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Using the spin-Hamiltonian formalism the magnetic parameters are introduced through the components of the Λ-tensor involving only the matrix elements of the angular momentum operator. The energy levels for a variety of spins are generated, and the modeling of the magnetization, the magnetic susceptibility, and the heat capacity is done. Theoretical formulae necessary for performing the energy level calculations for a multiterm system are prepared with the help of the irreducible tensor operator approach. The goal of the programming is to evaluate the entire relevant matrix element (electron repulsion, crystal field, spin–orbit interaction, orbital-Zeeman, and spin-Zeeman operators) in the basis set of free-atom terms. The modeling of the zero-field splitting is done at three levels of sophistication: (i) the differences in the crystal-field multiplets when the high-dimensional matrix in the complete space is diagonalized; (ii) the differences in the energy levels of a model subspace when the partitioning technique is applied in the first iteration; (iii) the differences in the energy levels as they are produced by the second-order perturbation theory for the spin Hamiltonian. The spin-Hamiltonian formalism offers simple formulae for the magnetic parameters (the D-factors , D-parameter, χTIP) by evaluating the matrix elements of the angular momentum operator in the basis set of the crystal-field terms. The magnetic functions for dn complexes are modeled for a wide range of crystal-field strengths. [ABSTRACT FROM AUTHOR]

Synthesis of high quality semiconductor nanocrystals is discussed in this chapter. Although the focus is on the recent progress in synthesis of colloidal semiconductor nanocrystals in non-aqueous solution, there is some review of the historical development of the aqueous and other approaches. Mechanisms for controlling the shape, size, and optical properties of colloidal semiconductor nanocrystals are described in detail, although for the most part this topic can only be treated on a qualitative level. Finally, one area of study, the formation of nanocrystals, is discussed in terms of its practical and fundamental significance. [ABSTRACT FROM AUTHOR]

The structures of ternary and quaternary silicates are revisited on the basis of the Zintl-Klemm concept and the Pearson's generalised octet rule. The three-dimensional skeletons formed by the Si atoms can be interpreted as if the Si atoms were behaving as Zintl polyanions, adopting the structure of either main-group elements or Zintl polyanions showing the same connectivity. The O atoms are then located close to both, the hypothetical two-electron bonds and the lone pairs, giving rise to a tetrahedral coordination. In some silicates, the Si atoms clearly show an amphoteric character so that some Si atoms act as donors (bases) adopting an octahedral coordination, whereas others behave as acceptors (acid), adopting a tetrahedral coordination. Although the octahedral coordination seems to be favoured by the application of pressure, the results reported here indicate that the coordination sphere of silicon is not a function of the assumed ionic radius of the Si4+ cations but it depends on the nature of the other cations accompanying them in the structure. [ABSTRACT FROM AUTHOR]

This chapter reviews intraband spectroscopy as it has been applied to semiconductor nanocrystals synthesized by colloid methods. [ABSTRACT FROM AUTHOR]

A retrospective overview of electrochemical studies of semiconductor nanocrystals (NCs) is given. The electrochemical behavior of monodisperse NCs in a non-aqueous supporting electrolyte can be derived from the electron quasiparticle energy and the electron self-energy. Coulomb blockade can sometimes be observed in Si NCs if the charged NCs are stable. Many NCs, such as Ge, CdS, CdSe, PbS and core-shell CdSe/ZnS undergo some electrochemical and chemical reactions such as the so-called EC reaction mechanism. Electrogenerated chemiluminescence (ECL) processes for elemental and II/VI compound NCs in solution (and their films) are found to follow the general ECL mechanisms of organic compounds. Most NCs emit ECL, which is red-shifted into the photoluminescence (PL) region. ECL studies of NC solid-state films, especially the combination of organic light-emitting diodes (LEDs) and NCs, have suggested the potential for real workable devices. Treated CdSe NC thin films also exhibit stable and fast electrochromic changes and ECL. [ABSTRACT FROM AUTHOR]

The properties of the actinides in the first half of the series are deduced from the characteristics of 5f wavefunctions, in many ways similar to the d-wavefunction in transition metals. Relativistic effects and the calculation of ground state properties are stressed. [ABSTRACT FROM AUTHOR]

Localization versus itineracy and the degree of hybridization of 5 f states with orbitals of the actinide atom (especially 6 d) as well as with those of the ligand in compounds are central questions for the understanding of bonding in actinide solids. Photoelectron spectroscopy provides answers to these questions. In narrow band solids, like the actinides, the interpretation of results requires the use of band calculations in the itinerant picture, as well as models of final state emission in the atomic picture. From the experimental viewpoint: l. the analysis of the variation of photoionization cross sections (affecting the intensities of photoelectron spectroscopy), gives an insight into the orbital composition of the bands of the solid; 2. the combination of direct and inverse photoemission provides a powerful tool to assess the structure of occupied and of empty states, and, in the case of localized 5 f states, permits the determination of a fundamental quantity, the Coulomb correlation energy, governing the physical properties of narrow bands. After a survey of the basic theory and some experimental aspects of photoelectron spectroscopy which are relevant to actinide solids, two systems are illustrated: elemental actinide metals, in which the Mott transition between plutonium and americium is evidenced in a "photographic" way by photoemission, and strongly ionic oxides, in which the 5 f localized behaviour is clearly seen, and indications of f-p or d-p covalent mixing are investigated. [ABSTRACT FROM AUTHOR]

The various magnetic properties observed in actinide solids are largely determined by the degree the unsaturated 5 f shells retain an atomic localized character or lose it in an itinerant mode. Hence the investigation of magnetism (and of related properties such as low temperature electrical resistivity) has played a central role in understanding bonding in actinide solids. Between clear cut cases of "localization" or "itineracy" intermediate situations are observed (leading for instance to spin fluctuating behaviour) which have made actinide magnetic research a rich but also controversial field. A short exposition of the basic concepts of actinide magnetic research and of the different aspects of magnetic ordering in actinide solids is followed by a discussion of experimental and interpretational aspects, in particular the difficulty always encountered of separating the different contributions. A number of examples have been chosen to illustrate the variety of magnetic behaviour and its meaning in establishing a unified picture of actinide solid state physics. One of the aims of the chapter is to inspire new research in actinide magnetism. To this purpose, tables collecting the main magnetic data for a large number of actinide binary compounds are provided. [ABSTRACT FROM AUTHOR]

The problem of bonding in actinide solids has been approached through the establishment of semi-theoretical correlations between thermodynamic and structural properties and the degree of participation to the bond of the different electronic orbitals of the actinide atom. The first part of the chapter is devoted to an analysis of these correlations, as well as to the presentation of the most important experimental results. In a second part the following stage of development is reviewed, i.e. the introduction of more quantitative theories mostly based on bond structure calculations. These theories are given a thermodynamic form (equation of states at zero temperature), and explain the typical behaviour of such ground state properties as cohesive energies, atomic volumes, and bulk moduli across the series. They employ in their simplest form the Friedel model extended from the d- to the 5 f-itinerant state. The Mott transition (between plutonium and americium metals) finds a good justification within this frame. In actinide binary compounds an equation of state can also be developed on the same lines. The difference in electronegativity of the actinide and the non-actinide element plays an important role, determining the degree of mixing between the actinide orbitals (5 f and 6 d) and the orbitals of the ligand. A mixture of metallic, ionic and covalent bond is then encountered. In the chapter, two classes of actinide compounds are reviewed: NaCl-structure pnictides or chalcogenides, and oxides. In oxides a large departure from the stoichiometric composition is usually met. The importance of this phenomenon, also for the understanding of the oxide bond, is highlighted at the end of the chapter. [ABSTRACT FROM AUTHOR]

Considerable progress has been possible in the determination of chemical and physical properties of actinide solids as a result of improved preparation and purification methods. The growing demands for well characterized samples have given the incentive to overcome the difficulties presented to the actinide chemist by the high radioactivity hazard and comparative rarity of the actinide elements. In this chapter, preparation and purification methods are reviewed. In view of the expected role of 5 f electrons in the metallic bond of actinides, methods for the preparation of metals have been particularly studied. There has also been important progress in the preparation of simple binary compounds. Special emphasis has been given to the growth of single crystals, particularly needed for the most refined physical techniques. The chapter underlines the interdisciplinary nature of actinide solid state research. [ABSTRACT FROM AUTHOR]

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]