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R H Holm

Publications and source records attributed to R H Holm.

88 records · Page 5Linked to original sources
PubMed1975

Structural and electronic effects on the polarographic half-wave potentials of copper (II) chelate complexes.

Polarographic measurements in dimethylformamide solution have been performed on 37 bis-chelate Cu(II) complexes containing primarily salicylaldimine, geta-ketoamine, beta-iminoamine, and pyrrole-2-aldimine ligand systems. The complexes were selected in order to reveal the effect of stereochemical and donor atom variations on half-wave potentials. Mono- and binuclear complexes undergo one-and two-electron reductions, respectively, which were established for representative cases by controlled potential coulometry. The response of Cu(II)/Cu(I) potentials to these variations reveals that nonplanar bis-chelate complexes are easier to reduce than their planar analogs, rigid planar tetradentate or related planar bis-chelate complexes, and complexes differing only in donor atoms are more readily reduced in the order N4 less than N2O2 less than N2S2. Binuclear complexes are reduced in one two-electron or two resolvable one-electron steps depending on ligand structure. Potentials of these and other complexes previously examined are considered in relation to the markedly positive potentials of "blue" copper proteins and ligand structural features which might afford Cu(II)/Cu(I) potentials in the protein range are noted.

Binding Sites↗
PubMed1975

Synthetic analogs of active sites of iron-sulfur proteins: bis (o-xylyldithiolato) ferrate (III) monoanion, a structurally unconstrained model for the rubredoxin Fe-S4 unit.

To complete the set of synthetic analogs of the three recognized types of active sites in iron-sulfur redox proteins, the compound (Et4N)[Fe((SCH2)2C6H4)2], derived from o-xylyl-alpha,alpha'-dithiol, has been prepared and its structure has been determined by x-ray diffraction. The bischelate anion contains a near-tetrahedral Fe(III)-S4 coordination unit with small rhombic distortions and all Fe-S bond distances in the range 2.252-2.279 A. Its electronic properties have been partially characterized by measurement of electronic absorption, paramagnetic resonance, Mössbauer spectra, and magnetic susceptibility. The analog, as the protein, exhibits the Fe(III)/Fe(II) redox couple. These results substantiate designation of [Fe((SCH2)2C6H4)2]- as a synthetic analog of the Fe(III)(S-Cys)4 center in oxidized rubredoxin proteins. Comparison of the analog structure with that of the Clostridium pasteurianum rubredoxin active site shows that the former is substantially less distorted from idealized tetrahedral symmetry, and is considered to represent an essentially unconstrained structural model of the latter. Provided the grossly distorted tetrahedral stereochemistry of the protein site persists through final structural refinement, the analog-protein structural comparison supports an entatic state description of oxidized rubredoxin.

Ferredoxins↗
PubMed1973

Synthetic analogs of the active sites of iron-sulfur proteins. Structure and properties of bis(o-xylyldithiolato-m2-sulfidoferrate (3)), an analog of the 2Fe-2S proteins.

The synthetic analog approach has been applied to a clarification of the active sites of 2Fe-2S(*) proteins. The compound (Et(4)N)(2)[FeS(SCH(2))(2)C(6)H(4)](2), derived from o-xylyl-alpha,alpha'-dithiol, has been prepared and its structure has been determined by x-ray diffraction. The centrosymmetric anion contains two tetrahedrally coordinated ferric ions bridged by two sulfide ions and separated by 2.70 A. Comparison of electronic, Mössbauer, and proton magnetic resonance spectra and magnetic susceptibility of the anion with the corresponding properties of the oxidized forms of the proteins reveals significant degrees of similarity. The anion also exhibits the essential redox capacity of the proteins. We conclude that [FeS(SCH(2))(2)C(6)H(4)](2) (2-) possesses the same total oxidation level and electronic configuration as the active sites of the oxidized proteins, and that its structure provides a feasible representation of the minimal structure of the active site. [FeS(SCH(2))(2)C(6)H(4)](2) (2-) is thus the first well-defined synthetic analog of the active sites of two-iron ferredoxins.

Binding Sites↗
PubMed1972

Structure and properties of a synthetic analogue of bacterial iron--sulfur proteins.

The compound (Et(4)N)(2)[Fe(4)S(4)(SCH(2)Ph)(4)] has been prepared and its structure determined by x-ray diffraction. The Fe(4)S(4) core of the anion possesses a configuration of D(2d) symmetry that is closely related to the Fe(4)S(4) active-site structures of the high-potential iron protein from Chromatium and the ferredoxin from Micrococcus aerogenes. Electronic properties of the tetrameric anion have been partially characterized by measurement of proton magnetic resonance, Mössbauer, photoelectron, and electronic spectra, and magnetic susceptibility. Comparison of corresponding properties of [Fe(4)S(4)(SCH(2)Ph)(4)](2-) and the proteins implies that the oxidation levels of the synthetic tetramer, the reduced form of the high-potential protein, and the oxidized form of the 8-Fe ferredoxins are equivalent. The tetramer possesses the one-electron redox capacity associated with the 4-Fe centers of the ferredoxins. The structural and collective electronic features of [Fe(4)S(4)(SCH(2)Ph)(4)](2-) reveal it to be the first well-defined synthetic analogue of the active site of an iron-sulfur protein.

Bacterial Proteins↗