X-ray crystal structure and conformation of N-(tertbutyloxycarbonyl)-L-methionyl-(1-aminocyclopent-3-ene-1-carbonyl)-L-phenylalanine methyl ester (Boc(0)-Met(1)-Cpg(2)-Phe(3)-OMe)

摘要

Molecular structure and conformation in the crystal of N-Boc-Met-Cpg-Phe-OMe, a conformationally restricted tripeptide derivative, structurally related to the chemotactic antagonist agent N-Boc-Met-Leu-Phe-OMe is reported. In the title compound the native central Leu residue has been replaced with the unusual cyclic and achiral quaternary amino acid residue of the 1-aminocyclopent-3-ene-1-carboxylic acid (Cpg). The peptide backbone adopts a type II beta-turn (C-10) conformation at the -Met-Cp- sequence with a distance, between the terminal pseudo-ring atoms O-6 center dot center dot center dot C-3, of 5.49(1)angstrom. The tripeptide crystallizes in the monoclinic crystal system, space group P2(1), cell parameter a = 5.867(7) angstrom, b = 23.54(2) angstrom, c = 10.34(1) angstrom, beta = 99.74(5)degrees, V = 1407(2) angstrom(3); empirical formula C26H37N3O6S; needle crystals were obtained by slow evaporatin of an ethyl acetate solution, after 4 days, at room temperature. The low diffraction power of the crystals, it was impossible to obtain crystal structure from normal diffractometer data, suggested to utilize X-ray synchrotron radiation.