Characterization of Secondary Radical Reactions in Irradiated Ribonuclease

Solid ribonuclease (RNase) has been irradiated in vacuum at 196°K and subsequently heated. The kinetics of formation of secondary radicals have been studied by following the growth of these radicals at different temperatures. Progressive power saturation studies have been applied to determine$(T_{1}T_{2})^{{\textstyle\frac{1}{2}}}$ for primary and secondary radicals. It is shown that the carbon radicals formed during irradiation at 196°K have$(T_{1}T_{2})^{{\textstyle\frac{1}{2}}}$ equal to 1.0 μsec at 77°K and the corresponding value for secondary carbon radicals at the same temperature is approximately the same. Evidence is presented which suggests that the primary organosulfur radicals relax at a faster rate than primary carbon radicals but slower than secondary neutral organosulfur radicals which have$(T_{1}T_{2})^{{\textstyle\frac{1}{2}}}$ equal to 0.18 μsec. Two statistically significantly different activation energies have been measured for the formation of both secondary sulfur and carbon doublet radicals, approximately 4 and 8 and 9 and 18 kcal/mole for the organosulfur and doublet radicals, respectively. Data are presented which indicate that the secondary carbon radical population is complex and probably consists of species producing both doublet and triplet spectra. Such data are consistent with carbon radicals on residues such as methionine and lysine in addition to glycine.