Induced conformational preferences in a non-chiral β-Ala residue:: X-ray diffraction, 1H NMR, FT-IR and CD studies of Boc-β-Ala-D-Ala-NHCH3 and Boc-β-Ala-L-Ala-NHCH3

In order to demonstrate the conformational features that can be induced by a chiral residue in a beta-Ala moiety, X-ray diffraction structure of Boc-beta-Ala-D-Ala-NHCH3 1 have been determined and compared with Boc-beta-Ala-L-Ala-NHCH3 2. An analysis of the crystal molecular conformations reveals the establishment of non-superimposable stereogeometrical features across beta-Ala residues. The supramolecular assembly of the highly ordered L-shaped molecules, generated by the parallel arrangements of the peptide molecules, is stabilised by a network of intermolecular H-bonds between the CO and NH groups. The chiroptical investigations, in solvents of varying polarities, provide experimental evidence which strongly supports that the stereochemical preferences of the beta-Ala residue can restrict significant conformational averaging. (C) 1999 Elsevier Science Ltd. All rights reserved.