SYNTHESIS, CRYSTAL-STRUCTURE, AND MOLECULAR-CONFORMATION OF N-AC-DEHYDRO-PHE-L-VAL-L-VAL-OCH3

The peptide N-Ac-dehydro-Phe-L-Val-L-Val-OCH3 (C22H31N3O5) was synthesized by the usual workup procedure and finally by coupling the N-Ac-dehydro-Phe-L-Val-OH to valine methyl ester. It was crystallized from its solution in acetonitrile-water mixture at 4-degrees-C. The crystals belong to the space group P1 with a = 8.900(3) angstrom, b = 11.135 (2) angstrom, c = 12.918(2) angstrom, alpha = 90.36(1)-degrees, beta = 110.14(3)-degrees, gamma = 90.10(3)-degrees, V = 1207.7 (6) angstrom, 3 Z = 2, d(m) 1.156 (5) Mgm-3, d(c) = 1.148(5) Mgm-3. The structure was determined by direct methods using SHELXS86. The structure was refined by full-matrix least-squares procedure to an R value of 0.077 for 3916 observed reflections. The molecular dimensions and conformations of the two crystallographically independent molecules are in good agreement. In the dehydro residues, the average C-alpha-C-beta distance is 1.31(2) angstrom whereas the bond angle C-alpha-C-beta-C-gamma is 132(1)-degrees. The average backbone torsion angles are omega-0 = 169(1)-degrees, phi-1 = -40(1)-degrees, psi-1 = -50(1)-degrees, omega-1 = -177(1)-degrees, phi-2 = 54(1)-degrees, psi-2 = 46(1)-degrees, omega-2 = -174(1)-degrees, phi-3 = 103(1)-degrees, psi-3T = -139(1)-degrees, and theta-3T = 176(1)-degrees. The acetyl group is in the trans conformation, while the backbone adopts a right-handed and left-handed helical conformation alternatingly. The two crystallographically independent molecules are held together by three hydrogen bonds: N21-H21-O-12' = 2.911(12) angstrom, N22-H22-O-11' = 2.941(12) angstrom and N-23-H23-O-11 = 3.005 (9) angstrom. The crystal structure is stabilized by van der Waals forces and three additional hydrogen bonds: N-11-H11-O22' = 2.896(15) angstrom, N-12-H12-O-21' = 2.947(14) angstrom and N-13-H13-O-21 = 2.987(15) angstrom.