Nuclear coupling of 33S and the nature of free radicals in irradiated crystals of N-acetyl-L-cysteine.

Hyperfine structure due to 33S in its natural abundance of 0.76% has been measured in the electron spin resonance of free radicals produced by x-irradiation of single crystals of N-acetyl-L-cysteine at 77 K. These measurements proved that the radicals produced at 77 K with principal g values of 1.990, 2.006, and 2.214 are monosulfide radicals with the 3p unpaired electron density of 0.70 on the S. They are believed to be negatively charged molecules RCH2S-H or neutral RCH2SH2 radicals in which 90% of the spin density of the captured electron is concentrated in a d-p hybrid orbital on the S. As the temperature is raised to 300 K, these, as well as the carbon-centered radicals produced at the lower temperature, are mostly converted to neutral disulfide radicals RCH2SS like those observed in irradiated cystine.

Nuclear coupling of 33S and the nature of free radicals in irradiated crystals of cysteine hydrochloride and N-acetyl methionine.

The 33S hyperfine structure has been observed in the electron spin resonance of irradiated crystals of cysteine-HCl-H2O and of N-acetyl DL-methionine. An analysis of the results shows that in both substances the free radicals that are stable at room temperature are disulfide radicals, RCH2SS. A possible mechanism for formation of these radicals is proposed.

Nuclear coupling of 33S and the nature of free radicals in irradiated crystals of cystine dihydrochloride: Part II, charged radicals.

The (33)S hyperfine structure has been observed in negatively charged cystine molecule-radicals RCH(2)(SS)(-)CH(2)R. The coupling, found to be axially symmetric about the SS bond, proved the orbital of the unpaired electron to be an antibonding SS sigma(*) orbital with the spin density shared equally between 3p(sigma(*)) orbitals of the two sulfurs. The (33)S hyperfine structure could not be detected for a second radical produced by irradiation of the cystine hydrochloride crystal at 77 degrees K, but interpretation of the proton hyperfine structure and g tensor observed by others led to the conclusion that this radical is the positively charged cystine molecule RCH(2)S(+)SCH(2)R with the spin density concentrated in a 3p orbital of one S and perpendicular to the SS bond.

Nuclear coupling of 33S and the nature of free radicals in irradiated crystals of cystine dihydrochloride.

The (33)S hyperfine structure has been observed and analyzed in the electron spin resonance of gamma-irradiated single crystals of cystine dihydrochloride at 77 degrees K and at room temperature. The radicals observed at room temperature and previously assigned as RCH(2)S are proved to be disulfide radicals, RCH(2)SS. However, the RCH(2)S radical is identified by the (33)S measurements in samples irradiated at 77 degrees K, annealed at 200 degrees K, and observed at 77 degrees K. This monosulfide radical appears to undergo reactions to form disulfide radicals at room temperature.