X-RAY STRUCTURES OF AZA-PROLINE-CONTAINING PEPTIDES

The aza-analogue of proline (AzPro) contains a nitrogen atom in place of the CHalpha of the cognate residue. The resolution of the crystal s tructures of seven AzPro-containing peptides, presenting a set of ten AzPro motifs, reveals the structural properties of this particular aza -residue. Because of sterical hindrances, both nitrogen atoms are out of planarity, and the reduced electronic conjugation in the two AzPro- adjacent amide groups probably explains the longer amide bond distance s and the weak proton-accepting character of the two pyrazolidine nitr ogens. The absolute configuration of both AzPro nitrogens depends on t he chemical nature of the sequence. In all cases, the AzPro residue as sumes the same intrinsic three-dimensional structure and presents fold ing tendencies opposed to those induced by proline. (C) Munksgaard 199 7.