AXIAL SUBSTITUTION OF NICKEL(III)-PEPTIDE COMPLEXES AND ELECTRON TRANSFER REACTIONS BETWEEN NICKEL(III)- AND NICKEL(II)-PEPTIDE COMPLEXES

CARL KIMBERLY MURRAY, Purdue University

Abstract

Equilibrium constants for axial substitution and kinetics of electron transfer reactions of nickel(III)-peptide complexes are described. Stability constants are determined by an electrochemical method for axial coordination of monodentate nitrogen ligands to nickel(III) peptide complexes in aqueous solution. The stability constants at 25(DEGREES)C vary from 1100 to 50 M('-1) (in the order: imidazole > NH(,3) (TURNEQ) N(,3)('-) > pyridine) but are relatively independent of the nature of the peptide. The substitution is too fast to observe by stopped-flow methods, and a lower limit for the axial water exchange rate constant is estimated as k(,H(,2)O) > 4 x 10('6) s('-1). There are no reactions observed with the corresponding nickel(II) peptides. Displacement of water from the second axial site of nickel(III) occurs in frozen aqueous solutions with ammonia but not at room temperature. The diglycylethylenediamine (DGEN) complex, Ni('III)(H(,-2)DGEN)('+), adds H('+) and Cl('-) to give one or two axially coordinated Cl('-) in frozen aqueous solution (123 K), but the complexes are too weak to be detected at room temperature. Cross reactions between nickel(III) peptides and nickel(II) peptides in aqueous solution are used to evaluate the self-exchange rate constants k(,11) of these species. The values of k(,11) for sixteen different peptide complexes vary with their structure but can be classified into the following groups: triply-deprotonated peptides, (1.2 x 10('5) M('-1) s('-1)) except for H(,-3)G(,5) (4.2 x 10('4) M('-1) s('-1)), and doubly-deprotonated peptides (1.3 x 10('4) M('-1) s('-1)) except for (alpha)-aminoisobutyryl tripeptides (5.5 x 10('2) M('-1) s('-1)). The cross reactions are catalyzed by bridging ligands with a 10-fold increase in the rate constant in the presence of 5 x 10('-3) M Br('-). The order of enhanced reactivity is Br('-) > Cl('-) > N(,3)('-) and there is no enhancement for F('-) or OH('-).

Degree

Ph.D.

Subject Area

Chemistry

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