L-ALANYL-L-ALANYL-L-ALANINE - PARALLEL PLEATED SHEET ARRANGEMENT IN UNHYDRATED CRYSTAL-STRUCTURE, AND COMPARISONS WITH THE ANTIPARALLEL SHEET STRUCTURE

The tripeptide L-alanyl-L-alanyl-L-alanine has been crystallized from a water/dimethylformamide solution in an unhydrated form, with cell dimensions a = 11.849, b = 10.004, c = 9.862 angstrom, beta = 101.30-degrees, monoclinic space group P2(1), with 4 molecules per cell (2 independent molecules in the asymmetric unit). The structure was determined by direct methods and refined to a discrepancy index R = 0.057. The tri-L-alanine molecules are packed in a parallel pleated sheet arrangement with unusually long amide nitrogen-carbonyl oxygen contacts within sheets. Comparisons are made with the antiparallel pleated sheet structure of tri-L-alanine hemihydrate, previously crystallized from the same solvent system.