KINETIC, THERMODYNAMIC, AND STRUCTURAL STUDIES OF NICKEL- AND COPPER-TRIPEPTIDE COMPLEXES OF ALPHA-AMINOISOBUTYRIC ACID (EXAFS, NICKEL(III))

WILLIAM RUSSELL KENNEDY, Purdue University

Abstract

Structural information is presented for the nickel(II), nickel(III), copper(II), and copper(III) complexes of the tripeptide of (alpha)-aminoisobutyric acid. This information is obtained by the use of single crystal X-ray diffraction, extended X-ray absorption fine structure (EXAFS), or a combination of the two techniques. The structural information is used to understand the changes which occur in the electron-transfer reactions between these two couples. The principal difference between the nickel(II) and nickel(III) structures is a change in coordination number. The nickel(III) complex has two axially bound water molecules (r(,av) = 2.18(2) (ANGSTROM)), whereas the nickel(II) complex has none. The equatorial interaction is approximately the same in the two cases (r(,av) = 1.84(2) (ANGSTROM)). The principal difference between the copper(II) and copper(III) complexes is a change in the equatorial bond distances (r(,av)('II) = 1.92(2) (ANGSTROM), r(,av)('III) = 1.83(2) (ANGSTROM)). The axial water interaction is negligible in both the copper(II) and copper(III) complexes. The change from a glycyl to an (alpha)-aminoisobutyryl peptide results in a decrease in the equatorial bond lengths of 0.03-0.06 (ANGSTROM) in complexes with nickel(II) and copper(II). This is a result of the enhanced basicity of the (alpha)-aminoisobutyryl peptides, which arises from the electron-donating ability of the methyl groups, which are on the peptide backbone. The kinetics and mechanisms of the reactions of acid and of cyanide with the nickel(II)-tripeptide complex of (alpha)-aminoisobutyric acid are presented. The reactions are two or more orders of magnitude slower than the corresponding reactions with glycyl peptide complexes. The reaction with acid involves a protonated intermediate, which has the proton coordinated on the peptide oxygen, outside the first coordination sphere. The reaction with cyanide also involves an intermediate, which has a cyanide coordinated in an equatorial site. The overall stability constant for the Ni('II)(H(,-2)Aib(,3))('-) ((beta)(,1-21)) is calculated from the equilibrium position of the acid dissociation reaction. The value of (beta)(,1-21) is 10('-9.65). This shows that the (alpha)-aminoisobutyryl peptide complex is 10('3) times more stable than the corresponding glycyl peptide complex.

Degree

Ph.D.

Subject Area

Analytical chemistry

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