Paramagnetic Resonance Spectrum of Irradiated Single Crystals of L-Cystine Dihydrochloride

The principal absorption in the ESR spectrum of single crystals of cystine dihydrochloride irradiated at 77°K is attributed to the free radical $$\mathrm{([–}\mathrm{S}–{\mathrm{CH}}_2–\mathrm{CH}({\mathrm{NH}}_3^{+}{\mathrm{Cl}}^{-})\mathrm{COOH}{]}_2{)}^{+}$$ in which the unpaired electron is localized primarily on the sulfur atoms. The principal values of the g tensor are 2.017, 2.018, and 2.002. Each of the four CH₂ protons contributes a nearly isotropic hyperfine splitting of approximately 8.5 G.