SOLUTION PROPERTIES OF NICKEL(III)-BIS(TRIPEPTIDE) COMPLEXES AND PLATINUM(II)-TRIPEPTIDE COMPLEXES

GEORGE EDWARD KIRVAN, Purdue University

Abstract

Bis(tripeptido)nickelate(III) complexes are formed on addition of excess peptide, L('-), to neutral or alkaline solutions of Ni('III)(H(,-2)L) (H(,-n) indicates n deprotonated peptide nitrogens). Ultraviolet-visible absorption spectra, as well as frozen and room temperature EPR spectra, indicate the presence of two forms of the bis complex. The predominant species is Ni('III)(H(,-2)L)(,2)('3-) above pH 11, and Ni('III)(H(,-2)L)(H(,-1)L)('2-) between pH 6 and 11. Methyl groups on the (alpha)-carbons of the tripeptide, particularly at the carboxylate terminal, inhibit formation of bis complexes. Acid dissociation to the mono complexes is observed. For L = glycyl-L-alanylglycine (GAG), rate constants vary between 71 and 12 s('-1) for 0.25 to 10('-5) M hydrogen ion. Both the bis and mono complexes undergo redox decomposition in base to give nickel(II)-peptide and oxidized peptide fragments. However, the Ni(III)-bis complexes are 10('3) to 10('6) times slower to decompose than the mono complexes. Electron-transfer reactions of Ni(III)-bis complexes occur by outer-sphere mechanisms. They are several orders of magnitude slower than the corresponding electron-transfer reactions with the mono complexes. The product Ni(II)-bis complexes rapidly dissociate (k = 150 s('-1) at pH 9 for Ni('II)(H(,-2)G(,3))(H(,-1)G(,3))('3-)) to nickel(II)-mono complexes or, depending on the concentration of L, rearrange to Ni(II)-bis and -tris complexes of the form Ni('II)(H(,-1)L)(,2)('2-) and Ni('II)H(,-1)(L)(,3)('2-). The stability constants of the latter two complexes are log (beta)(,1-22) = -11.79 and log (beta)(,1-13) = -0.247 for L = G(,3). Upper limits of the reductional potentials of the complexes Ni('III/II)(H(,-2)G(,3))(,2)('3-,4-) and Ni('III/II)(H(,-2)G(,3))(H(,-1)G(,3))('2-,3-) are 0.19 and 0.24 V (vs. NHE), respectively. Triglycine reacts with PtCl(,4)('2-) to form three complexes, Pt('II)(H(,-2)G(,3))Cl('2-), Pt('II)(H(,-2)G(,3))('-), and Pt('II(H)(,-2)G(,3))OH('2-). The ('195) Pt NMR chemical shifts of these complexes are -7517, -7144, and -7146 ppm, respectively (referenced to K(,2)PtCl(,6)). The pK(,a) of the peptide nitrogens is between 1 and 2 in the presence of Cl('-). UV-vis spectra of the platinum(II) complexes all have shoulders in the region 230-250 nm with molar absorptivities of (TURN)5100 M('-1) cm('-1). Circular dichroism (CD) spectra of the optically active GAG and GGA complexes are similar with maxima in the range 243-247 nm and (DELTA)(epsilon) = 1.36 M('-1) cm('-1). The CD results together with ('13)C, ('1)H, and ('195)Pt NMR results and the determination of released Cl('-) and H('+) indicate that the main species present in solution is Pt('II)(H(,-2)L)Cl ('2-).

Degree

Ph.D.

Subject Area

Analytical chemistry

COinS