Copper(II)-tripeptide complexes in aqueous solution. Effects of the C-terminal chelate ring size on the coordination structure of doubly deprotonated complex species

Stabilities and coordination modes of the Cu(TI) complexes of tripeptides, glycylglycyl-X (GlyGly-X), where X denotes Gly, beta-alaninate, gamma-aminobutyrate, delta-aminovalerate, epsilon-aminocaproate, and sarcosinate, have been studied by pH-titration and electronic absorption and electron spin resonance (ESR) spectroscopies. On the basis of the stability constants and spectral data, the structure-stability relationship was determined, and the complexation pathways have been proposed. The stability constants determined at 25 degrees C and I = 0.1 M (KNO3) indicated that Cu(II) reacted with tripeptides (L) to form CuL and deprotonated species CuH-1L, CuH-2L, and CuH-3L (charges are omitted). The stability constants for CuH-2L were found to be primarily in the order of the deprotonation from the C-terminal peptide bond and thus to be determined by the accessibility of the carboxylate group to Cu(II). The coordination mode of CuH-2L depended on the size of the chelate ring formed by X with different lengths of the chain -NH(CH2)(n)COO-. CuH-2L with n less than or equal to 3 formed 5-5-5- - 5-5-7-membered fused rings with an amino nitrogen, two deprotonated peptide nitrogens, and a carboxylate oxygen, exhibiting the d-d transition band centered at 550-560 nm. The same species with n greater than or equal to 3 showed the absorption peak at similar to 650 nm and was concluded to be Cu(H-1L) (OH-) having a 5-5-membered fused ring with an amino nitrogen, a deprotonated peptide nitrogen, a peptide carbonyl oxygen, and a hydroxide ion as donors. The structures for CuH-3L were also discussed. (C) 1999 Elsevier Science Inc. All rights reserved.