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Research has been conducted on the cis (hydroxo)-Fe (super)III-(carboxylato) configuration. The X-ray analysis of this configuration has been carried out and its details are reported.

The copper(II) complexes supported by the N,N-di(2-pyridylmethyl)benzylamine tridentate ligand or its derivatives having m-substituted phenyl group at the 6-position of pyridine donor groups are prepared and their reactivity towards [H.sub.2][O.sub.2] is examined at low temperature. These complexes have afforded simple copper(II)-hydroperoxo complexes L[Cu.sup.II]-OOH in the reaction with [H.sub.2][O.sub.2], which showing the solvent effect on the reaction between copper(II) complexes and [H.sub.2][O.sub.2].

The electronic and steric effects that lead to distorted trigonal-bipyramidal geometry using series of sterically hindered mesityl iron salen complexes and another series of less-hindered Xylyl salen complexes is reported. The close inspection of X-ray crystal structures of protocatechuate 3,4-dioxygenase(3,4 PCD) and 3,4-PCD-inhibitor complexes suggests that distorted trigonal-bipyramidal iron site is not preorganized by a 3,4-PCD protein but is electronically induced on binding of an external hydroxide ligand to the iron(III) center.

Mössbauer spectroscopy has been used to characterize oxygenated myoglobins (oxy Mbs) reconstituted with native and chemically modified 57 Fe-enriched heme cofactors with different electron densities of the heme Fe atom (ρ Fe ) and to elucidate the effect of a change in the ρ Fe on the nature of the bond between heme Fe and oxygen (O 2 ), i.e., the Fe-O 2 bond, in the protein. Quadrupole splitting ( ΔE Q ) was found to decrease with decreasing ρ Fe , and the observed ρ Fe -dependent ΔE Q confirmed an increase in the contribution of the ferric-superoxide (Fe 3+ -O 2 - ) form to the resonance hybrid of the Fe-O 2 fragment with decreasing ρ Fe . These observations explicitly accounted for the lowering of O 2 affinity of the protein due to an increase in the O 2 dissociation rate and a decrease in the autoxidation reaction rate of oxy Mb through decreasing H + affinity of the bound ligand with decreasing ρ Fe . Therefore, the present study demonstrated the mechanism underlying the electronic control of O 2 affinity and the autoxidation of the protein through the heme electronic structure. Carbon monoxide (CO) adducts of reconstituted Mbs (CO-Mbs) were similarly characterized, and we found that the resonance between the two canonical forms of the Fe-CO fragment was also affected by a change in ρ Fe . Thus, the nature of the Fe-ligand bond in the protein was found to be affected by the ρ Fe .

New bis(ene-1,2-dithiolato)-oxido-alcoholato molybdenum(VI) and -oxido-thiolato molybdenum(VI) anionic complexes, denoted as [Mo(VI)O(ER)L2](-) (E = O, S; L = dimethoxycarboxylate-1,2-ethylenedithiolate), were obtained from the reaction of the corresponding dioxido-molybdenum(VI) precursor complex with either an alcohol or a thiol in the presence of an organic acid (e.g., 10-camphorsulfonic acid) at low temperature. The [Mo(VI)O(ER)L2](-) complexes were isolated and characterized, and the structure of [Mo(VI)O(OEt)L2](-) was determined by X-ray crystallography. The Mo(VI) center in [Mo(VI)O(OEt)L2](-) exhibits a distorted octahedral geometry with the two ene-1,2-dithiolate ligands being symmetry inequivalent. The computed structure of [Mo(VI)O(SR)L2](-) is essentially identical to that of [Mo(VI)O(OR)L2](-). The electronic structures of the resulting molybdenum(VI) complexes were evaluated using electronic absorption spectroscopy and bonding calculations. The nature of the distorted O(h) geometry in these [Mo(VI)O(EEt)L2](-) complexes results in a lowest unoccupied molecular orbital wave function that possesses strong π* interactions between the Mo(d(xy)) orbital and the cis S(p(z)) orbital localized on one sulfur donor from a single ene-1,2-dithiolate ligand. The presence of a covalent Mo-S(dithiolene) bonding interaction in these monooxido Mo(VI) compounds contributes to their low-energy ligand-to-metal charge transfer transitions. A second important d-p π bonding interaction derives from the ∼180° O(oxo)-Mo-E-C dihedral angle involving the alcoholate and thiolate donors, and this contributes to ancillary ligand contributions to the electronic structure of these species. The formation of [Mo(VI)O(OEt)L2](-) and [Mo(VI)O(SEt)L2](-) from the dioxidomolybdenum(VI) precursor may be regarded as a model for the active-site formation process that occurs in the dimethyl sulfoxide reductase family of pyranopterin molybdenum enzymes.

We analyzed the oxygen (O2) and carbon monoxide (CO) binding properties of the H64L mutant of myoglobin reconstituted with chemically modified heme cofactors possessing a heme Fe atom with a variety of electron densities, in order to elucidate the effect of the removal of the distal His64 on the control of both the O2 affinity and discrimination between O2 and CO of the protein by the intrinsic heme Fe reactivity through the electron density of the heme Fe atom (ρFe). The study revealed that, as in the case of the native protein, the O2 affinity of the H64L mutant protein is regulated by the ρFe value in such a manner that the O2 affinity of the protein decreases, due to an increase in the O2 dissociation rate constant, with a decrease in the ρFe value, and that the O2 affinities of the mutant and native proteins are affected comparably by a given change in the ρFe value. On the other hand, the CO affinity of the H64L mutant protein was found to increase, due to a decrease in the CO dissociation rate constant, with a decrease in the ρFe value, whereas that of the native protein was essentially independent of a change in the ρFe value. As a result, the regulation of the O2/CO discrimination in the protein through the ρFe value is affected by the distal His64. Thus, the study revealed that the electronic tuning of the intrinsic heme Fe reactivity through the ρFe value plays a vital role in the regulation of the protein function, as the heme environment furnished by the distal His64 does.