Structure and conformation of peptides involving prolyl residue. IV. L-Prolyl-L-leucine monohydrate

The dipeptide, L-prolyl-L-leucine monohydrate (C11H20N2O3.H2O, molecular weight, 246.3) crystallizes in the monoclinic space group P2(1), with cell constants: a = 6.492(2)A, b = 5.417(8)A, c = 20.491(5)A, beta = 96.59(2) degrees, Z = 2, Do = 1.15 g/cm3, and Dc = 1.142 g/cm3. The structure was solved by SHELX-86 and refined by full matrix least squares methods to a final R-factor of 0.081 for 660 unique reflections (I greater than 2 sigma (I)) measured on an Enraf Nonius CAD-4 diffractometer (CuK alpha, lambda = 1.5418 A, T = 293 K). The peptide linkage exists in the trans conformation. The pyrrolidine ring exists in the envelope conformation. The values of the sidechain torsion angles are: chi 1 = -59.3(13) degrees, chi 21 = -63.1(16) degrees and chi 22 = 174.8(15) degrees for leucine (C-terminal). The crystal structure is stabilised by a three-dimensional network of N-H ... O, O-H ... O, and C-H ... O hydrogen bonds.