STERIC EFFECTS OF CIS-TRANS ISOMERISM ON NEIGHBORING RESIDUES IN PROLINE OLIGOPEPTIDES - C-13-NMR STUDY OF CONFORMATIONAL HETEROGENEITY IN LINEAR TRIPEPTIDES

A series of proline-containing linear oligopeptides (4 dipeptides and 15 tripeptides) were synthesized and examined in aqueous and nonaqueous solutions using 13C-NMR spectroscopy. Spectra of linear tripeptides showing cis-trans isomerism about the X-Pro bond (X = Pro, Gly and Ala) also show neighboring effects on the chemical shifts of residues preceding and following the prolyl moiety. The extent of cis-trans isomerism observed about the X-Pro peptide bond correlates not only with the nature of X, but also depends on the size of the residue following proline; the larger substituents favor an increase in cis content about the X-Pro bond.