Proton-transfer reactions of copper(II)- and nickel(II) tetrapeptide complexes with bulky α-carbon substituents

Abstract

Protonation of copper(II)- and nickel(II) tetrapeptide complexes with bulky α-carbon substituents has been studied. The pK_a-values for the second and terminal metal–N(peptide) bond formation have been determined spectrophotometrically. More than one deprotonated peptide nitrogen exists in a complex but the individual protonation steps for the different Cu^II–N(tetrapeptide) positions could not be accounted for by the variation of the hydrogen ion concentration in the same experiment, as the protonation of the different metal–N(tetrapeptide) positions takes place at different wavelengths. For the Ni^II-tetra-alanine complex, the proton transfer to the terminal and second deprotonated peptide nitrogens have been detected by varying the hydrogen ion at the same wavelength. The proton transfer to the terminal metal–N(tetrapeptide) of Cu^II- and Ni^II-tetrapeptide complexes show first order kinetics with respect to the hydrogen ion and the peptide complex concentration and proceed via an outside protonation pathway. The relatively high pK_a values for the terminal deprotonated peptide nitrogen indicate the instability of this metal–N(peptide) bond due to strain in the chelate ring because of atom overcrowding of the bulky α-carbon substituents.