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-NHCH 3 and Boc-β-Ala-L-Ala-NHCH 3

Abstract

In order to demonstrate the conformational features that can be induced by a chiral residue in a β-Ala moiety, X-ray diffraction structure of Boc-β-Ala-D-Ala-NHCH 3 1 have been determined and compared with Boc-β-Ala-L-Ala-NHCH 3 2. An analysis of the crystal molecular conformations reveals the establishment of non-superimposable stereogeometrical features across β-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 β-Ala residue can restrict significant conformational averaging.