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) A, b = 11.135(2) A, c = 12.918(2) A, alpha = 90.36(1) degrees, beta = 110.14(3) 14(3) degrees, V = 1207.7(6) A, 3Z = 2, dm = 1.156(5) Mgm-3, dc = 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) A 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) degree, psi 1 = -50(1) degree, omega 1 = -177(1) degree, phi 2 = 54(1) degree, psi 2 = 46(1) degree, omega 2 = -174(1) degree, phi 3 = 103(1) degree, psi T3 = -139(1) degree, and theta T3 = -176(1) degree. The acetyl group is in the trans conformation, while the backbone adopts a right-handed and left-handed helical conformation alternatingly.(ABSTRACT TRUNCATED AT 250 WORDS)