This paper describes the chemical synthesis and crystal molecular conformation of a non-chiral beta-Ala containing model peptide Boc-beta-Ala-Acc(5)-OCH3. The analysis revealed the existence of two crystallographically independent molecules A and B, in the asymmetric unit. Unexpectedly, while the magnitudes of the backbone torsion angles in both molecules are remarkably similar, the signs of the corresponding torsion angles are reverse therefore, inclining us to suggest the existence of non-superimposable stereogeometrical features in a non-chiral one-component beta-Ala model system. The critical mu torsion angle around (CH2)-H-beta-(CH2)-H-alpha bond of the beta-Ala residue represents a typical gauche orientation i.e., mu = 67.7degrees in A and mu = -61.2degrees in B, providing the molecule an overall crescent shaped topology. The observed conformation contrasts markedly to those determined for the correlated non-chiral model peptides: Boc-beta-Ala-Acc(6)-OCH3 and Boc-beta-Ala-Aib-OCH3 signifying the role of stereocontrolling elements since the stereochemically constrained C-alpha,C-alpha-disubstituted glycyl residues (e.g., Acc(5), Acc(6), and the prototype Aib) are known to strongly restrict the peptide backbone conformations in the 3(10)/alpha-helical-regions (phi approximate to +/-60 +/-20degrees, Psi approximate to +/- 30 +/-20degrees) of the Ramachandran map. Unpredictably, the preferred, phi, Psi torsion angles of the Acc(5) residue fall outside the helical regions of the Ramachandran map and exhibit opposite-handed twists for A and B. The implications of the semi-extended conformation of the Acc(5) residue in the construction of backbone-modified nouel scaffolds and peptides of biological relevance are highlighted. Taken together, the results indicate that in short linear beta-Ala containing peptides specific structural changes can be induced by selective substitution of non-coded linear- or cyclic symmetrically C-alpha,C-alpha-disubstituted glycines, reinstating the hypothesis that in addition to conformational restrictions, the chemical nature of the neighboring side-chain substituents and local environments collectively influences the stabilization of folding-unfolding behavior of the two methylene units of a beta-Ala residue. (C) 2004 Published by Elsevier Inc.