Peptides containing the sequence Arg-Gly-Asp antagonize binding of fibrinogen to its platelet GPIIb/IIIa receptor, thereby inhibiting platelet aggregation. Incorporation of the sequence into cyclic pentapeptide disulfides has been reported to yield effective antagonists. The conformations in solution of two such antagonists, (2-mercaptobenzoyl)-N(alpha)-methylArg-Gly-Asp-2-mercaptoanilide cyclic disulfide (1) and Ac-Cys-N(alpha)-methylArg-Gly-Asp-Pen-NH2 cyclic disulfide (2), have been studied using a constrained distance geometry search procedure in conjunction with proton NMR data, and a structure of 1 has been determined from single-crystal X-ray diffraction data. NMR spectra of the cyclic diaryl disulfide 1 at 203 K in methanol show two slowly exchanging conformations. The Arg-Gly-Asp region of the major form is characterized, inter alia, by an extended Gly residue flanked by an N(alpha)-methylArg residue in a conformation roughly consistent with the i + 2 position of a beta-turn and an Asp residue in a C-7 like conformation. In the minor component, the Asp residue is near the alpha(R) conformation. The barrier to exchange between the two forms is estimated at 11 kcal/mol. NMR data and analysis of the constrained distance geometry search results suggest that, at room temperature in dimethyl sulfoxide-sulfolane, the dominant conformation of the Arg-Gly-Asp regions of both 1 and 2 is like that in the major component of 1 at 203 K. (2-Mercaptobenzoyl)-N(alpha)-methylArg-Gly-Asp-2-mercaptoanilide cyclic disulfide (1) was crystallized from aqueous ethanol as a solvated nitrate salt in a cell of dimensions a = 27.919 (16) angstrom, b = 7.552 (3) angstrom, c = 16.3131 (10) angstrom, and beta = 108.79 (5)-degrees with four formula units in space group C2. The structure was solved by direct methods and refined to R = 0.057 for 2869 observations (I greater-than-or-equal-to 3 sigma(I)). The crystal structure of 1 and the most probable conformation of its minor form in solution agree closely.
