Abstract: | Conformations of the cyclic tetrapeptide cyclo(L -Pro-Sar)2 in solution were studied by 1H- and 13C-nmr spectrometry and model building. The nmr data provide definite evidence that this cyclic peptide exists chiefly in two conformations, namely, a C2-symmetric conformation and an asymmetric structure. The former was demonstrated to be predominant in polar solvents (100% in Me2SO-d6). This structure contains all cis-peptide bond linkages and all trans′ Pro Cα?CO bonds. It represents the first cyclic tetrapeptide in which all four peptide bonds have been found in the cis-conformation. As the polarity of the solvent decreases, the population of C2-symmetric conformers decreases (88% in CD3CN and 65% in CDCl3). At the same time, a minor asymmetric conformer, characterized by cis-cis-cis-trans peptide bond sequences (two cis Sar-Pro bonds, one cis Pro-Sar bond, and one trans Pro-Sar bond), is seen to increase (9% in CD3CN and 30% in CDCl3). A proposed predominant conformation in solution for cyclo(L -Pro-Sar)2 was compared with a crystal structure, as reported in an accompanying paper. Both structures show striking overall similarities. |