Conformational Preferences of Heterochiral Peptides. Crystal Structures of Heterochiral Peptides Boc-(D) Val-(D) Ala-Leu-Ala-OMe and Boc-Val-Ala-Leu-(D) Ala-OMe-Enhanced Stability of β-sheet Through C-H…O Hydrogen Bonds

Abstract The crystal structures of Boc-(D) Val-(D) Ala-Leu-Ala-OMe (vaLA) and Boc-Val-Ala-Leu-(D) Ala-OMe (VALa) have been determined. vaLA crystallises in space group P212121 with a = 9.401 (4), b = 17.253 (5), c = 36.276 (9)A, V = 5884 (3) A3, Z = 8, R = 0.086. VALa crystallises in space group P21 with a = 9.683 (9), b = 17.355 (7), c = 18.187 (9) A, β = 95.84 (8)°, V = 3040(4) A3, Z = 4, R = 0.125. There are two molecules in the asymmetric unit in antiparallel β-sheet arrangement in both the structures. Several of the Cα hydrogens are in hydrogen bonding contact with the carbonyl oxygen in the adjacent strand. An analysis of the observed conformational feature of D-chiral amino acid residues in oligopeptides, using coordinates of 123 crystal structures selected from the 1998 release of CSD has been carried out. This shows that all the residues except D-isoleucine prefer both extended and αL conformation though the frequence of occurence may not be equal. In addition to this, D-leucine, valine, proline ...