1985 Volume 58 Issue 12 Pages 3436-3443
Complex formation of synthetic cyclic octapeptide, cyclo[Gly–L-Lys(Z)–Sar–L-Pro]2, with alkali and alkaline earth metal ion was investigated by CD and NMR spectroscopy. Titration curves obtained from CD data revealed three kinds of CGLSP2(P)/cation(C) complexes. P2C (“peptide sandwich”), PC (“1:1”), and PC2 (“cation sandwich”) complexes occurred in the complexation between CGLSP2 and Ba2+. In the presence of Li+, Mg2+, or Ca2+, the existence of PC and PC2 was assumed. K+ and Na+ form only a PC species with CGLSP2. By analyzing the titration curves, 1 :1 complex-formation constants (K1) were evaluated. The K1 values decrease in the order Ba2+>Ca2+>Mg2+>Li+>>Na+, K+. 1H- and 13C-NMR data showed that CGLSP2 in acetonitrile exists in at least five different conformational states in a free state. A predominant conformer had a C2-symmetric structure containing two cis Lys–Sar peptide bonds. After the addition of equimolar amounts of Ba(ClO4)2, these conformations converged into a single C2-symmetric one with all-trans peptide bonds. This corresponds to a PC species. On further addition of the salt, CGLSP2 changed the conformation into an asymmetric structure with one cis Lys–Sar peptide bond. This corresponds to a PC2 species.
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