Nickel(II) and nickel(III) bis(dipeptido) complexes of alpha-aminoisobutyric acid and nickel-peptide complexes with sulfur-containing ligands

Sally Lyn Anliker, Purdue University

Abstract

Nickel(II) forms bis complexes with Aib$\sb2$ (the dipeptide of $\alpha$-aminoisobutyric acid) that change from a high-spin form, Ni$\sp{\rm II}$(H$\sb{-1}$Aib$\sb2$)(Aib$\sb2$)$\sp-$, to a low-spin form, Ni$\sp{\rm II}$(H$\sb{-1}$Aib$\sb2$)$\sb2\sp{2-}$, with loss of a proton (pK$\sb{\rm a}$ 10.1 at 25.0$\sp\circ$C) and the formation of a second N(peptide)-Ni(II) bond. Other dipeptides form tetragonally-compressed 6-coordinate bis-complexes, but change of spin with pH is observed only when Aib is in the second residue. This is attributed to the electron-donating effect of the $\alpha$-carbon methyl groups on the ligand field strength rather than to a change in coordination number. The low-spin six-coordinate structure of Ni$\sp{\rm II}$(H$\sb{-1}$Aib$\sb2$)$\sb2\sp{2-}$ is unusual for nickel(II) and unprecedented for nickel(II) peptide complexes. Oxidation of Ni$\sp{\rm II}$(H$\sb{-1}$Aib$\sb2$)$\sb2\sp{2-}$ gives a tetragonally-compressed Ni$\sp{\rm III}$(H$\sb{-1}$Aib$\sb2$)$\sb2\sp-$ complex that is very stable in neutral and basic solutions. The standard reduction potential for the Ni(III,II) couple is only 0.34 V (vs. NHE). A self-exchange rate constant of 48 M$\sp{-1}$ s$\sp{-1}$ (25.0$\sp\circ$C, I = 0.1) is determined for Ni$\sp{\rm III,II}$(H$\sb{-1}$Aib$\sb2$)$\sb2\sp{-,2-}$. This is 500 times larger than values found for outer-sphere electron-transfer reactions of Ni(III,II)tripeptide complexes. Ni$\sp{\rm III}$(H$\sb{-1}$Aib$\sb2$)$\sb2\sp-$ is reduced to the low-spin Ni$\sp{\rm II}$(H$\sb{-1}$Aib$\sb2$)$\sb2\sp{2-}$ complex by hydroperoxyl anion, HO$\sb2\sp-$ (k = 4.26 M$\sp{-1}$ s$\sp{-1}$). Acid reacts with Ni$\sp{\rm III}$(H$\sb{-1}$Aib$\sb2$)$\sb2\sp-$ in a three-step process to give two other Ni(III) complexes before decomposition to Ni(II) occurs. The last reaction (k$\sb{\rm obsd}$ = 0.002 s$\sp{-1}$) is independent of pH and is 100 times slower than the corresponding reaction of the bis(diglycinato)nickel(III) complex. Nickel(III) forms a complex with M$\sb3$ (the tripeptide of L-methionine) with the stoichiometry: Ni$\sp{\rm III}$(H$\sb{-2}$M$\sb3$). The EPR spectrum of Ni$\sp{\rm III}$(H$\sb{-2}$M$\sb3$) has g$\perp$ = 2.180, which is unusually low for Ni(III) tripeptides. This is attributed to axial bonding of a thioether sulfur. The acid decomposition behavior of Ni$\sp{\rm III}$(H$\sb{-1}$GM)$\sb2\sp-$, where GM is glycyl-L-methionine, also implicates nickel(III) bonding to thioether sulfurs. Ni(II) forms square planar complexes with cysteine methyl ester (COMe), glycyl-L-cysteine methyl ester (GCOMe) and N-mercaptoacetyl-glycylglycine that contain Ni$\sp{\rm II}$-S(sulfhydryl) bonds. The Ni(III) complexes of these ligands are not isolable, but the redox potential for Ni$\sp{\rm III,II}$-COMe is estimated to be 0.53 V (vs. NHE).

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Analytical chemistry

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