[Carbon 13 nuclear magnetic resonance conformation studies of peptides with analgesic activity]

13C Nuclear magnetic resonance studies of proline peptides with central antalgic activity (Pro-Arg, Arg-Pro, Pro-Arg-Pro, Pro-Lys-Pro) indicate a predominance of trans conformation at all pH values; but their physiological activities appear to be related to the ability of the C-terminal proline peptides to assume the cis conformation.