Stability of Fused Rings in Metal Chelates. XI. Stability of Dipeptide Amide-Copper(II) Complexes

Abstract The structure-stability relationship between the copper(II) complexes of several dipeptide amides consisting of glycine and/or β-alanine has been investigated by potentiometric titration. The amides of glycylglycine, glycyl-β-alanine, and β-alanylglycine (abbreviated as H2L) have been found to form complexes of the type Cu(H2L)2+ with logK1 values 4.80–5.22. The −logKC1 values for deprotonation of the peptide group are 5.05–5.42 and comparable to those of tripeptides, whereas the −logKC2 values for the terminal amide group are 7.96 for glycylglycine amide and 8.99 for glycyl-β-alanine amide. This indicates that, at neutral pH, the –CONH2 group coordinates to copper(II) mainly through the carbonyl oxygen. β-Alanyl-β-alanine amide-copper(II) system gave precipitates at an early stage of titration. The structures and relative stability of the complexes of types Cu(H2L)2+, Cu(HL)+, and CuL have been discussed from comparative studies of their stability constants.