Conformationally restrained peptides: crystal structure of tert-butyloxycarbonyl-L-alanyl-D-alanyl-D-aminosuccinyl-glycyl-L- alanine methyl ester.

The solid-state structure of a heterochiral peptide embodying a D-aminosuccinyl peptide (D-Asu) and a D-Ala was studied in order to analyse the effects of Asu and amino acids with inverse chirality on peptide conformation. The crystal structure has been determined by X-ray diffraction techniques and refined to a final R factor of 0.043. The molecule adopts an unusual overall "S-shape' conformation due to two consecutive type II beta-turns. In this molecule it is possible to compare a type II beta-bend conformation (L-Ala1-D-Ala2) favoured by the presence of a D-residue at second corner to a type II beta-turn (D-Asu3-Gly4) favoured by the presence of a D-Asu residue at first corner. In agreement with previous studies, this structure confirms that the Asu has a high propensity to adopt a type II or II' beta-bend conformation and that it may be used as a strong determinant of these structural motifs.