1H‐ and 13C‐NMR. Studies of the molecular conformations of cyclo‐tetraglycyl [1]

The 1H‐ and 13C‐NMR. spectra of cyclo‐tetraglycyl show that the four peptide groups are magnetically equivalent, and different from either a standard trans or a standard cis peptide group. It is suggested that the observed NMR. features correspond to a non‐planar form of the peptide groups. On the one hand these data confirm the earlier conclusions from theoretical investigations of the molecular geometry, that cyclic tetrapeptides could not contain four standard trans peptide groups. On the other hand they are not consistent with a previously suggested alternative molecular conformation according to which cyclo‐tetraglycyl would adopt a conformation similar to cyclo‐tetrasarcosyl, with two cis and two trans peptide bonds. The different behaviour of glycine and sarcosine under the steric strains of tetrapeptide ring closure would appear to suggest that with the exception of the X‐Pro bonds, transoid peptide groups in polypeptide chains of the common amino acids should be more likely to occur than the cis form, which has as yet apparently not been observed for N‐unsubstituted peptide groups in natural peptides or proteins.