L‐Alanyl‐L‐alanyl‐L‐alanine: Parallel pleated sheet arrangement in unhydrated crystal structure, and comparisons with the antiparallel sheet structure

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Abstract
The tripeptide L‐alanyl‐L‐alanyl‐L‐alanine has been crystallized from a water/dimethyl‐formamide solution in an unhydrated form, with cell dimensions a = 11.849, b = 10.004, c = 9.862 Å, β = 101.30°, monoclinic space group P21, 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.