ion from the Sugar Moiety Wendy Knapp Pogozelski† and Thomas D. Tullius*,‡ Department of Chemistry, State University of New York at Geneseo, Geneseo, New York 14454, and Department of Chemistry, Boston University, Boston, Massachusetts 02215 Received August 27, 1997 (Revised Manuscript Received February 26, 1998)

Oxidative Strand Scission of Nucleic Acids: Routes Initiated by Hydrogen Abstraction from the Sugar Moiety.

Transition-metal ions in aluminophosphate and silicoaluminophosphate molecular sieves: location, interaction with adsorbates and catalytic properties.

Defect centers induced by ionizing radiation (50–100‐keV x rays, 60Co γ rays) in high purity P‐doped silica glass have been observed and elucidated by ESR spectroscopy. Four generic species are well characterized on the basis of the observed 31P hyperfine splittings and g values as defects analogous to PO2−3 (phosphoryl), PO4−4 (phosphoranyl), PO2−2 (phosphinyl), and PO2−4 radicals. The latter species, also termed the phosphorus‐oxygen‐hole center (POHC), is shown to occur in two variants comprising holes trapped on one or two nonbridging oxygens. Radiation‐induced Si E′ centers with and without P next‐nearest‐neighbors were also identified, and a singlet resonance S due to E′ type defects such as (OSi2)Si⋅ and/or (O2Si)Si⋅ was observed to grow in with annealing above ∼800 K, regardless of whether or not the sample was irradiated. The structures, formation mechanisms, and precursors of these defects have been determined or inferred for all centers. Radiation‐induced optical absorption spectra over the ran...

Fundamental defect centers in glass: Electron spin resonance and optical absorption studies of irradiated phosphorus‐doped silica glass and optical fibers

Interplay of electronic, environmental, and vibrational effects in determining the hyperfine coupling constants of organic free radicals.

Electron affinities and ionization potentials of nucleotide bases

Single crystals of the amino acid l-α-alanine have been X-irradiated at room temperature, and the free radical formation has been studied using X-band and K-band EPR, ENDOR, and EIE (ENDOR-induced EPR) spectroscopy in the temperature interval 220−295 K. Aided by the results from EIE, as well as ENDOR from selected magnetic field positions, nine hyperfine coupling tensors were obtained and assigned to three different radicals. Room-temperature relaxation behaviors characterized by efficient W1x and W1e and by slow W1n relaxation rates allowed for determination of the signs of the various hyperfine couplings from the ENDOR spectra obtained at room temperature. The temperature dependence of the W1x relaxation is qualitatively discussed. The EPR spectra from alanine are dominated by the well-known resonance of the “stable alanine radical”, SAR, formed by a net deamination of the protonated alanine anion. Precise hyperfine coupling tensors due to the α-proton coupling, the methyl group coupling, and a dipolar ...

Alanine Radicals: Structure Determination by EPR and ENDOR of Single Crystals X-Irradiated at 295 K

Single crystals of 2'-deoxyguanosine 5'-monophosphate were X-irradiated at 10 K and at 65 K, receiving doses between 4.5 and 200 kGy, and studied using K-band EPR, ENDOR, and field-swept ENDOR (FSE) spectroscopy. Evidence for five base-centered and more than nine sugar-centered radicals was found at 10 K following high radiation doses. The base-centered radicals were the charged anion, the N10-deprotonated cation, the C8 H-addition radical, a C5 H-addition radical, and finally a stable radical so far unidentified but with parameters similar to those expected for the charged cation. The sugar-centered radicals were the H-abstraction radicals centered at C1', C2', C3', and C5', an alkoxy radical centered at O3', a C5'-centered radical in which the C5'-O5' phosphoester bond appears to be ruptured, a radical tentatively assigned to a C4'-centered radical involving a sugar-ring opening, as well as several additional unidentified sugar radicals. Most radicals were formed regardless of radiation doses. All radicals formed following low doses (4.5-9 kGy) were also observed subsequent to high doses (100-200 kGy). The relative amount of some of the radicals was dose dependent, with base radicals dominating at low doses, and a larger relative yield of sugar radicals at high doses. Above 200 K a transformation from a sugar radical into a base radical occurred. Few other radical transformations were observed. In the discussion of primary radicals fromed in DNA, the presence of sugar-centered radicals has been dismissed since they are not apparent in the EPR spectra. The present data illustrate how radicals barely traceable in the EPR spectra may be identified due to strong ENDOR resonances. Also, the observation of a stable radical with parameters similar to those expected for the charge guanine cation is interesting with regard to the nature of the primary radicals stabilized in X-irradiated DNA.

Free radical formation in single crystals of 2'-deoxyguanosine 5'-monophosphate tetrahydrate disodium salt: an EPR/ENDOR study.

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 study of the guanine cation

Single crystals of thymidine and 1-methylthymine were x-irradiated at 10 K and studied by using K-band ESR, ENDOR, and FSE spectroscopy. In thymidine, six primary radicals were identified. The major species were the O4-protonated anion (TI), the radical formed by net H addition to the C6 position of the base (TII), the radical formed by net H abstraction from the methyl group (TIII), and the alkoxy radical (TVI). The two minority radicals were one formed by net H addition to the C5 position of the base (TIV) and one in which the unpaired spin was localized on the sugar moiety, thought to be a C1{prime} H abstraction radical (TV). A small, nonexchangeable coupling in radical TI was shown to be due to a coupling to the methyl group. Evidence is presented that the characteristics of this coupling tensor indicates the protonation state of the thymine anion radical. Upon annealing, the anion decayed at about 40 K with no detectable successor. The alkoxy radical decays in the same temperature range. In 1-methylthymine, three major radical species were identified.

Primary reduction and oxidation of thymine derivatives: ESR/ENDOR of thymidine and 1-methylthymine x-irradiated at 10 K

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.

William H. Nelson

Papers

Alanine Radicals: Structure Determination by EPR and ENDOR of Single Crystals X-Irradiated at 295 K

Free radical formation in single crystals of 2'-deoxyguanosine 5'-monophosphate tetrahydrate disodium salt: an EPR/ENDOR study.

ESR study of the guanine cation

Primary reduction and oxidation of thymine derivatives: ESR/ENDOR of thymidine and 1-methylthymine x-irradiated at 10 K

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