Preparation and Complex Formation of Cyclic Octapeptides Containing L-Proline Residues with Phenylalanine Methylester Hydrochloride

Abstract

Cyclic octapeptides, cyclo[L-Tyr(Bzl)-L-Pro]₄ (2) and cyclo(L-Tyr-L-Pro)₄ (3), were prepared and their conformations investigated. A C₂-symmetric conformation containing two cis peptide bonds was found in 2 in CDCl₃ and 3 in CD₃OD. Cyclo(L-Phe-L-Pro)₄ (1) formed a 1 : 1 complex with L-phenylalanine methylester hydrochloride (L-PheOMe·HCl). The ¹³C-NMR spectra of the complexes of 1 and 2 with 1 eq of D- and L-PheOMe·HCl (D/L ratio=1/2) in CDCl₃ displayed separate resonances for several carbon atoms of D-PheOMe·HCl and L-PheOMe·HCl owing to the formation of diastereomeric pairs of the complexes.