NMR studies on the conformation of derivatives of the side chain of oxytocin: examples of cis-trans isomerism.

The 220 MHz spectra reported in this paper show the existence of cis-trans isomerism about the Cys-Pro bond in (S-benzyl)-L-Cys-L-Pro-L-Leu-Gly(NH(2)) and about the Z-Pro bond in (N-benzyloxycarbonyl)-L-Pro-L-Leu-Gly(NH(2)). These peptides are derivatives of the side chain of oxytocin, which has the structure L-Pro-L-Leu-Gly(NH(2)). Taken in water-free (CD(3))(2)SO, the spectra also show differences between the two isomers in the amide chemical shifts, which indicate interaction of the terminal end of the peptides with the rest of the residues. The ratio of the isomeric forms is about 2:3 for the S-benzylcysteinyl peptide, while it is 1:1 for the N-benzyloxycarbonyl-protected peptide. The probable assignment of peaks in isomers is discussed, in addition to routine spectral assignments based on extensive decoupling experiments.