This updating review contains the results of recent ESR/ENDOR experiments on DNA constituents in the solid state. The compounds reviewed include 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 these primary products undergo. A critical review of previous ESR experiments is considered to see if these results conflict with the newer ENDOR results. Finally, a summary is provided to show which of the numerous results discussed may have some relevance to understanding the radiation chemistry of DNA.

Single crystals of adenosine were X-irradiated at 10 K and investigated between 10 and 300 K using K-band ESR and ENDOR spectroscopy. Two free radicals were analyzed. Radical I exhibits small hyperfine couplings to the C8-H, C2-H, and a N3-H protons, and was identified as the N3 protonated base anion radical. Radical II exhibits small hyperfine couplings to a C8-H and an exocyclic - N10-H proton. It is suggested that this is before the N10 deprotonated base cation radical. Enough data were not available to analyze a third primary radical believed to be located on the ribose moiety. Upon warming Radical I decays at ca. 40 K with no apparent successor. Likewise, no successor was identified for Radical II, which decays at ca. 100 K. At ca. 200 K there is ESR evidence for the C2 and C8 H-addition radicals. Their precursors have not been identified.

This work involves an ESR and ENDOR study of the reactions of the three primary radicals observed in X-irradiated single crystals of guanine hydrochloride monohydrate. Radical I, the O6-protonated anion, decays at 250 K yielding a stable room temperature radical (Radical V). The experimental evidence indicates that Radical V results from H-abstraction at N9 of a neighboring molecule. Radical II, the N7-deprotonated cation, decays at 60 K with no detectable successor. Radical IV, a C8 H-addition radical, is formed when an imidazole ring-opened radical (Radical III) decays at 150 K. The added H-atom was found to be from an easily exchangeable source. It is proposed that Radical III decays by the formation of a diamagnetic formamida molecule and an H-atom. It is important to note that Radical IV, the purine H-addition radical, is the result of basic oxidation events. Previous assumptions have been that purine H-addition radicals result either from reduction, i.e., protonation of a pristine anion, or from the 'excitation pathway', by addition of H-atoms dissociated from superexcited purine bases.

In a study of guanine·HCl·H2O (Gm) single crystals X-irradiated at temperatures between 10 and 77 K, three radical species were found and characterized by ESR and ENDOR spectroscopy. All three are primary products in that they were present immediately following irradiation at T < 10 K. Radical I, which apparently can exist in two slightly different conformations, was identified as the product of electron gain by the parent molecule and subsequent protonation at O6. Radical I decayed only after warming the crystals beyond 250 K. Radical II was the guanine cation previously reported (D. M. Close, E. Sagstuen, and W. H. Nelson, J. Chem. Phys. 82, 4386 (1985)); however, ENDOR data are reported here which confirm the previous results. The guanine cation in Gm resulted from electron loss from the parent and subsequent deprotonation at N7. It is proposed that Radical III results from OH attack at C8 of the parent molecule, followed by rupture of the C8-N9 bond and ring opening. The OH radicals thought to produce Radical III result from electron loss by the cocrystallized water molecules. The reaction leading to Radical III, unusual in solid-state radiation chemistry, is thought to be mediated by the specific hydrogen bonding network in this crystal.

Single crystals of guanine hydrochloride dihydrate, in which the guanine base is protonated at N7, were X-irradiated at 20K, 65K and 150K. Study with K-band ESR and ENDOR techniques indicated at least four radical species to appear in the temperature range 20-300K. Three of the radical species (Radicals 1, 2, and 3) were present immediately following irradiation at 20K. Radical 1 was identified as the molecular anion protonated at O6. Hyperfine couplings in Radical 1 to HC8, HN1, and HN7 were fully characterized with ENDOR spectroscopy. The data indicated this product to be trapped in four different conformations which coalesced into the most stable form as the sample temperature was raised to ca. 180K. Radical 2 was the C8 H-addition radical. For this radical, hyperfine couplings to HN7, HN9, and the two βmethylene protons were fully characterized with ENDOR spectroscopy. Radical 3 was the result of hydroxyl addition to C8 of the guanine base. For Radical 3, full characterization by ENDOR was possible for couplings to HN7, HN9 and the proton at C8. Annealing the samples beyond 250K for several hours led to disappearance of Radical 1, and appearance of Radical 4. The evidence indicated that Radical 4 was the result of net hydrogen abstraction from N9 of the guanine base. The ENDOR results permitted full characterization of hyperfine couplings to HN7 and the two amino protons (the HN10s). From these results, and those from two other systems containing N7-protonated guanine bases, reaction mechanisms are proposed to account for formation of the products.

No description available.

No description available.

No description available.

The EPR spectra of single crystals of deoxycytidine 5′-phosphate monohydrate (5′dCMP), X-irradiated at 10 K, exhibit signals from several distinct radical species. Analysis of the ENDOR spectra from two of these radicals indicates that these result from oxidation and reduction of the cytosine base. The reduced species exhibits hyperfine coupling to the C6-Hα proton, and an additional small exchangeable hyperfine coupling from the N3-H proton. No additional couplings that may be associated with protonation of the amino group have been observed. Since the native molecule is protonated at N3, it appears that reduction of the cytosine base does not result in any further protonation. The oxidized species exhibits hyperfine couplings to C5-Hα and C1′-Hß, and two small exchangeable couplings from the C4-NH2 protons. Since no hyperfine coupling to N3-H was observed, the oxidized species is believed to be the N3-H deprotonated cation. At high X-ray dose there is also evidence for both C5 and C6 H-addition radicals at 10 K. The fate of these radicals has been studied under controlled warming conditions. Attempts have been made to relate the fate of the low-temperature radicals with several radicals that have been detected previously at 77 K and at room temperature.

o,o′-Diphenyleneiodonium cations (DPI) convert relatively harmless radiation-produced electrons into efficient DNA cleaving agents. The cleavage products are unaltered DNA bases, 5-methylenefuranone (5-MF), and a complete set of 3′ and 5′-phosphorylated DNA fragments. The production of alkali-labile sites is a minor factor in the process. Based on the production of 5-MF, it is concluded that DNA cleavage by DPI cations involves (but may not be limited to) the C1′ chemistry. The loss of 3-aminoDPI (ADPI) cations bound to highly polymerized calf thymus DNA appears to be due to a short-chain reaction with an apparent length of up to 2.1 ADPI cations consumed for each radiation-produced electron. The suggested chain reaction mechanism includes the one-electron oxidation of DNA radicals (including the C1′ sugar radical) by ADPI cations bound to the same duplex. The yields of DNA loss in complexes formed by ADPI with short synthetic duplexes indicate that there is more than a 60% probability of DNA damage after one-electron reduction of ADPI.