Lavoisier paradigm was invalidated (by the characterization of the electron in 1897 and by the discovery of radioactive isotopes), Crookes suggested that a primordial material first formed hydrogen and then all the heavier elements by consecutive polymerization at exceedingly high temperatures, in remarkable agreement with recent discoveries in astrophysics. As early as 1860 Marignac had suggested that the heavier elements have slight negative deviations from additive mass, as quantitatively elaborated by Harkins and Wilson in 1915. The Z protons and N = (A - Z) neutrons assumed in 1932-1964 to occur in nuclei were superseded by d, u, s, c, b (and perhaps t) quarks, the nuclear ground state being u2z+Ndz+2N. Though single quarks may be inconceivable, the vacuum may "dress" them with (a fluctuating or invariant number) x additional quarks and x anti-quarks; and unsaturated systems (likely to contain comparable numbers of u, d, and s quarks) of (3A+2) quarks have Z = (Z0 + 1/3) whose chemical properties can be predicted. Astrophysical arguments about nonbaryonic matter and the hot quark soup (2 terakelvin) coagulating 20 microseconds after the Big Bang are closely related to this problem. [ABSTRACT FROM AUTHOR]

Since two-digit Z elements are only known to be produced in supernovae, the chemically peculiar main-sequence stars (being 6 to 8 percent of Sirius-like objects within a rather narrow temperature interval) are enigmatic. The hypothesis of magnetic accretion is compared to other explanations, and it seems likely that the intensification of Fraunhofer lines (by factors 10 to a million) is indeed predominantly due to factual high abundances of a few elements including europium (and other lanthanides), uranium, bismuth, mercury, platinum, osmium, gallium and manganese in individual stellar photospheres. A given supernova seems to provide a quite specific selection of elements, and the accretion of the locally enriched interstellar gas (and probably protoplanetary grains) enhances it. Sidelights are thrown from silicon-rich stars on the striking difference between the Sun and (the Earth, its moon, and Mars), supernova dust contributing significantly to the inner planets, with 1300 ppm of weight having Z above 30 (but less than 0.6 ppm of solar matter). The lanthanides represent 120 and 0.04 ppm, respectively. A new parameter of atomic spectra is proposed as being relevant to magnetic accretion. [ABSTRACT FROM AUTHOR]

The chemistry of inorganic vapors is for the most part the chemistry of high-temperature vapors. This article essentially reports on the gaseous species observed in the equilibrium vapors and their dissociation or atomization energies. Most of the results were obtained by Knudsen effusion mass spectrometry. The fundamentals of this method are described with respect to the recent methodic developments. Accounts of the investigations of metals, alloys, oxides, metal halides, and technical materials are given and the results are reviewed. Vaporization processes and thermochemical properties of condensed phases evaluated from gas phase data are also considered. The results are for example of interest for the active fields of energy, high-temperature materials, and cluster research. An extended summary with outlook is given in Sect. 7-"Summary and conclusions"-of this article. [ABSTRACT FROM AUTHOR]

The results of 65 years of experimental and theoretical research in light noble gas chemistry is reviewed, with particular emphasis on recent quantum chemical studies on the structures, stabilities and bonding of molecules containing He, Ne, or Ar. The scattered experimental results reported mainly for cations are interpreted using a chemical bonding model which is based on donor-acceptor interactions. A systematic view of bonding in Ng compounds (Ng = He, Ne, Ar) is presented that allows the prediction of new compounds which are theoretically predicted to be stable or metastable. The nature of the Ng,X interactions is studied with the help of the analysis of the electron density distribution and its associated Laplace field. Covalent noble gas bonds are found for many cations and dications, while closed-shell interactions are responsible for the unusually stable van der Waals compounds NgBeO. [ABSTRACT FROM AUTHOR]

The closed-shell configuration of noble gas atoms Ng does not prevent formation of compounds, either as even, positive oxidation states of xenon, isosteric with iodine complexes (and to a smaller extent by krypton and radon) or functioning as Lewis bases. In condensed matter, Ar, Kr, and Xe form distinct NgCr(CO)5 and ArCr(NN)5 complexes. Gaseous noble gas molecular ions, especially HeX+ and ArX+, numerous organo-helium cations, and some neon-containing cations are calculated to be quite stable, and several of them are indeed detected in mass-spectra. The history of Ng chemistry and its relations with the Periodic Table, atomic spectra, and ionization energies, are discussed. [ABSTRACT FROM AUTHOR]

The cytochrome P450 superfamily of heme monooxygenases catalyse many reactions involved in the biosynthesis and degradation of endogenous compounds and in the oxidative metabolism of xenobiotics. These enzymes all share the same overall catalytic mechanism, and their varied substrate specificities arise from differences in their active site topologies, suggesting that it should be possible to engineer P450 enzymes for the oxidation of unnatural substrates. A review of the active site protein engineering experiments on cytochrome P450cam reported to date is presented here. Cytochrome P450cam is a stable, soluble enzyme isolated from Pseudomonas Putida, and is an excellent system for rational protein engineering. The current understanding of the details of the P450cam catalytic mechanism, including the uncoupling pathways, is first discussed. The factors controlling substrate access, binding and turnover activity, and the coupling efficiency have been investigated by site-directed mutagenesis. The results of this work have enabled the rational redesign of P450cam to broaden the substrate range, to improve the activity and coupling efficiency towards unnatural substrates, and to engineer the regioselectivity of substrate oxidation. The construction of a "self-sufficient" fusion protein containing all three proteins of the P450cam system brings one step nearer the in vitro and in vivo biotechnological applications of P450cam. [ABSTRACT FROM AUTHOR]

Ribonucleotide reductases catalyze an essential reaction in all living cells — the production of deoxyribonucleotide precursors for DNA synthesis. An intricate allosteric regulation enables one and the same enzymic component to accept both purine and pyrimidine ribonucleotide substrates and to furnish growing cells with balanced deoxyribonucleotide pools. The reaction, which is based on radical chemistry, is common to all ribonucleotide reductases, whereas the mode of furnishing the radical differs and is currently a basis for distinguishing at least three different classes of ribonucleotide reductases. The radical generating components, although of seemingly different evolutionary origin, all include transition metal binding sites. Studies on the ribonucleotide reductase family highlight exciting mechanistic and evolutionary implications of far-reaching significance. [ABSTRACT FROM AUTHOR]

Structural, spectroscopic and chemical features of the metal centres of non-haem-iron-containing cytochrome b557 are described. This haemoprotein is a member of the ferritin family of proteins and thus is also called bacterioferritin. It consists of 24 polypeptide subunits and, in addition to its inter-subunit bis-methionine coordinated haem b groups, it also contains intra-subunit dinuclear metal centres and has the capacity for a central non-haem-iron deposit of 4,500 Fe(III) ions per molecule. The dinuclear site can be occupied by Co(II) or Mn(II), and probably by Zn(II) and Fe(II) as well. Mixed metal centres also appear to be formed. The non-haem-iron core can be either amorphous, as customarily found with native proteins, or crystalline. Crystalline cores can be laid down in vitro. This latter type of core is often superparamagnetic and this phenomenon is decribed, particularly with regards to its characterisation by 57Fe Mössbauer spectroscopy, EPR spectroscopy and Magnetic circular dichroism spectroscopy. Finally, possible functional roles for the metal centres are described. [ABSTRACT FROM AUTHOR]

Serum transferrin is an 80 kDa glycoprotein which transports Fe3+ and a variety of metal ions of diagnostic, therapeutic and toxic importance. Methods for the study of the differences between the two metal binding sites (the N- and C-lobe sites), and mechanisms for the uptake and release of both metal ions and synergistic anions are discussed. The strength of metal binding to transferrin can be rationalised on the basis of metal ion acidity (strength of hydroxide binding). Similarly the strength of metal binding to the enzymes carbonic anhydrase and carboxypeptidase can be correlated with the binding of the same metal ions to imidazole. Such correlations of metal binding to proteins and low molecular mass ligands may provide insight into the preorganisation of metal binding sites in proteins. [ABSTRACT FROM AUTHOR]

Iron-porphyrin complexes, known as hemes, form the prosthetic groups of a number of proteins. These hemoproteins exhibit an impressive range of biological functions. These include: Simple electron transfer reactions, oxygen transport and storage, oxygen reduction to the level of hydrogen peroxide or water, oxygenations of organic substrates, and the reduction of peroxides. This diversity of function is often extended further by combining heme groups with other cofactors, e. g. flavins and/or metal ions. Such combinations frequently allow heme cofactors to couple electron transfers with other processes, such as the translocation of protons or the reduction/oxidation of other molecules. This versatility in function is made possible by a combination of differences in both the polypeptide and heme constituents of the various hemoproteins. The aim of this article is to illustrate how nature has used different protein frameworks to exploit the redox properties of heme. This is done by focusing on a carefully chosen selection of hemoproteins which exemplify the numerous redox functions performed by heme in biology. [ABSTRACT FROM AUTHOR]

Iron biominerals are ubiquitous and fulfil many functions. These are often iron oxide, oxyhydroxide and hydroxide phases, many of which are common in the lithosphere, but some of which, such as the mineral found within the iron storage protein ferritin, are metastable. This article takes the specific example of loaded ferritin as a biomineral and explores the use of model compounds to elucidate the formation, structure and properties of the iron oxyhydroxide mineral contained within the protein. The utility of various compounds is evaluated by considering how closely they match a number of essential criteria defining the composition and properties of loaded ferritin. Those compounds with well-defined structures which can be regarded as "scale models" of the loaded ferritin protein are found to be the most realistic mimics. [ABSTRACT FROM AUTHOR]

Phthalocyanines and related complexes are important compounds due to their special electrical and optical properties. This article describes, after a more preparative part on the synthesis of porphyrins and phthalocyanines, the different ways to use the macrocycles for the mentioned properties. Phthalocyanines and related compounds can be assembled in a stacked structure by using the so-called "shish-kebab" approach, taking advantage of the possibility to form discotic mesophases or simply by doping the mononuclear metal complexes. The different systems e.g. main group metal complexes or transition metal complexes are discussed with regard to their structure. The methods for increasing the solubility by using differently substituted macrocycles and the electrical properties of these compounds are described in detail. A separate section is devoted to phthalocyanines and porphyrins as discotic liquid crystals. The nonlinear optical effects as well as photoconductivity and the possibility of the formation of Langmuir-Blodgett-films of phthalocyanines and derivatives is discussed at some length. Several practical applications of these unconventional materials are mentioned. [ABSTRACT FROM AUTHOR]

The application of extended X-ray absorption fine structure (EXAFS) spectroscopy to the structural characterization of the active sites of heme-containing oxygenases and peroxidases is reviewed. Metal-ligand bond distances for first shell atoms can be established to an accuracy of ±0.02 Å and coordination number to ±25%. EXAFS spectroscopy is particularly well suited for establishing the presence of a sulfur donor axial ligand to the heme iron, as is shown to be the case for cytochrome P-450 and chloroperoxidase. In addition, the technique has been used to demonstrate that very short Fe=O bonds (∼ 1.65 Å) are consistently present in high-valent oxo-iron intermediates, such as those in the reaction cycle of horseradish peroxidase. Structural information derived from the use of EXAFS spectroscopy, especially of unstable intermediates for which crystallographic analysis is difficult, provides the foundation for a more thorough understanding of the mechanisms of dioxygen and peroxide activation by heme enzymes. [ABSTRACT FROM AUTHOR]

This review is concerned with the properties of the type 1 blue copper proteins [1–4], a class of protein incorporating a single Cu atom and involved in redox processes. Within the class most information is available for plastocyanin, a component of photosynthetic electron transport, which has proved to be a particular focus of recent research. Relevant to the whole area of metalloprotein study is the more general question of how and over what distance electrons are transferred. Rapid, i.e. efficient, long-distance (> 10 Å) electron transfer between metal centres is known to occur in biological systems, and attempts to better understand such processes is a subject of widespread current interest [7–14]. For reactions of two large biomolecules, specific relative orientations are required at the time of reaction, and questions relating to the docking process and association prior to electron transfer are highly relevant. For example, plastocyanin, which is a solute in the inner thylakoid of the chloroplast, is required to associate and electron transfer with its redox partners. In the case of single proteins having more than one active site [5], and membrane bound complexes made up of different protein molecules, intramolecular electron transfer is of prime concern, and the orientation of domains and molecules within these units is of considerable importance [15]. [ABSTRACT FROM AUTHOR]

Some of the factors which control molecular recognition and binding between electron transfer proteins have been elucidated from studies of the interaction between cytochrome c and its redox partners, particularly cytochrome c peroxidase (ccp). Data from a number of labs point towards a new motif for (macro)molecular recognition: not the classical lock and key of enzymology, but a variety of overlapping binding sites which create broadly complementary patches of charge and hydrophobicity on the partners, which consequently stick on contact, like “velcro’. The initially formed complex may then undergo dynamic reorientation to obtain an optimal orientation for electron transfer. If slow, this reorientation may be rate limiting (gating), while if fast, may contribute to the observed reorganization energy. A biological rationale for this motif is briefly discussed. [ABSTRACT FROM AUTHOR]

The investigation of metalloprotein electron transfer mechanisms through structural and functional characterization of modified proteins produced by site-directed mutagenesis provides a systematic means of gaining new, often quantitative, insight into the structural characteristics that regulate the electrochemical, kinetic and protein-binding properties of such proteins. This review discusses aspects of the experimental strategies that have been employed in the application of site-directed mutagenesis as a means of addressing these mechanistic issues, some of the known limitations of this approach, and strategies for expression of recombinant electron transfer proteins. In addition, a detailed summary of the current literature concerning functional studies of engineered mutant forms of cytochrome c is presented to illustrate the manner in which the technique has been employed in studies of this protein as a representative member of this large category of proteins. [ABSTRACT FROM AUTHOR]

Studies of the dependence of the rate of electron transfer (ET) on donor-acceptor separation and reaction driving force ( − †G°) in ruthenated derivatives of myoglobin (Mb) and cytochrome c (cyt c) have been performed. Measurements of MP* → Ru3+ ET (MP* is an electronically excited metalloporphyrin) at four fixed distances (His48, 81, 116, 12) in modified sperm whale Mb have defined limits for long-range heme: Ru electronic couplings. The lnkET values decrease exponentially with the donor to acceptor edge-edge distance; from a plot of InkET vs. d, a β of ∼ 0.9 Å−1 is obtained. The 3ZnP* → Ru3+ and Ru2+ → ZnP+ rates in Ru(His33)Zncyt c derivatives have been analyzed in terms of λ ∼ 1.2 eV and electronic couplings of roughly 0.1 cm−1. The electronic couplings for Ru(His39)Zncyt c reactions (∼ 0.2 cm−1) are twice as large as those for Ru(His33)Zncyt c, but the experimentally derived electronic coupling in Ru(His62)Zncyt c ( ∼ 0.01 cm−1) is much smaller than the couplings in either Ru(His33)Zncyt c or Ru(His39)Zncyt c. The donor-acceptor electronic coupling order, His39 > His33 ≪ His62, accords well with the effective lengths of calculated ET pathways for these derivatives. The evidence suggests that shortcuts in ET pathways that involve hydrogen bonds lead to stronger couplings than those requiring through-space jumps. [ABSTRACT FROM AUTHOR]

The connections between basic electron transfer theory and the methods currently available for analyzing quantum electronic interactions of long-range electron transfer within and between proteins are examined. Both traditional basis set expansion as well as quantum path integral Monte Carlo descriptions of propagation through the intervening medium are described and compared. The development of the concepts and technical aspects of superexchange are traced from simple to complex implementations, with an emphasis on the unifying framework of the sum over tunnelling path amplitudes. Directions for the future refinement of quantitative biomolecular computations are identified. [ABSTRACT FROM AUTHOR]

In biological systems, the mechanisms of conversions and storage of energy involved sequences of oxido-reduction reactions in which electrons are transferred along a chain of redox centers embedded in a protein medium. The theoretical interpretation of the kinetics of these transfers pertains to Quamtum Mechanics, and was developed by chemists and physicists. However, owing to the fundamental importance of these processes, many biochemists are also concerned with these theories and their practical application to biological systems. This introductory chapter is an attempt to clarify the physical basis of current theoretical interpretations of biological electron transfers. It comprises an account of the standard formalism appropriate for non-adiabatic processes, and a analysis of kinetic data. Important advances in this field have resulted, on the one hand, from precise theoretial calculations based on molecular structures, and on the other hand, from implementation of elaborate experimental methods based on efficient chemical and biochemical techniques. This topic is illustrated by many examples taken from the recent literature which concern redox proteins as well as photosynthestic systems. [ABSTRACT FROM AUTHOR]

Based on the observations of the cooperative E ⊗ ε Jahn-Teller effect (JTE) in compounds with the hexagonal perovskite structure (ABX3 2L compounds with an h-stacking) the occurrence and non-occurrence of helical structures as e.g. in CsCuCl3 and [N(CH3 )4]CuCl3 (TMCuCl3) are discussed. In particular the relative magnitude of the radii of Rb+ and Cs+ with respect to the radius of CI can explain the difference in structure of RbCuCl3 and CSCuCl3. This can be shown by comparing new neutron diffraction measurements of the acoustic phonon modes in CsFeCl3 with those previously determined for RbFeCl3. The acoustic phonon branches also demonstrate the phenomenon of structural resonance, which leads to energy lowering in cases of formation of helices. It is further shown that, in order to form a structural helix, displacements of ions related to ferro-electricity as well as to ferro-elasticity are necessary. The quadrupolar distortions resulting from the E ⊗ ε JTE are therefore complemented with dipolar shifts of layers perpendicular to the c-axis of the structure of CsCuCl3. The structure of CsCuBr3 is also shown to belong to this family of helices. It arises through distortion waves at the A-point in the first Brillouin zone and changes from an h-stacking to an hc-stacking of layers. Although a phase transition between the chloride and the bromide has not been observed, the deformation in CsCuBr3 reminds us of a Spin-Peierls transition. A new mechanism connected with ferromagnetic exchange along the columns of octahedra is proposed to explain the structure of β-RbCrCl3 and the low temperature structures (space group C2) of CsCrCl3 and RbCrCl3. The larger radius of Cs+ can explain why the β-RbCrCl3 structure does not occur for CSCrCl3. The structures having the C2 spacegroup resemble racemic dl-compounds. The question about the stability of the helix in β-CsCuCl3 remains difficult to answer without calculations. As experiment shows, other influences can easily disturb its formation. Probably because of the smaller radius of Cue+ with respect of Cr2+, the electrostatic energy stabilizes β-CsCuCl3 with respect to γ-CsCrCl3. [ABSTRACT FROM AUTHOR]

Iron-sulfur polymetallic centers pose interesting questions on the oxidation state of each metal ion, on the possibility of electron delocalization over the metal ions versus the possibility of existence of equilibria among different localized-valence states, and on the factors determining if, and to what extent, electron delocalization is present. By interpreting the hyperfine coupling of unpaired electrons with 57Fe as well as with 1H, and by analyzing the EPR spectra, the valence distributions of Fe-S polymetallic centers in proteins and model systems are obtained. The systems analyzed here are: [Fe2S2]2+/+, [Fe3S4]+/0, [Fe4S4]3+/2+/+. The understanding of the parameters describing the exchange coupling within pairs of spin vectors, as well as the double exchange coupling, is attempted on the basis of the oxidation states of the various ions. Upon proper scaling of these parameters, we show that it is possible to transfer them from one polymetallic center to another. The electronic properties are tentatively related to the microscopic reduction potentials of each iron in the center. [ABSTRACT FROM AUTHOR]

In the last few years interest in chelating agents has expanded rapidly with increases in the number and variety of chelating agents. Chelating agents are no longer used in medical sciences primarily to remove toxic metals from the body. Some of their new uses are to be found in non-invasive diagnostic medicine, where they are used to carry either γ-emitting or paramagnetic cations to specific parts of the body, and in the manipulation of nucleic acids, a process which facilitates fragmentation into designated sequences of nucleotides. As antimetabolites, chelating agents might find uses in the chemotherapy of protozoa, viruses, microorganisms and cancer. The development of NMR for nuclei of the metallic elements will lead to the use of new chelating agents as models of metalloproteins. Future developments in the use of chelating agents in the medical sciences will rest upon the preparation of new chelating agents. Of particular value for understanding the binding of metals by proteins will be the preparation of chelating agents containing mixed functional groups. [ABSTRACT FROM AUTHOR]

The majority of ruthenium(II) compounds which have been observed to be luminescent are complexes of aromatic heterocyclic chelating ligands. Methods of synthesizing such complexes are reviewed and recommendations made (where considered possible) as to the best procedures. Models for the emitting excited state are reviewed, particularly with regard to the effects of molecular structure on excited state behavior. Data are tabulated for known emitting complexes. While a wide variety of luminescent ruthenium(II) complexes cannot be designed at present, some guidelines are beginning to emerge. In the future we look for continued development of the excited state model. Further structure-emission efficiency correlations and correlation with solvent properties are seen as bein important. [ABSTRACT FROM AUTHOR]

An overview is presented of platinum amine compounds, used as antitumor drugs. The history of the discovery is briefly described, including a discussion of the synthesis of platinum amine coordination compounds. Attention is given to the relationship between biological activity and structure of the compounds, especially to the nature of the ligands coordinated to platinum. The primary biological event appears to be the interaction with nucleic acids, and therefore the major part of the review is deals with studies of the binding interactions of platinum amine compounds (active and inactive ones) to nucleic acids and nucleic acid fragments. Studies on mononucleic acid fragments have made clear that a strong preference exists for binding at guanine-N7 sites. Studies on oligonucleotides have shown that, when two neighboring guanines are present, chelation of the cis-Pt(NH3)2 unit is strongly favored above all other possibilities. However, binding at neighboring AG-units also occurs readily. Studies on DNA (in vivo and in vitro) have shown that similar bindings occur in DNA and oligonucleotides, and that the DNA is distorted in a similar manner to double-stranded oligonucleotides. [ABSTRACT FROM AUTHOR]

This paper surveys several aspects of metal-to-metal charge-transfer transitions. Species of interest originate from non-molecular and molecular solids and from solutions. The parallel in the different approaches is stressed. In addition to the spectroscopy of these transitions, their influence or role in other phenomena is also discussed. [ABSTRACT FROM AUTHOR]

Spin-state transitions in metal complexes which are driven by a change of temperature T or pressure p are always associated with a considerable reorganization of molecular geometry. The change involves metal-ligand bond lengths R, bond angles, and a variation of ligand orientation. In particular, the elongation 4R by up to ∼ 10% occurring in the course of the LS → HS conversion produces an expansion of molecular volume ΔV ≌ 25 Å3 per metal atom. The average crystal structure for given values of T and p is reproduced by the fractional occupancy of the individual structures of the high-spin (HS) and low-spin (LS) isomer. The transitions are reasonably well described by a number of theoretical models which are equivalent to the Bragg and Williams approximation of the Ising model. The dynamics of the spin-state transitions in solution, based on measurements by ultrasonic and photo-perturbation techniques, is in general rapid with rate constants between 4 × 105 and 3 × 108 s−1. Similar results are obtained for the spin conversion in solid complexes where the line shape analysis of Mössbauer spectra based on the theory of Blume and Tjon is applied. The dynamics may be rationalized employing one-dimensional cross sections through Gibbs free-energy surfaces G = G(R), an alternative being the comparison of the results with quantum-mechanical calculations for a radiationless non-adiabatic multiphonon process. [ABSTRACT FROM AUTHOR]

The nature of metal complexes of heterocyclic thiosemicarbazones reported in the literature through 1989 have been reviewed with an emphasis on variations in stoichiometry and stereochemistry. A brief introduction is followed by a survey of the biological activity of the uncomplexed thiosemicarbazones. Next is a section on the chemical nature of thiosemicarbazones including a discussion of preparative methods, modes of coordination and isomerism. This is followed by a discussion of structural information and biological activity of the iron(III), iron(II), copper(II), cobalt(III), cobalt(II) and nickel(II) complexes of heterocyclic thiosemicarbazones. Short sections on the biological activity of other chelating thiosemicarbazones and metal complexes are also included. [ABSTRACT FROM AUTHOR]

Arsenic is a potentially essential element for mammals and is abundant in certain marine organisms. Arsenic compounds have been widely used as drugs in the past and were used until recently as growth promoters for some farm animals. Here we review the structures, reactivities and biological and pharmacological activities of arsenic compounds, emphasising both the similarities to and differences from other Group V elements (P, Sb, Bi). There appear to be no crystal structures of As-containing peptides or proteins, and the inhibition of thiol-enzymes by As(III), although widely quoted, is poorly understood. There is much scope for the design of arsenic compounds as drugs, but progress depends on the discovery of new probes for investigating the structural and redox chemistry and biochemistry of arsenic, especially in aqueous media. [ABSTRACT FROM AUTHOR]

Cisplatin, cis-diamminedichloroplatinum(II), and carboplatin, cis-diammine(cyclobutane-1,1-dicarboxylato)platinum(II), are the first drugs from inorganic chemistry to have come under routine clinical use in medical oncology. Their antitumor activity ranges from testicular carcinomas, ovarian carcinomas, and tumors of the head and neck to bladder tumors. However, the spectrum of indication is fairly limited. There is no or only insufficient antitumor activity in tumors which account for the major share of cancer mortality today, e.g. lung tumors and gastrointestinal tumors. Direct derivatives of cisplatin such as carboplatin have only led to a limited reduction or change in drug toxicity. In most cases, the toxicity pattern has changed from nephrotoxicity to myelotoxicity. New metal complexes are now being developed which are designed to supplement the spectrum of indication of platinum complexes. Among non-platinum complexes, budotitane (INN), cis-diethoxybis(1-phenylbutane-1,3-dionato)titanium(IV), is among the most advanced. It is undergoing clinical trials today. Extensive investigations into structure-activity relations have clearly shown a dependence of the activity on the central metal and the diketonato ligand. The tumor-inhibiting effect decreases in the order titanium > zirconium > hafnium > molybdenum > tin > germanium. Antitumor activity is also highly dependent on the nature of the diketonate used. Ligands substituted with planar aromatic ring systems such as the phenyl groups in budotitane are advantageous. Most of the tumor-inhibiting bis(β-diketonato) complexes are cis-configurated. The cis-configurated compounds with an unsymmetrically substituted β-diketonate as ligand are in an equilibrium between three possible cis-isomers in solution at room temperature, due to the fact that the diketonate can rotate via a twist mechanism. The easily hydrolizable group in these complexes does not play a major role in antitumor activity, but it is important for the galenic formulation in the clinic. The ethoxy group as leaving group in budotitane hydrolizes at a slower rate than the corresponding halides. The best antitumor effects could be obtained with titanium and a diketonato ligand substituted with phenyl groups. Budotitane is highly active in several transplantable tumors and shows promising effects in an autochthonous colorectal tumor model, which is highly predictive for the clinical situation. Side-effects include mild hepatotoxicity and nephrotoxicity. These findings have been confirmed in clinical phase I studies. A phase II study is now in preparation. If preclinical antitumor activity in colorectal tumors can be confirmed in the clinic, this would lead a considerable step forward in the chemotherapy of cancer. [ABSTRACT FROM AUTHOR]

The three methods covered by this review — Mössbauer spectroscopy, electron paramagnetic resonance and magnetic susceptibility-provide a powerful set of tools for detailed studies of electronic structure and molecular magnetism of iron-containing proteins and related analogs. The interpretation of measured data is based on the spin-Hamiltonian concept which is described in detail. A short introduction to the principles of the three methods as well as a basic description of the spectrometers is given. The major part of the review deals with applications, e.g. to mononuclear iron complexes and to spin-coupled iron complexes. Wherever possible, the complementarity in applying the three methods is described. [ABSTRACT FROM AUTHOR]

Dynamic electrochemical methods, which have long held an important place among the techniques of the coordination chemist, have generally remained unexploited by those seeking to understand the complex and often elusive chemistry of metal centres in proteins. For a number of reasons, electron transfer between electrodes and proteins' has been regarded as being too slow and irreversible to provide useful information. This article seeks to counter such a view and outlines the advances that have been made towards achieving and interpreting voltammetric responses from metal-containing active sites. The main theme, exploitation, is developed through discussion of several investigations that demonstrate the advantages that are now on offer from electrodes that "talk" to metalloproteins. Far from being restricted to measurements of stable equilibria, voltammetry can address reactive states; species which are thermodynamically inaccessible by normal chemical titration or which display interesting yet complicating dynamic properties such as structural change or catalytic activity. In such cases, the coupled processes are visualised and may be investigated quantitatively and under controlled conditions. The resolution of chemical activity which is thus afforded extends even to multi-site enzyme complexes. [ABSTRACT FROM AUTHOR]

The theoretical approach to the understanding of the NMR parameters in paramagnetic exchange coupled dimers is discussed and simplified formulas are provided. The effect of magnetic coupling on electron relaxation times in magnetic coupled heterodimetallic systems is also discussed. The 1H NMR spectra of two oxidized and reduced Fe2S2-ferredoxins are interpreted and structural information is obtained. Other systems encountered in the literature are briefly reviewed and the guidelines for the interpretation of the 1H NMR spectra of Fe4S4 systems are given as an extension of the above model. [ABSTRACT FROM AUTHOR]

The important hybridization schemes for co-ordination and organometallic compounds have been derived using a methodology based on a spherical harmonic expansion of the hybridized orbitals. For spherical co-ordination compounds MLn it is possible to define a set of n hybrids which have their maxima in the metal-ligand directions and 9-n d orbitals or hybrids which have nodes along the metal-ligand bonds. The latter are important for π-bonding to the ligands. [ABSTRACT FROM AUTHOR]

The synthetic, structural, and coordination chemistry of crown thioethers with both transition and p-block metal ions is reviewed comprehensively through December 1988. Emphasis falls upon the electronic structures and redox properties induced in metal ions by coordination to crown thioethers. Examples include stabilization of mononuclear Rh(II), Pt(III), and low-spin octahedral Co(II). A subsidiary theme concerns the influence of ligand conformation in determining both the binding efficacy and the qualitative coordination chemistry associated with a given crown thioether. The review concludes with a view toward potential future applications of crown thioethers in catalysis, in sequestration or biological delivery of heavy metal ions, and in fundamental studies directed toward rational design of ligands. [ABSTRACT FROM AUTHOR]

Cyclopentadienyl metal complexes are organometallic compounds which exhibit antiproliferative properties in vivo and in vitro. They are represented by compounds of various structural type. The metallocene diacido complexes (C5H5)2MX2 contain early transition metal atoms such as titanium(IV) and vanadium(IV), the ionic metallicenium salts [(C5H5)2M]+X− include medium transition metals, e.g. iron(III), whereas the uncharged decasubstituted metallocenes (C5R5)2M comprise the main group elements tin(II) and germanium(II) as central metal atoms M. A variety of biological data are available about metallocene diacido complexes, especially titanocene dichloride. These substances exhibit antitumor properties against numerous experimental tumors, e.g. Ehrlich ascites tumor, B 16 melanoma, colon 38 carcinoma, Lewis lung carcinoma, as well as against various human tumors heterotransplanted to athymic mice. Biological experiments using (C5H5)2TiCl2 and (C5H5)2VCl2 pointed to the nucleic acids as a probable target for metallocene diacido complexes within the cells, revealed a main accumulation of titanium or vanadium in the liver and the intestine and unfolded a pattern of organ toxicity which fundamentally differs from that of other cytostatic drugs. These biological features confirm the cyclopentadienyl metal complexes to be an independent group of non-platinum-group metal antitumor agents being characterized by unusual biological properties. They represent interesting candidates for future biological and clinical investigations. [ABSTRACT FROM AUTHOR]

Current interest in the biological chemistry of phosphines and their metal complexes ranges from the widespread application of PH3 as a fumigant, to the clinical use of a Au(I)PEt3 complex as an anti-arthritic drug. Metal phosphine complexes also offer potential as heart-imaging agents and anti-cancer drugs. Data on cytotoxicity and anticancer activity are surveyed in detail with particular emphasis on diphosphines and their copper, silver and gold complexes. Chemistry of likely relevance to pharmacology is discussed: especially redox reactions (disulphide reduction, oxygen abstraction), radical reactions and metal binding. Much phosphine chemistry of relevance to biology has yet to be explored. [ABSTRACT FROM AUTHOR]

The purple acid phosphatases comprise a group of proteins distinguished by their enzymic activity, distinctive color and, most importantly, the presence of a spin-coupled binuclear iron center. This center exists in two stable interconvertible states: pink, reduced, EPR-visible and enzymically active, with an antiferromagnetically exchanged Fe(II)-Fe(III) cluster, and purple, oxidized, EPR-silent and inert, with the binuclear pair as Fe(III)-Fe(III). In uteroferrin, the purple acid phosphatase of uterine secretions, a transient intermediate species has also been identified. Engendered by the interaction of phosphate with the pink form of uteroferrin, this transient intermediate is purple, EPR-silent and devoid of the contact-shifted proton resonances seen in its pink parent. Nevertheless, it is paramagnetic, with an Fe(II)-Fe(III) couple demonstrable by Mössbauer spectroscopy. Although considerable progress toward characterizing the properties of uteroferrin and the purple acid phosphatases has been achieved in recent years, enigmas persist. The identity of the ligands of each iron atom and how these change in nature or arrangement during redox transitions is still unknown. Perhaps most interestingly, the mechanism of enzymic activity, and the relation of the redox and enzymatic properties of the purple acid phosphatases to their yet uncertain physiological roles, remain to be established. [ABSTRACT FROM AUTHOR]