Stereochemical behavior of acyclic peptide-cation complexes.

The role of the beta-turns in the peptide interaction with several cations was investigated. In this work we report the solution studies of four linear peptides: Z-Aib-Aib-Aib-L-Val-OMe, Boc-D-aIle-L-Ile-D-aIle-L-Ile-OMe, Boc-L-Leu-L-Leu-L-Leu-L-Leu-OMe, and Boc-L-Phe-D-Phe-L-Phe-D-Phe-OMe. CD and 1H-nmr spectra reveal the presence of multiple ion-bonding equilibria. The stoichiometry and binding constant of the four peptides in the presence of Ca2+ ions in acetonitrile solution has been determined. Variable-temperature nmr spectra in the absence and in the presence of a large excess of cation have shown that the complexation process is not critically dependent on the conformation of the free peptide.