Copper(II) complexation by pituitary adenylate cyclase activating polypeptide fragments

Abstract

Results are reported from potentiometric and spectroscopic (UV–Vis, CD, and ESR) studies of the protonation constants and Cu 2+ complex stability constants of pituitary adenylate cyclase activating polypeptide fragments (HSDGI–NH 2, TDSYS–NH 2, RKQMAVKKYLAAVL–NH 2). With HSDGI–NH 2, the formation of a dimeric complex Cu 2H −2L 2 was found in the pH range 5–8, in which the coordination of copper(II) is glycylglycine-like, while the fourth coordination site is occupied by the imidazole N 3 nitrogen atom, forming a bridge between two copper(II) ions. The formation of dimeric species does not prevent the deprotonation and coordination of the amide nitrogen, and in pH above 8 the CuH −2L complex is formed. Aspartic acid in the third position of peptide sequence stabilizes the CuH −2L species and prevents the coordination of a fourth nitrogen donor. Aspartic acid residue in the second position of TDSYS–NH 2 stabilizes the CuL (2N) complex but does not prevent deprotonation and binding of the second and third peptide nitrogens to give 3N and 4N complexes at higher pH. The tetradecapeptide amide forms with copper(II) ions unusually stable 3N and 4N complexes compared to pentaalanine amide.