Conformational studies of retro-inverso peptides: the crystal and molecular structure of the hydantoin from H-Ala-g-Ala-mGly-OBzl.

The crystal structure of the hydantoin 1-[(S)-1'-aminoethylmalonyl benzyl ester]-(S)-4-methylimidazolidin-2,5-dione (1) derived from the peptide H-Ala-gAla-mGly-OBzl, having the retro-inverso modification of the Ala-Gly bond, has been determined by x-ray diffraction analysis. The crystals are orthorhombic, space group P2(1)2(1)2(1) with a = 6.539, b = 14.721, c = 17.101 A, z = 4. The structure was solved by direct methods and refined with anisotropic thermal factors to a final R value of 0.067 for the 947 observed reflections. Reversal of the Ala-Gly amide bond perturbs the folding tendency of the backbone shown by the parent peptide t-BuCO-Ala-Gly-NHiPr. The gem-diamino residue, gAla, and the malonyl moieties are found in the helical and the extended conformations, respectively. Intramolecular hydrogen bonding is not observed. The molecules in the crystal are held together by the formation of two intermolecular hydrogen bonds of the N-H ... O=C type with N ... O distances of 2.86 and 3.17 A, respectively.