13C‐ and 1H‐nmr studies of cis‐trans conformers of oligoprolines

In aqueous solutions, 13C‐ and 1H‐nmr studies show that the percentage of trans conformation of proline oligomers +H2H Pro‐(Pro)n‐CO increases substantially from n = 1 (65% trans) to n = 2 (90% trans). The relatively low percentage of trans structure for the dimer (n = 1) very likely is caused by the extra stability acquired by the end‐to‐end intramolecular H‐bonding of the cis dimer. As n increase from 2 to 3 (or 5) in +H2N‐Pro‐(Pro)n‐CO , the percentage of trans conformation stays more or less constant (∼0.9). A high salt concentration (4M CaCl2) causes a conformation randomization, so that the short‐chain oligomer (n = 1, 3, 5) and the long‐chain poly (L‐proline) all show about the same frantion of trans conformation (0.7‐0.8).