ESR and ENDOR studies of irradiated cysteine HCl: Decarboxylation

K-band ESR and ENDOR spectra of irradiated single crystals of L-cysteine hydrochloride monohydrate at 65 K provide evidence for an unstable carbon-centered radical produced by decarboxylation. The unpaired electron is localized in a 2p orbital on the alpha carbon. An α-proton hyperfine coupling with principal values −104.8, −65.4, and −33.1 MHz has been measured. Five β-proton couplings have also been observed, of which three are exchangeable.