CONFORMATIONAL STUDIES OF CYCLO(L-PHE-L-PRO-GLY-L-PRO)2 BY C-13 NUCLEAR-MAGNETIC-RESONANCE

The C-13-NMR spectrum (Fig. 2, 1) of cyclooctapeptide cyclo(L-Phe-L-Pro-Gly-L-Pro)2 (A) in CDCl3 suggested that its conformation involved the coexistence of two kinds of C2-symmetric conformation with trans-trans-trans-trans and cis-trans-trans-trans forms. Adding 0.5 equivalent of CsSCN or one equivalent of DL-Phe-OMe.HCl to the solution of cyclopeptide (A) in CDCl3 yielded C-13-NMR spectra (Fig. 2,2 and Table I) which suggested a single C2-symmetric conformation with trans-trans-trans-trans form, resulting from the formation of complexes with CsSCN or DL-Phe-OMe.HCl. The C-13-NMR spectrum of complexes of A with DL-Phe-OMe.HCl displayed separete resonances 2) for C-gamma, C(o),1) C(m),1) C-alpha, and C-beta of D-Phe-OMe.HCl and L-Phe-OMe.HCl (Table I).