Polycrystalline samples of amino acids and proteins were exposed to electrons and beams of helium, carbon, neon, and argon ions from the Berkeley heavy-ion linear accelerator. Both the irradiations and the electron-spin resonance observations were carried out at 77°K. The results showed that the same types of radicals were formed independent of the stopping power of the bombarding particle. The results for the amino acid DL-valine have been interpreted in relation to the "thermal spike model." The irradiation of DL-valine at 77°K results in the formation of a radical which converts upon heat treatment into a secondary radical. On the basis of the thermal spike model and the kinetics for the radical conversion, it is possible to calculate the expected amount of secondary radicals formed at 77°K along the tracks of the heavy ions. The observation that no spectral changes take place with increasing stopping power is clearly in contrast to the predictions from the thermal spike model. Therefore, the model in its present form does not apply to radical formation or secondary radical reactions.

The radicals induced in single crystals of thymine monohydrate by ionizing radiation have been studied by ESR. Both at 77°K and at room temperature three different radicals are formed. The spectral changes observed at different temperatures are due to variations in the relative yields of these three radicals. In addition to the previously known 5-thymyl (5,6-dihydrothymin-5-yl) radical, two other radicals have been identified. The hyperfine splitting tensors and the directions of the principal axes show that for both radicals the unpaired spin density is mainly in a $p_{\pi}$ orbital on C 6 . One radical has hyperfine coupling to one α and one β proton; the other radical has coupling to one α proton only. The radicals are the 6-thymyl (5,6-dihydrothymin-6-yl) radical formed when a hydrogen atom is added to $C_{5}$ , and a radical in which a hydrogen atom is added to the oxygen atom bonded to C 4 . The latter radical is presumably formed by electron capture with subsequent proton tunneling in the hydrogen bond to the water molecule.

Frozen solutions of dihydrothymine in H 2 SO 4 , irradiated at 77°K, contain large amounts of thymine after melting. Concentration studies show that the thymine must be produced mainly by indirect mechanisms. Low-temperature UV absorption measurements reveal that thymine is not present after irradiation at 77°K, but is formed upon warming of the samples to about 160° to 200°K. ESR experiments suggest that the 5-thymyl radical is an intermediate in the production of thymine. At 77°K the irradiated samples contain only H atoms and some other unidentified H 2 SO 4 radicals. The 5-thymyl radicals are formed upon warming to temperatures above 105°K, and they disappear in the temperature range at which thymine is formed.

Hydrogen abstraction from ${\rm N}_{(1)}$ occurs when single crystals of orotic acid are exposed to ionizing radiation at room temperature. The stable free radical has unpaired spin density on both ${\rm N}_{(1)}$ and ${\rm C}_{(5)}$ of the pyrimidine ring. Hyperfine coupling to the nitrogen nucleus shows axial symmetry, with $A_{\parallel}{}^{{\rm N}}=15.0$ gauss and $A_{\perp}{}^{{\rm N}}=0$ . The hydrogen bonded to ${\rm C}_{(5)}$ has principal hyperfine values, in gauss, of $A_{{\rm x}}{}^{{\rm H}}=7.3$ , $A_{{\rm y}}{}^{{\rm H}}=22.2$ , and ${\rm A}_{{\rm z}}{}^{{\rm H}}=15.1$ . From the hyperfine couplings, the spin densities calculated for ${\rm N}_{(1)}$ and ${\rm C}_{(5)}$ , respectively, are 0.29 and 0.64. Principal values of the g-tensor are $g_{{\rm u}}=2.0059$ , $g_{{\rm v}}=2.0052$ , and $g_{{\rm w}}=2.0023$ . The ESR analysis can be used to predict the molecular orientation in orotic acid crystals.

Irradiation of 9-methyladenine at room temperature produces two stable free radicals, one by a hydrogen addition reaction at either C-2 or C-8 of the purine ring, and one by a hydrogen abstraction reaction from the methyl group. In this paper the hydrogen abstraction radical is analyzed in detail by using electron spin resonance (ESR) spectroscopy. Hydrogen hyperfine interactions with the two protons have principal values, in gauss, of $A_{x}{}^{{\rm H}(1)}=10.5,A_{y}{}^{{\rm H}(1)}=30.0,A_{z}{}^{{\rm H}(1)}=18.5,A_{x}{}^{{\rm H}(2)}=8.5,A_{y}{}^{{\rm H}(2)}=32.5$ , and $A_{z}{}^{{\rm H}(2)}=19.5$ . The principal values for the hyperfine interaction with N-9 are $A_{x}{}^{{\rm N}}=3.5$ , $A_{y}{}^{{\rm N}}=3.9$ , and ${\rm A}_{z}{}^{{\rm N}}=3.0$ . In nondeuterated crystals, an additional hyperfine splitting due to the C-8-H proton is observed at some orientations. The g tensor is axially symmetric, with $g_{\parallel}=2.0021$ and $g_{\perp}=2.0029$ . Some effects of observation temperature on the ESR spectra are also reported.

Kinetics for free radical production and decay in γ-irradiated single crystals of L-alanine have been studied by electron spin resonance spectroscopy. By selectively deuterating only the radicals in irradiated crystals of deuterated alanine, it has been established experimentally that radical saturation in γ-irradiated single crystals of alanine is due to a process of radical destruction by the radiation. At saturation, radical production is not zero, but rather radicals are being produced and destroyed at the same rate. The biological implications are that the phenomenon of low-level radical saturation cannot be used as an argument against the possible role of free radicals in biological radiation-damaging processes.