N-Methyl peptides. IV. Water and beta-turn in peptides. Crystal structure of N-pivaloyl-L-prolyl-N,N'-dimethyl-D-alaninamide in the anhydrous and monohydrated states.

The model tripeptide tBuCO-L-Pro-Me-D-Ala-MHMe crystallizes in both anhydrous (1) and monohydrated (2) states: 1, monoclinic space group C2 with a = 20.030 (2) A, b = 5.836 (2) A, c = 14.958 (3) A and beta = 94.11 (1) degrees; 2, orthorhombic space group P212121 with a = 6.971 (6) A, b = 11.766 (3) A, and c = 22.394 (8) A. Both crystal structures were solved by X-ray diffraction in order to characterize the influence of water on the molecular structure. The anhydrous molecule accommodates the well-known, beta II-folded conformation with three trans amide functions and an intramolecular i + 3 leads to hydrogen bond. In the hydrated molecule, water is inserted in a loop containing 12 atoms and induces some conformational changes of the peptide backbone.