A crystal structure with features of an antiparallel α‐pleated sheet

A single‐crystal x‐ray diffraction analysis of Boc‐ L ‐Ala‐ D ‐aIle‐ L ‐Ile‐OMe has been carried out. The analysis has shown (a) that the tripeptide molecules have in part an α‐extended conformation, the torsion angles of the L ‐Ala and D ‐aIle residues being φ 1 = −75.1° and ψ 1 = −25.8° and φ 2 = 67.3° and ψ 2 = 44.1°, respectively, and (b) that the molecules are organized in rippled planes where they occur in relative antiparallel orientation linked together side by side by H bonds. This molecular organization of the tripeptide corresponds closely to that of an antiparallel α‐pleated sheet, and likely constitutes the first example of a structure of this kind for which a characterization at the atomic level has been achieved. A molecular dynamics study has shown that the molecular conformation of the tripeptide in the crystalline state is determined primarily by intermolecular interactions. © 1994 John Wiley & Sons, Inc.