Abstract: | The crystal structure of Boc-Pro-Val-Gly-NH2 has been determined: monoclinic; P21; a = 9.331 (3) Å, b = 9.532 (4), c = 23.080 (9), β= 91.33 (3)R, Z = 4; R = 0.053 for 3400 reflections with ˙Fo˙,>α(Fo). There are two independent but very similar molecules in the crystal. The peptide main chains are in an extended form, and packed in two kinds of antiparallel β sheets, the (φ, Φ) angles of the central Val residues are (-156°, 146°) and (-139°, 155°), and the mean length of the N- H . 0 hydrogen bonds in the sheets is 2.965 Å. A detailed study of the conformations of the Val residues in oligopeptide crystals shows that the preferred conformation of Val in peptides is: the (φ, Φ) angles close to those of the antiparallel β sheet, and Cγ1 and Cγ2, against N with respect to the Cα– Cβ bond, at either (trans, gauche) or (-gauche, gauche). The mean π(NCαC') angle of such Val residues is 107.9(9)°. A twisting in the β sheets is also discussed. |