MECHANISMS OF ELECTRON-TRANSFER AND INDUCED OXYGEN REACTIONS OF COPPER(II) AND COPPER(III) PEPTIDE COMPLEXES

JOHN MATTHEW ANAST, Purdue University

Abstract

The electron-transfer reactions of copper(III)-peptide complexes with SO(,3)('2-), Fe(CN)(,6)('4-), and Ru(II) ammine complexes are reported. Copper(III) tetraglycine reacts rapidly with sulfite in two reversible one-electron steps to give Cu(II) tetraglycine and aquated SO(,3), which hydrolyzes to sulfate. Suppression of the rate by the copper(II) complex indicates SO(,3)('-) is an intermediate and must have E(DEGREES) < 0.89 V. Oxygen intercepts the SO(,3)('-) to produce oxidizing intermediates which in turn can regenerate Cu('III)(H(,-3)G(,4))('-) by reactions with Cu('II)(H(,-3)G(,4))('2-) causing a net gain of Cu(III) to occur. Observed oscillations in the Cu('III)(H(,-3)G(,4))('-) concentration suggest that non-equilibrium pathways alternate during the reaction. The Cu(III,II) tetraglycine couple catalyzes the autoxidation of sulfite to sulfate. The rate constants for electron-transfer reactions of Fe(CN)(,6)('4-) with a series of Cu(III)-peptide complexes vary from 2.6 x 10('5) M('-1) s('-1) to 8 x 10('7) M('-1) s('-1). The reaction with a neutral Cu(III) complex proceeds more slowly in the presence of added alkali metal cations and is attributed to the formation of ferrocyanide-cation complexes. The electron-transfer rates are much larger than is predicted from an outer-sphere mechanism, suggesting that cyanide bridges axially to the square-planar Cu(III) complex facilitating the reaction. The electron paramagnetic resonance data are consistent with a bridged complex as the Cu(II) peptide signal is quenched upon addition of Fe(CN)(,6)('3-). Bulky substituents in the Cu(III)-peptide ligand sterically block formation of the cyano bridge and yield slower rates which are more in accord with an outer-sphere mechanism. The self-exchange rate constants for five copper(III,II)-peptide complexes have been determined from the reactions of copper(III) di(-(alpha)-aminoisobutryl-)-(alpha)-aminoisobutric acid with the copper(II)-peptide complexes. Copper(III) peptides are rapidly reduced by the complexes of Ru(NH(,3))(,6)('2+), Ru(NH(,3))(,5)pyridine('2+), Ru(NH(,3))(,5)picoline('2+). The rates and activation parameters are accurately predicted with the known Ru(III,II) and the determined Cu(III,II) peptide self-exchange values using Marcus theory. Evidence strongly supports electron transfer by an outer-sphere mechanism.

Degree

Ph.D.

Subject Area

Chemistry

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