A study of the conformational states of cyclopeptide systems V. NMR spectra of cyclohexapeptides constructed from alanine and glycine residues, the spin-spin coupling constants of the NH-CH protons, and the "pleated-sheet" structure

Summary 1. On the basis of an analysis of the experimental and calculated data on peptide systems containing IMHBs of the 4 1 type, it has been shown that positions 2 and 5 in the preferred conformations of the cyclohexapeptides must be characterized by low values of the³J_NH-CH constants (1–5 Hz), and positions 3 and 6 by large values of these constants (7–10.5 Hz).2. The positions of the IMHBs in cyclohexapeptides including L-alanine and glycine residues have been refined, and the signals of the NH groups in the NMR spectra have been assigned to the individual amino acid residues.3. In the dominating conformations of the cyclohexapeptides, the lateral methyl groups of the alanine residues generally occupy the pseudoaxial orientation in positions 1, 3, 4, and 6 and the pseudoequatorial orientation in positions 2 and 5.For the preceding communication, see 1].M. M. Shemyakin Institute of the Chemistry of Natural Compounds of the Academy of Sciences of the USSR. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 339–346, May–June, 1971.