cis-helix. The differing strength of the bonding between solvent and peptide carbonyl groups could be determined from small differences in the T values of the C,-H resonances (CF,COOD: 5.10ppm; CD,COOD: 5.19ppm; DzO: 5.24ppm; CF3CD20D: 5.26ppm; C D 3 0 D : 5.31 ppm). In CD,COOD and CF3COOD all oligopeptides have the trans-configuration. We found that there is a distinct dependence of the configuration on chain length in the other solvents, di-and triprolines behaving differently from the higher oligoprolines: in D 2 0 , di-and triproline are in cis-trans equilibrium; from n=4 onwards only trans peptide bonds are formed. In CD,OD, diproline is completely and triproline extensively cis configurated, respectively. The proportion of cis bonds (T= 5.50ppm) passes through a minimum at n = 4 and increases rapidly with increasing chain length; the all-cis configuration is reached at ri>10. The reverse behavior was observed in CF3CD20D: here diproline is likewise completely cis configurated; in the oligomers with r1=3 to 10, both cis and trans peptide bonds occur. From n = 12 onwards we found exclusively trans peptide bonds. In the case of C-and N-protected oligoprolines, however, apart from in the first disordered four members, only all-trans peptide bonds a r e foundL4]. The ability of di-and triprolines to form cis peptide bonds is consistent with the pronounced tendency to form cyclotriprolyls and, particularly, cyclodipr~lyls[~~.
While only the signal .of the cis-or trans-isomer appears in the range of C, protons in the 'H-NMR spectra of the polyproline and the oligomers with n > 15, the spectra of the lower oligomers coptain additional resonances which arise from the N-and C-terminal proline residues. The T values of the C-terminal proline residue depend on the position of the dissociation equilibrium of the carboxyl group, and hence on the solvent (CF3COOD: 5.1 ppm; CD,COOD: 5.4 ppm; D 2 0 , CF3CD20D, C D 3 0 D : 5.7 ppm).