X-ray crystal structure and conformation of N-(tert-butyloxycarbonyl)-L-methionyl-(1-aminocyclopent-3-ene-1-carbonyl)-L-phenylalanine methyl ester (Boc0-Met1-Cpg2-Phe3-OMe)

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 b-turn (C10) conformation at the -Met- Cpg- sequence with a distance, between the terminal pseudo- ring atoms O6 C3, of 5.49(1)A. The tripeptide crystallizes in the monoclinic crystal system, space group P21, cell parameter: a ¼ 5.867(7) A, b ¼ 23.54(2) A, c ¼ 10.34(1)A, b ¼ 99.74(5), V ¼ 1407(2) A3; empirical formula C26H37N3O6S; needle crystals were obtained by slow evaporation 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.