A new cyclolinopeptide containing nonproteinaceous amino acid N-methyl-4-aminoproline

Summary We have found that besides the known cyclolinopeptides A (CLA) and B (CLB), there is a new cyclic peptide in linseed mill cake that we have named CLX. Its composition is very similar to that of CLA, a cyclic peptide with a distinct immunosuppressive activity. The sequence of this peptide has been established ascyclo(PPFFILLX), where X is a non-proteinaceous amino acid,N-methyl-4-aminoproline. this amino acid substitutes for two amino acid residues of CLA, mimicking a dipeptide moiety with a nonplanarcis amide bond. The non-proteinaceous amino acid X may mimic a transition state of the peptide bond which exists in such processes as, e.g., PPIase-catalysedcis/trans amide-Pro bond isomerisation.