David M. Close

ESR study of the guanine cation

It has been proposed that the primary direct radiation damage products in DNA are guanine cations and thymine anions. Experiments reported here characterize a guanine cation observed in a single crystal of guanine:HCl:H2O. ESR experiments were performed by x‐irradiating and observing the crystals at 15 K. Spectral parameters for the cation include N3 and N10 hyperfine couplings, a C8–Hα hyperfine coupling, and two small exchangeable couplings presumably from the N10 protons. The computed spin densities of ρ(N3)=0.283, ρ(N10)=0.168, and ρ(C8)=0.182 agree nicely with those observed for the guanine cation in DNA. In the single crystal the native molecule is protonated at N7. It is proposed that once the native molecule is oxidized it rapidly deprotonates at N7 to form the cation observed.

ESR and ENDOR study of the guanine cation: Secondary product in 5’‐dGMP

Previous ESR studies of x‐irradiated single crystals of 2’‐deoxyguanosine‐5’‐monophosphate have indicated the presence of a radical thought to be formed by deprotonation of a primary base cation at N1. In this communication are reported some results of detailed ESR and ENDOR experiments at 10 K conflicting with the above results. One of the radicals detected exhibited two α‐proton type couplings. The data analysis shows that one coupling is due to the exchangeable proton of the extra‐annular NH2 group, while the other is due to the proton bonded at C8. The experimental spin densities were ρ(N10)=0.33, and ρ(C8)=0.18. The results agree reasonably well with the INDO calculated spin density distribution of a radical formed by deprotonation at N10 of a primary cation radical. The radical is stable on warming to about 200 K where it anneals rapidly.

Ionization Energy Thresholds of Microhydrated Adenine and Its Tautomers

In the present work the vertical and adiabatic ionization energy thresholds (IET) of adenine, and its amino and imino tautomers complexed with 1-3 water molecules are presented. The vertical and adiabatic IETs have been calculated at the B3LYP and P3 levels of theory, using the standard 6-31++G(d,p) basis set. The results show that there is hardly any effect of microhydration on the vertical DeltaIET of adenine, which is at odds with the experimental values determined by Kim et al. (J. Phys. Chem. 1996, 100, 7933). In an attempt to assign the experimental DeltaIET values, calculations have been performed on the microhydrated amino and imino tautomers of adenine. Vertical DeltaIET calculations and adiabatic DeltaIET calculations on adenine N7H tautomers complexed with water are in better agreement with the experimental results than are calculations involving the canonical (N9H) form of adenine.

Low temperature intrinsic defects in x‐irradiated hydroxyapatite synthetic single crystals

ESR studies of radiation‐induced defects have been conducted on synthetic calcium hydroxyapatite single crystals. For a room temperature x‐irradiation a major defect (labeled A) was reported to be an O− ion. X irradiation at 6 K shows defect A, trapped atomic hydrogen, and a nonaxial holelike center (labeled I). These new centers are stable at 6 K but anneal near 77 K. Observations at 9 and 35 GHz indicate that the I center is a spin‐1/2 defect located in six inequivalent sites. In the ab plane, spectra exhibit an isotropic hyperfine doublet (∼13 G splitting) and an anisotropic doublet (17–27 G splitting) in three symmetry‐related sites. For other orientations additional site splitting and ’’forbidden transitions’’ make the spectra very complex. The hyperfine coupling tensor for the anisotropic doublet has diagonal elements −29.5, −19.2, and +3.11 G. The g tensor for this defect has diagonal elements 2.0068, 2.0032, and 2.0148. The sets of directional cosines associated with the minimum g value and the in...

Radiation chemistry of purines in the solid state: ESR and ENDOR studies of x-irradiated xanthosine dihydrate single crystals

By means of ESR and ENDOR spectroscopy, we were able to detect and characterize four products of the purine riboside xanthosine in x‐irradiated xanthosine dihydrate single crystals. Product I was identified (at 65 K) as the π cation of the xanthine base deprotonated at N3. Product II was identified (at 65 K) as the π anion of the xanthine base protonated at O6. Product III was identified as the result of the hydrogen abstraction from C5′ of the sugar group (probably from a primary cation of the ribose). Product IV was identified as the result of hydrogen addition at C8 of the purine ring. Although studied in detail at 65 K, products I and II were present after irradiation and study at 15 K. Slow warming of the crystal beyond 100 K led to gradual and irreversible decay of the signals from products I and II. No clear‐cut evidence for new products appeared until the crystals were warmed beyond 250 K, where spectra from the C8 H adduct (product IV) appeared. At 65 K, signals from product III were present, but...

From the Primary Radiation Induced Radicals in DNAConstituents to Strand Breaks: Low Temperature EPR/ENDOR Studies

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

Theoretical elucidation of conflicting experimental data on vertical ionization potentials of microhydrated thymine.

In a recent article we reported calculations of the ionization energy thresholds (IET) of microhydrated thymine (Close; et al. J. Phys. Chem. A, 2006, 110, 7485). Calculations showed a distinct effect of microhydration on the IETs of thymine. The first water molecule was seen to decrease the IET by about 0.1 eV, and the second and third water molecules caused a further decrease of less than 0.1 eV each. These changes in IET calculated for the canonical form of thymine with 1-3 waters of hydration are smaller than the experimental values determined by Kim et al. (J. Phys. Chem. C 1996, 100, 7933). In the present study it has been shown that there is considerable reorientation of the water molecules in microhydrated thymine upon ionization. This leads to the expectation that the experimental ionization energies may therefore represent an adiabatic process. The results presented here show that the changes in experimental ionization energies determined by Kim et al. for microhydrated thymine are in good agreement with the calculated adiabatic ionization energies.

ESR study of irradiated single crystals of the cocrystalline complex of cytidine: Salicylic acid

Irradiation at 77 K of single crystals of the 1:1 complex of cytidine and salicylic acid produces a phenoxyl radical formed by oxidation of the salicylic acid. Anisotropic hyperfine coupling tensors have been determined for this radical which are associated with the para and ortho hydrogens. No cytidine oxidation products (alkoxy or hydroxyalkyl radicals) were observed at 77 K. Following the decay of the phenoxyl radical at room temperature, four radicals were detected. These include the cytosine 5–yl and 6–yl radicals, formed by H addition to the cytosine ring, and an anisotropic doublet. By UV irradiation at room temperature, it is possible to convert a significant fraction of 6‐yl radicals into 5‐yl radicals. Hyperfine coupling and g tensors determined for the anisotropic doublet indicate that this radical is formed in the C1‐C2 region of the sugar moiety. These results indicate a shift in radiation damage away from the salicylic acid upon warming, and show that the radiation chemistry of the cocryst...

Alternating linewidth effects in the ESR spectrum of 5‐6‐dihydro‐6‐methyluracil

Variable temperature ESR spectra of radicals formed by x‐irradiation of single crystals of DH6MU are reported. (AIP)

Low‐temperature thermoluminescent behavior of 5′dCMP single crystals

Thermoluminescence (TL) and emission spectra studies of x‐irradiated single crystals of deoxycytidine‐5′‐phosphate monohydrate (5′ dCMP) were conducted in the interval 10–300 K. Two intense glow peaks were observed at 42 and 74 K with additional weaker peaks at 112, 140, and 200 K. By applying the Randall–Wilkins theory of glow curves to our data we obtained thermal activation energies (E) and frequency factors (s) characterizing each of the glow peaks, except the one at 200 K—it was of insufficient intensity. The parameters are: 42 K (E = 0.032±0.009 eV, s = 316 s−1); 74 K (E = 0.095±0.035 eV, s = 105 s−1); 112 K (E = 0.080±0.02 eV, s = 20 s−1); and 140 K (E = 0.12±0.04 eV, s = 56 s−1). As a check on the validity of the Randall–Wilkins theory we generated theoretical glow curves using the experimental parameters of the 42 K peak. Excellent agreement between the two glow curves was found. Typical spectral emission from each glow peak consists of band emission exhibiting maxima at 428 and 463 nm. We attrib...