Conformation of L-cystathionine, a carba analog of cystine, and stereochemistry of hormone-receptor interactions

Abstract

The conformation of L-cystathionine, a carba analog of L-cystine has been studied in the solid state using X-ray diffraction techniques. Crystals of L-cystathionine are tetragonal, space group P4 1 with cell constants a = 6.691(1)Å, c = 21.998(3) Å and Z = 4. From diffractometer data to the limit of 2 π = 162° for Cukα, the structure was refined using full-matrix least-squares to an R value of 0.061. L-Cystathionine is isostructural chemically to L-cystine and its crystal structure is isomorphous to tetragonal L-cystine (Chaney, M.O. and Steinrauf, L.K. (1974) Acta Crystallogr. 1330, 711–716). The crystal structure of L-cystathionine is disordered, leading to two slightly differing conformers of L-cystathionine (each with half occupancy) with same helical sense but running in opposite directions and occupying the locations of L-cystine molecules in tetragonal L-cystine structure. Their conformational similarity, even when no sterical constraints such as cyclization are present, offers an explanation of the activities of the carba analogs of neuro-hypophysial hormones in terms of the structural integrity of the disulfide-like bridges.