(Z)-Aib-Gly-L-Ile-L-Leu-OMe.H2O, C27H42N4O7.H2O, is a protected analogue of the C-terminal sequence of the membrane-active peptaibol antibiotic trichogin A IV. The peptide backbone is folded. The urethane carbonyl O atom acts as the acceptor of two intramolecular hydrogen bonds which give rise to a beta bend and to an alpha bend. The geometry of the latter is significantly distorted from that observed in alpha helices. This structure represents the first observation of an alpha bend in a protected tetrapeptide sequence.