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)

Abstract

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 β-turn (C₁₀) conformation at the –Met-Cpg- sequence with a distance, between the terminal pseudo-ring atoms O₆ · · · C₃, of 5.49(1)Å. The tripeptide crystallizes in the monoclinic crystal system, space group P2₁, cell parameter: a = 5.867(7) Å, b = 23.54(2) Å, c = 10.34(1) Å, β = 99.74(5)°, V = 1407(2) ų; 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.