Electron spin resonance spectroscopy was used to study freehyphen;radical formation in single crystals of cysteine HCl exposed to ultraviolet irradiation. Irradiation at 77deg;K and momentary warming produced the free radical Ssngbnd;CH2sngbnd;CH(NH3Cl)COOH. Further irradiation at 77deg;K using longer wavelengths causes a quantitative transformation of the paramagnetic absorption which is attributed to the formation of the radical ionminus;Ssngbnd;CH2sngbnd;CHsngbnd;NH3Cl. The transformation is reversible and, upon warming, the initial radical is reconstituted.
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