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Within the last five years significant advances have been made in elucidating the chemistry of technetium. While this element is continually produced in some abundance during the operation of fission nuclear reactors, its practical application has been limited to nuclear medicine. Owing to the development of the "technetium generator" almost two decades ago, 99mTc is now the preferred isotope for many diagnostic studies involving the imaging of internal organs by radioscintigraphic techniques; however, many of the agents employed have yet to be well characterized from a chemical standpoint. Owing to its position in the middle of the periodic table, the chemistry of this element has already proven rich, even when limited to aqueous solution. Subtle chemical differences often control the relative stabilities of the various oxidation states and each oxidation state exhibits more than one geometry. Coordination complexes involving Tc(III), Tc(IV) and Tc(V) tend to predominate in preparations used for diagnosis. Recent synthetic advances now pave the way for new series of technetium compounds with well-described chemistry. Of these, methods for coordinating chelating nitrogen ligands to Tc(V) offer substantial versatility. (This review covers the literature through August, 1981.) [ABSTRACT FROM AUTHOR]

Electron nuclear double resonance (ENDOR) of transition metal complexes with organic ligands is considered from an experimental and theoretical point of view. In the experimental sections, instrumentations used in conventional ENDOR and multiple resonance spectroscopy and in some new double resonance methods are discussed. Particular emphasis is placed on various experimental techniques which supplement the conventional ENDOR experiment. The different methods are illustrated by a number of applications on transition metal complexes1. In the theoretical sections, the analysis of ENDOR spectra of complex spin systems is described including second order effects, transitions probabilities and determination of signs of the magnetic parameters. Some of the approaches used in ENDOR to interpret ligand hyperfine and quadrupole data are outlined. Finally, a review of the ENDOR literature on transition metal complexes with organic ligands is presented. [ABSTRACT FROM AUTHOR]

The purpose of this review is to describe some spectacular effects of many-electron interactions in certain core levels in heavy atoms, as well as to give an overview of closely related phenomena in molecules, solids and adsorbates. The central concept will be what we shall call giant Coster-Kronig (gCK) fluctuation and decay of a hole level, involving ineraction of a single hole with configurations with primarily two holes and one excited electron. In systems with an open "valence shell" structure. i.e. with empty levels spacially as compact as the occupied ones, the interaction process can become extremely strong and lead to a breakdown of the one-electron picture. In some cases, there is even a complete breakdown of the quasi-particle picture, in which case the spectral strength has no prominent discret features and rather shows a continuum-like distribution. The breakdown of the one-electron picture can be associated with symmetry breaking and localization, in a wide sense, of the hole. Due to the gCK fluctuation process (configuration interaction), a hole has to be described in terms of a wave-packet of one-electron symmetry states. As a consequence, in a number of cases, an atomic hole cannot be confined to a proper subshell but will move in a polarized subshell. In molecules, a hole will often not be confined to a molecular symmetry orbital but will be localized to varying degrees. Finally, in a metal, a hole in a narrow band often cannot be described in terms of extended states, in which case the non-validity of a ground state band picture may show up as band narrowing or shifted band structure. Particularly spectacular effects of localization occur in the case of two holes in a narrow band. Examples of atomic levels showing very strong many-electron effects are 4 s, 4 p-like holes in Cd to Gd and 5 s, 5 p-like holes in Bi to Pu, where the gCK process takes the form ns ⇆ np5nd9nf,ɛf and np5 ⇆ nd8nf,cf, with n=4 and 5, resp. These spectra represent a partial or complete breakdown of the quasi-particle picture. In molecules one has found a large number of cases of partial or complete breakdown of the quasi-particle picture in the inner valence region of e.g. N2, N2O4, C2H2, CS2, to mention only a few examples. This type of behaviour seems to be the rule rather than the exception due to the high density of two-hole-one-electron levels in the inner valence region. Finally, in metallic Ni a hole in the 3 d-valence band has been found to show pronounced effects of localization, giving rise to a narrowing of the 3 d-band in comparison with the calculated ground-state band structure. [ABSTRACT FROM AUTHOR]

This article is thought to be a supplement to an earlier one "Mechanism of Hydrogen Transfer in Redox Enzymes"33). Most of the basic concepts and hypotheses put forward then, are still valid. Substantial new evidence supporting our views has been advanced since then. The new data presented here concern modified or as we call them "mutilated flavins", namely 5-deaza- and 5-thiaflavin, which upon incorporation into apoflavoprotein retain some, but not all of the activities encountered with the native flavoproteins. [ABSTRACT FROM AUTHOR]

Although the process of light energy conversion to chemical form by specialized plant cell organelles, the chloroplasts, is well understood, many contemporary investigations have been concerned with the catalytic role played by manganese in oxygen evolution. Current views of the photosynthetic electron transport chain or the so-called "Z-scheme" intermediates identify the cytochromes b6, b-559 and f, the chlorophyll linked primary electron acceptor and donors of photosystems -I and -II and a number of e.p.r. distinct iron-sulfur centres involved in non cyclic and possibly cyclic electron transfer pathways. The accumulation of four redox equivalents leads to the oxidation of water to yield molecular oxygen by a water splitting metalloenzyme (designated -Y) which involves at least two manganese atoms per molecule of enzyme. During the four electron oxidation of water, e.p.r. and proton relaxation evidence suggest the two manganese atoms undergo redox changes between manganese(II) and manganese(III) states. A number of proposed manganese model systems exhibiting the features required by the well known S-state mechanism of oxygen evolution are discussed. The modelling of both the charge separation function of the photosystems and the properties of the catalytically active manganese is of current interest from an energy production point of view. [ABSTRACT FROM AUTHOR]

The major solid state principles involved in mineralization in biological systems are discussed. Three major biological control factors of mineralization are described; structural, spatial, and chemical control. Factors determining nucleation on organic surfaces, mineral growth, mineral structure, and morphology are reported. Oriented growth of minerals on organic matrices can occur by three processes; (i) lattice matching (epitaxis) between the organic matrix and depositing crystal faces, (ii) surface structural relationships between the organic matrix and depositing crystal faces, (iii) ordered aggregation of preformed mineral particles. Several a priori reasons indicate that epitaxis is unlikely to be a major process of oriented growth in biomineralization. The presence and importance of biogenic amorphous minerals is also described. [ABSTRACT FROM AUTHOR]

The redox chemistry of chromium(VI) is discussed with respect to the cellular metabolism of the carcinogen chromate in vivo. Possible sites for cellular reduction of chromium(VI) to chromium(III) are considered. The reactions of amino acids, ascorbic acid, carboxylic acids, thiol-containing mole-cules and other small molecules with chromate under physiological conditions are presented. In general only ascorbate and those molecules containing sulfhydryl groups are capable of easily reducing chromate at pH 7.4. Thus, in the cytoplasm, glutathione, cysteine and ascorbate are likely candidates to react with chromate. While most proteins are unreactive toward chromate, certain redox proteins are active in reducing chromate. The heme proteins hemoglobin and cytochrome P-450 possess chromate-reductase activity, whereas cytochrome c and myoglobin are inactive. The NADPH-dependent flavoenzymes glutathione reductase and NADPH-cytochrome P-450 reductase also possess chromate-reductase activity. However, the NAD(P)H enzymes, isocitrate dehydrogenase, glutamate dehyrogenase and malate dehydrogenase do not reduce chromate. Both microsomes and mitochondria possess chromate-reductase activity. The microsomal activity is accounted for by the NADPH-cytochrome P-450 reductase/cytochrome P-450 system. The enzyme(s) responsible for the mitochondrial reduction of chromate have not been identified. Chromium(VI) and its metabolite chromium(III) inhibit the normal activities of enzymes which bind chromium(III) or reduce chromate. The metabolism of chromate involves the generation of reactive intermediates which ultimately bind to cellular constituents and damage their function in the cell. [ABSTRACT FROM AUTHOR]

Reductive elimination of the 2′-hydroxyl group from ribonucleotides to yield 2′-deoxyribonucleotides, the monomeric precursors of DNA, requires an uncommon type of enzyme catalysis in which the transition metals, manganese, iron, or cobalt, and free radical intermediates cooperate. In the group of deoxyadenosylcobalamin (coenzyme B 12)-dependent ribonucleotide reductases the coenzyme supplies a transient radical pair of deoxyadenosyl· and cob(II)alamin whereas in the nonheme-iron group of enzymes a protein subunit carries a stable tyrosyl radical coordinated to a binuclear iron(III) complex; the manganese-dependent enzymes are less precisely known. The radicals are thought to function in hydrogen transfer from cysteine SH to the ribonucleotide substrate, and the metal complexes are apparently needed to generate and stabilize the radicals. In aerobic organisms oxygen also plays a critical role in these processes and hence in DNA synthesis and cell proliferation. In addition to the transition metals, Mg2+ or Ca2+ are required by several ribonucleotide reductases for structural integrity. However the most potent inhibitors of deoxy-ribonucleotide biosynthesis (of potential interest in chemotherapy) are not metal chelators but radical scavengers. Cell cycle arrest and cell death produced by simple chemicals like N-hydroxyurea and hydroxamates can be traced back to their reaction with ribonucleotide reductase. Evidence is accumulating that independent enzymes of deoxyribonucleotide and DNA synthesis are functionally coupled in a novel type of supramolecular structure. [ABSTRACT FROM AUTHOR]

A brief discussion of the vast and increasingly important area of organoselenium chemistry is presented as it relates to the biochemistry of this element. Organo selenoamino acids are the basic building blocks of many selenium-containing macromolecules, which are the primary focus of this article. First to be discussed are synthetic selenium analogues of sulfur-containing natural products such as selenocoenzyme A and selenobiotin. Secondly, are those enzymes which will indiscriminately incorporate selenium and the most important of these are considered. Finally, the most important and interesting class of selenium-containing macromolecules are the naturally occurring selenoproteins, of which there are eight currently known and only three which have been extensively studied. Physical studies which relate to or have the potential to aid in investigations of selenium biochemistry constitute the final topic. [ABSTRACT FROM AUTHOR]

Both natural and polluting metal ions are taken up by marine organisms. For many metal, and metal-containing ions, mechanisms exist which allow these organisms to discriminate between essential and non-essential ions, and reject the latter. In this article we examine the accumulation mechanisms and effects of metal and metal-containing ions on bivalve molluscs and tunicates (sea squirts). Special emphasis is put on the internal or physiological environment of metal ions in marine organisms. A molecular mechanism based on experimental studies is presented to explain the extraordinary accumulation of vanadium in tunicate blood cells called vanadocytes. Vanadium in the form of vanadate anion is transported into the blood cells by passage through anionic channels. Inside the cell it is reduced to cationic V(III) and V(IV) ions which are trapped inside. [ABSTRACT FROM AUTHOR]

Vanadium is an essential element. It is found widely distributed in minor and trace amounts throughout the lithosphere and biosphere. Vanadium is present in all mammalian tissues at concentrations of about 10 µM or less. Numerous physiological effects of this element are known at both the organismic and cellular levels, and in some instances these effects can be explained in terms of its biochemistry. Vanadate(V) and/or vanadyl(IV) ions are very potent inhibitors (K1 ≲ 10−6 M) of certain phosphatases, ATPases, phosphotransferases, nucleases, and kinases, among others. Vanadate(V) activates adenylate kinase. These ions have been quite useful as kinetic and spectroscopic (EPR) probes of complex cellular processes. Vanadium is an insulin mimetic agent. The ability of vanadium at low concentrations to influence enzymes is probably important to its function in vivo. Much of the biochemistry of vanadium is intricately related to the redox and coordination chemistry of this element. In particular, the parallel between the chemistries of vanadates and phosphates can account for the observed inhibition of various phosphohydrolases. However, there is ample evidence that vanadium in the body exists primarily as the vanadyl(IV) ion, VO2+, complexed to proteins and other cellular components; several biological reducing agents can convert vanadium(V) to this form. Serum transferrin appears to be involved in the metabolism of vanadium, perhaps as the transport protein for VO2+. Despite recent major advances in our understanding of the biochemistry of vanadium, the delineation of the physiological function(s) of this important element has remained an elusive goal. [ABSTRACT FROM AUTHOR]

Studies on the enzyme nitrogenase have shown that it contains two novel metal-sulfur clusters: the iron-molybdenum cofactor (FeMo-co), which contains one Mo, 6-7 Fe, and ∼ 4 S atoms in a spincoupled paramagnetic (S=3/2) cluster; and the P clusters, variants of normal but highly reduced 4 Fe-4 S clusters in which three Fe atoms are somehow distinguished from the fourth and which, when oxidized, exhibit an usually high spin (S=5/2 or 7/2) ground state. No structures consistent with all available spectroscopic data have been proposed for either unit. Synthetic efforts aimed at developing models for FeMo-co have utilized tetrathiomolybdate as starting material, and have resulted in two well-characterized classes of Mo-Fe-S cluster: those containing the MoFe3S4 "cubane" core and those containing a "linear" MoS2Fe unit. The former apparently contain Mo in a relatively reduced valence state (ca. + 3) and exhibit Mo-S and Mo-Fe distances in good agreement with those found for the Mo site of nitrogenase; the latter contain Mo in a higher oxidation state (ca. + 6) and have significantly shorter Mo-S distances. Synthetic work related to the P clusters has centered on the properties of the [Fe4S4(SR)4]3− ions, for which two different structures with different magnetic properties are found, and on the properties of 4 Fe-4 S clusters with non-thiolate ligands. [ABSTRACT FROM AUTHOR]

A detailed picture of the unique electronic structure of active sites in copper proteins is essential to our understanding of their biological functions. This has been achieved through a combination of complementary spectroscopic techniques, chemical perturbations, and parallel studies on copper complexes which serve as spectral analogues. These studies provide a basis for the generation of a "spectroscopically effective" picture of the active site and allow a correlation of changes in geometric and electronic structure with variation in function. When combined with high-resolution crystallographic information, single crystal spectral studies allow a correlation of electronic features with specific geometric features and provide a great deal of insight into the bonding at the site. [ABSTRACT FROM AUTHOR]

Theoretical aspects of mineral flotation in particular the adsorption of flotation reagents at the mineral-water interface, were reviewed in 1959 for the last time. Several papers have been published since on this topic. The aim of this chapter is to survey recent publications that have contributed to the development of the flotation theory. [ABSTRACT FROM AUTHOR]

Studies on the solvent extraction of actinide ions by different combinations of extractants have been reviewed. Various equilibria involved in the extraction processes and the formation of the extractable complexes have been considered along with their equilibrium constant data. Various methods which are useful in establishing the composition and the nature of the extractable complexes are presented. The data on isolation and structural studies of some complexes, involved in synergic extraction, are also included. A brief description of the different areas in which synergic extraction is finding application is also given. Many combinations of extractants, where the studies conducted are very few but, which are likely to yield enhanced extractions are indicated. Areas of research, both from the academic and applied points of view, which require attention are suggested. [ABSTRACT FROM AUTHOR]

In a homogeneous chemical reaction maintained far from equilibrium where the reaction mechanism contains coupled feedback steps, oscillations in the concentrations of certain intermediates are possible. Until recently, the few known chemical oscillators were discovered accidentally, were variants of these reactions, or were extracted from biological systems. To provide new chemical oscillators for theoretical and experimental study, a program has been developed to design homogeneous oscillators based on reactions of inorganic compounds and ions. Mechanistic detail is presented for the chlorite-iodide-arsenite oscillator, and other newly designed oscillators are discussed more briefly. A classification scheme for grouping these and related oscillators is presented. [ABSTRACT FROM AUTHOR]