The crystal structure of pivaloyl-D-prolyl-L-prolyl-L-alanyl-N-methylamide
Pivaloyl-D-prolyl-L-prolyl-L-analyl-N-methylamide (I), C19H32N4O4, crystallizes in the orthorhombic space group P212121 with four molecules in a unit cell of dimensions a= 9.982 (1), b= 10.183 (3), c= 20.746 (2)Å. The structure has been refined to R 0.048 for 1 745 observed reflections. All the peptide bonds in the molecule are trans and both the proyl residues are in the Cγ-exo-conformation. The molecule assumes a highly folded conformation in which a Type II′DL bend is followed by a Type I LL bend, both stabilised by intramolecular 4 → 1 hydrogen bonds. This conformation, which has been observed for the first time, is of interest in relation to the structure of gramicidin S.