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When the thymine base is oxidized, the resulting cation may deprotonate reversibly at N3, or irreversibly at >C5-CH3. In all thymine derivatives studied so far in the solid state, there is always a significant concentration of a radical formed by net H-abstraction from the >C5-CH 3. DFT calculations on this allyl-like radical are in good agreement with the experimental results for both the isotropic and anisotropic hyperfine couplings. There is a tendency for the thymine cation to deprotonate at N3 in solution. Calculations on the N3 deprotonated thymine cation yield two structures, one planar radical with an unusually large N1-C2 bond length, and one nonplanar radical with the N3 more than 25° out of the molecular plane. Calculations show that the structure with the lowest energy is the allyl-like radical.

This review contains the results of EPR/ENDOR experiments on DNA constituents in the solid-state. Most of the results presented involve single crystals of the DNA bases, nucleosides and nucleotides. The emphasis is on low-temperature ENDOR results. Typical experiments involve irradiations at or near helium temperatures in attempts to determine the primary radiation induced oxidation and reduction products. The use of the ENDOR technique allows one to determine the protonation state of the initial products. Subsequent warming of the sample facilitates a study of the reactions that the primary products undergo. A summary of the results is provided to show the relevance the study of model compounds has in understanding the radiation chemistry of DNA.

Detailed EPR and ENDOR experiments on the cocrystalline complex of 1-methyluracil:9-Ethyladenine (MUEA) have revealed that the major radiation-induced products observed at 10 K on MU are: MUEA1, a radical formed by net hydrogen abstraction from the N1-CH3 methyl group, MUEA2, the MU radical anion, and MUEA3, the C5 H-addition radical. The following four products were observed on the adenine moiety at 10 K, MUEA4, the N3 protonated adenine anion, MUEA5, the native adenine cation, MUEA6, the amino deprotonated adenine cation, and MUEA7, the C8 H-addition radical formed by net H-addition to C8 of the adenine base. The geometries, energetics, and hyperfine properties of all possible radicals of MU and EA, the native anions and cations, as well as radicals formed via net hydrogen atom abstraction (deprotonated cations) or addition (protonated anions) were investigated theoretically. All systems were optimized using the hybrid Hartree-Fock-density functional theory functional B3LYP, in conjunction with the 6-31G(d,p) basis set of Pople and co-workers. Calculations of the anisotropic hyperfine couplings for all the radicals observed in MUEA are presented and are shown to compare favorably with the experimentally measured hyperfine couplings. The calculated ionizations potentials indicate that EA would be the preferred oxidation site. In MUEA, both the adenine cation and its N4-deprotonated derivative were observed. The calculated electron affinities indicate that MU would be the preferred reduction site. In MUEA radical, MUEA2 is a uracil reduction product, however the protonation state of this radical could not be determined experimentally. Calculations suggest that MUEA2 is actually the C4=O protonated anion.