Interaction of zinc(II) with the cyclic octapeptides, cyclo[Ile(Oxn)-D-Val(Thz)]2 and ascidiacyclamide, a cyclic peptide from Lissoclinum patella

The interactions between zinc salts and the naturally occurring cyclic octapeptide ascidiacyclamide in methanol, as well as a synthetic analogue cyclo[Ile(Oxn)-D-Val(Thz)]2, were monitored by 1H NMR and CD spectroscopy. Three zinc complexes were identified, their relative amounts depending on the nature of the anion (perchlorate, triflate or chloride) and the presence or absence of base. Binding constants for two of the zinc species were calculated from CD or 1H NMR spectra, [Zn(L – H)]+ (KZn(L–H) = [Zn(L – H)+]/[Zn2+][(L – H)–] = 107 ± 2 M–1; 95% methanol/5% water, 298.0 K, NEt3/HClO4 buffer 0.04 M) and [ZnLCl]+ (KZnClL = [ZnClL+]/[Zn2+][Cl–][L] = 107.2 ± 0.1 M–2; d3-methanol, 301 K).