A Raman spectroscopic investigation of the disulfide conformation in oxytocin and lysine vasopressin.

Abstract

The conformation of the CCSSCC moiety in oxytocin and lysine vasopressin is investigated using laser Raman spectroscopy. The Raman spectra of solutions of these hormones in water and in dimethyl sulfoxide show an intense band at 508 cm-1 which is assigned to the S-S stretching mode. The presence of shoulders on this band between 490 and 525 cm-1 shows that there is an equilibrium among several conformations for the disulfide unit of these hormones in solution. Most of the CS-SC dihedral angles are within 30 degrees of +/-90 degrees, but some of the molecules have CS-SC dihedral angles strained away from this value by more than 30 degrees. The previously published circular dichroism spectra of these hormones are reinterpreted, and it is shown that the circular dichroism spectra indicate the presence of more than one conformation for the disulfide unit, in agreement with the Raman results.