The palladium(II) promoted hydrolysis of the methyl esters of glycyl-L-leucine, glycyl-L-alanine and L-alanylglycine

Abstract

The palladium(II)-promoted hydrolysis of the methyl esters of glycyl-L-leucine, glycyl-L-alanine and L-alanylglycine have been studied at 25 °C and I=0.1 M in the pH range 4–5. At a 1:1 metal to ligand ratio the peptide esters act as tridentate ligands, donation occurring via the terminal amino group, the deprotonated amide nitrogen, and the carbonyl group of the ester. Due to the high Lewis acidity of Pd(II) rapid hydrolysis of the ester function by water and hydroxide ion occurs. Rate constants k OH and k H 2O have been obtained for base hydrolysis and water hydrolysis of the coordinated peptide esters at 25 °C. The rate constants for base hydrolysis are 3.4 X 10 6 M −1 s −1 (L-alaglyOMe), 6.4 X 10 6 M −1 s −1 (gly-L-alaOMe) and 2.3 X 10 7 M −1 s −1 (gly-L-leuOMe). Base hydrolysis of the coordinated peptide esters is at least 10 6 times that of the free unprotonated ligand. Activation parameters have been obtained for both water and base hydrolysis of the Pd(II) complex of methyl L-alanylglycinate and possible mechanisms for the hydrolyses are considered.