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 φ1 = −75.1° and ψ1 = −25.8° and φ2 = 67.3° and ψ2 = 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. Copyright © 1994 John Wiley & Sons, Inc.