CHEMISTRY OF COPPER COMPLEXES OF TRI - ALPHA-AMINOISOBUTYRYL AMIDE: PHOTOCHEMICAL REDOX DECOMPOSITION OF COPPER(III) AND KINETIC AND MECHANISTIC STUDIES OF COPPER(II,III) WITH OXYGEN AND ACTIVATED OXYGEN SPECIES

JOANNA PEYRAUD HINTON, Purdue University

Abstract

Tri-(alpha)-aminoisobutyryl amide, Aib(,3)a, forms three very stable square-planar copper(III) complexes in aqueous solution: (UNFORMATTED TABLE FOLLOWS) pK(,a) = 0.25 pK(,a) = 12.5 Cu('III)(H(,-2)Aib(,3)a)('+) (DBLHARR) Cu('III)(H(,-3)Aib(,3)a) (DBLHARR) Cu('III)(H(,-4)Aib(,3)a)('-) (TABLE ENDS) where H(,-n) refers to the number of coordinated deprotonated nitrogens. In contrast to their stability with respect to thermal redox decomposition, they show varied photochemical sensitivity upon irradiation into their ligand-to-metal charge-transfer (LMCT) bands. The quantum yield for loss of copper(III), (PHI), and the photodecompo- sition products are wavelength dependent. For irradiation in the UV-LMCT band, (PHI) = 0.34 and 0.30, and in the visible band, (PHI) = 0.16 and 0.22 for Cu('III)(H(,-3)Aib(,3)a) and Cu('III)(H(,-2)Aib(,3)a)('+), respectively. Deprotonation of the terminal amine nitrogen causes a dramatic decrease in (PHI), with (PHI) (LESSTHEQ) .08 in the UV and (LESSTHEQ) .004 in the visible region for Cu('III)(H(,-4)Aib(,3)a)('-). Substituted hydantoins are the principal peptide oxidation products from photolysis of Cu('III)(H(,-3)Aib(,3)a) and Cu('III)(H(,-4)Aib(,3)a)('-) and are proposed to form by a metal-assisted intramolecular nucleophilic reaction. Oxygen does not affect (PHI) for these complexes. However, oxygen quenches the photoredox decomposition of the copper(III) complexes of Aib(,3)G, G(,2)AibG and AAib(,2). The copper(II) complex, Cu('II)(H(,-2)Aib(,3)a) reacts with O(,2) in aqueous solution pH 7-10 to form Cu('III)(H(,-3)Aib(,3)a) with a second-order rate constant of 0.072 M('-1)s('-1). No peptide oxidation is observed. The reaction is proposed to proceed by an inner-sphere path, and the intermediacy of superoxide is kinetically confirmed. Oxidation of Cu('II)(H(,-2)Aib(,3)a) by superoxide occurs with a rate constant of 1.8 x 10('6) M('-1)s('-1). The catalytic activity of Cu(III/II)-Aib(,3)a for dismutation of O(,2)('-) is evaluated to be 3 orders of magnitude smaller than for CuZn superoxide dismutase. Two equivalents of Cu('II)(H(,-3)Aib(,3)a)('-) are oxidized by H(,2)O(,2) to give Cu('III)(H(,-3)Aib(,3)a) in aqueous solution pH 7.5-8.5. The reaction is proposed to proceed by the formation of a peroxide-copper-peptide adduct, with a stability constant of 260 M('-1). In alkaline solution, OH('-) = 0.02-1.0 M, two equivalents of Cu('III)Aib(,3)a are reduced by the hydroperoxide anion via a proposed inner-sphere path, k(,max) = 315 M('-1)s('-1) at OH('-) (GREATERTHEQ) .1 M. The optimum pH for catalytic decomposition of H(,2)O(,2) by a Cu(III/II) redox cycle is 9.7.

Degree

Ph.D.

Subject Area

Chemistry

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