A CRYSTAL-STRUCTURE WITH FEATURES OF AN ANTIPARALLEL ALPHA-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 tripepti de molecules have in part an alpha-extended conformation, the torsion angles of the L-Ala and D-aIle residues being phi(1) = -75.1 degrees a nd psi(1) = -25.8 degrees and phi(2) = 67.3 degrees and psi(2) = 44.1 degrees, respectively, and (b) that the molecules are organized in rip pled planes where they occur in relative antiparallel orientation link ed together side by side by H bonds. This molecular organization of th e tripeptide corresponds closely to that of an antiparallel alpha-plea ted sheet, and likely constitutes the first example of a structure of this kind for which a characterization at the atomic level has been ac hieved. A molecular dynamics study has shown that the molecular confor mation of the tripeptide in the crystalline state is determined primar ily by intermolecular interactions. (C) 1994 John Wiley and Sons, Inc.