Proline conformations in linear peptides. Structure determination of the methyl ester of N-benzyloxycarbonyl-L-prolyl-D-alanine (N-Z-L-Pro-D-Ala-OMe).

N-Z-L-Pro-D-Ala-OMe, C17H22N2O5, Mr = 334.38, crystallizes in the orthorhombic space group P2(1)2(1)2(1), with the cell dimensions a = 5.005 (5), b = 17.690 (9) and c = 18.70 (1) A3, V = 1656.3 A3, Z = 4, Dx = 1.341 g cm-3, Cu K alpha, lambda = 1.54184 A, mu = 7.830 cm-1, F(000) = 712, T = 198 K, final R (on F) = 0.036 for 1575 observed reflections with I greater than or equal to 3 sigma(I). The pyrrolidine ring takes on the C2-C gamma-endo conformation. The urethane bond is in the cis conformation [omega 0 = 6.0 (3) degrees] while the peptide bond is in the trans conformation [omega 1 = 170.8 (2) degrees]; phi 1/psi 1 values are -88.0 (3) degrees and 151.3 (2) degrees. Intermolecular hydrogen bonding occurs between the C-terminus and the symmetry-related amide. Systematic examination of the pyrrolidine ring in linear peptides reveals no correlation exists between the cis-trans orientation of the proline and the conformation of the pyrrolidine ring.