Free-Radical Formation by Ultraviolet Irradiation in Single Crystals of Cysteine HCl

Abstract
Electron spin resonance spectroscopy was used to study free‐radical formation in single crystals of cysteine HCl exposed to ultraviolet irradiation. Irradiation at 77°K and momentary warming produced the free radical S–CH2–CH(NH3Cl)COOH. Further irradiation at 77°K using longer wavelengths causes a quantitative transformation of the paramagnetic absorption which is attributed to the formation of the radical ion S–CH2–CH–NH3Cl. The transformation is reversible and, upon warming, the initial radical is reconstituted.