title: Observation of a sterically unfavorable side-chain conformation in a leucyl residue: crystal and molecular structure of L-leucyl-L-leucine.DMSO solvate.
abstract: 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) A, b = 15.712(7) A, c = 11.275(2) A, 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 < or = 0.60 A-1) with I > or = 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 21 = 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.
authors.affiliation: Department of Crystallography and Biophysics, University of Madras, India.
authors.forename: S N
authors.initials: SN
authors.lastname: Mitra
authors.forename: E
authors.initials: E
authors.lastname: Subramanian
mesh_terms: D003460:Crystallization; D018360:Crystallography, X-Ray; D004121:Dimethyl Sulfoxide / Q000737:chemistry*; D004151:Dipeptides / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation; D012996:Solutions / Q000737:chemistry; D013237:Stereoisomerism
chemical_list: D004151:Dipeptides; D010455:Peptides; D012996:Solutions; C038361:leucylleucine; D004121:Dimethyl Sulfoxide