CONFORMATIONAL STUDIES OF CYCLO(L-Phe-L-Pro-Gly-L-Pro)₂ BY ¹³C NUCLEAR MAGNETIC RESONANCE

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The ¹³C-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-One·HCl to the solution of cyclopeptide (A) in CDCl₃ yielded ¹³C-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-One·HCl. The ¹³C-NMR spectrum of complexes of A with DL-Phe-OMe·HCl displayed separete resonances²⁾ for C_γ, C_o, ¹⁾ Cm, ¹⁾ C_α, and C_β of D-Phe-OMe·HCl and L-Phe-OMe·HCl (Table I).