OBSERVATION OF A STERICALLY UNFAVORABLE SIDE-CHAIN CONFORMATION IN A LEUCYL RESIDUE - CRYSTAL AND MOLECULAR-STRUCTURE OF L-LEUCYL-L-LEUCINE-CENTER-DOT-DMSO SOLVATE

The crystal structure of a dipeptide L-leucyl-L-leucine (C12H24N2O3) has been determined. The crystals are monoclinic, space group P2(1), with a = 5.434(4) Angstrom, b = 15.712(7) Angstrom, c = 11.275(2) Angstrom, beta = 100.41(1)degrees, and Z = 2. The crystals contain one molecule of dimethyl sulfoxide (DMSO) as solvent of crystallization for each dipeptide molecule. The structure has been solved by direct methods and refined to a final R index of 0.059 for 920 reflections (sin theta/lambda less than or equal to 0.60 Angstrom(-1)) with I greater than or equal to 2 sigma (I). The trans peptide unit shows substantial degree of non-planarity (Delta omega = 14 degrees). The peptide backbone adopts an extended conformation with torsion angles of psi(1) = 138(1)degrees, omega(1) = 166(1)degrees, phi(2) = -149.3(7)degrees, psi(21), = 164.2(7)degrees, and psi(22) = -15(1)degrees. For the first leucyl residue, the side-chain conformation is specified by the torsion angles (1) chi(1) = 176.7(7)degrees, (1) chi(22) = 62(1)degrees, (1) chi(22) = -177.4(8)degrees; the second leucyl residue adopts a sterically unfavorable conformation with (2) chi(1) = 61(1)degrees, (2) chi(21) = 97(1)degrees, and (2) chi(22) = -151(1)degrees. The packing involves head-to-tail interaction of peptide molecules and segregation of polar and nonpolar regions. The DMSO molecule is strongly hydrogen bonded to the terminal NH3+ group. (C) 1994 John Wiley & Sons, Inc.