Circular dichroism of crystalline l-cystine in potassium bromide disc. Possibility of exciton splitting

Abstract

Circular dichroism (CD) of crystalline l-cystine and l-cystine dihydrochloride in a potassium bromide disc was studied to clear up the cause of the abnormal CD spectrum found for the former by Imanishi and Isemura 9, and to relate CD spectra to the conformations of disulfide groups in the crystals. Inspection of published crystalline structures showed that disulfide groups are linearly arrayed with van der Waals contacts in l-cystine crystal but not in l-cystine · 2HCl crystal. It was inferred that the abnormal CD is the result of an exciton splitting due to the characteristic array of cystine disulfides in the crystal. Supporting evidence was obtained by measurements of polarized ultraviolet and CD spectra with a potassium bromide disc in which cystine crystals were preferentially oriented in one direction. The maximum CD intensity of the longest wavelength band expected for a potassium bromide disc of l-cystine, free from exciton splitting but retaining the intact conformation, was roughly estimated. The obtained results seem to be useful to understand the CD of cystine in solution.