Structure of radicals from X-irradiated guanine derivatives: an experimental and computational study of sodium guanosine dihydrate single crystals

In sodium guanosine dihydrate single crystals, the guanine moiety is deprotonated at N1 due to growth from high-pH (>12) solutions. Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) studies of crystals X-irradiated at 10 K detected evidence for three radical forms. Radical R1, characterized by two proton and two nitrogen hyperfine interactions, was identified as the product of net hydrogenation at N7 of the N1-deprotonated guanine unit. R1 exhibited an unusually distorted structure leading to net positive isotropic components of the hydrogen alpha-couplings. Radical R2, characterized by one proton and one nitrogen hyperfine coupling, was identified as the primary electron-loss product. This product is equivalent to that of deprotonation at N1 by the guanine cation and represents the first ENDOR characterization of that product. Radical R3, characterized by a single hydrogen hyperfine coupling, was identified as the product of net dehydrogenation at C1' of the ribose moiety. The identification of radicals R1-R3 was supported by density functional theory (DFT) calculations on several possible structures using the B3LYP/6-311G(2df,p)//6-31G(d,p) approach. Radical R4, detected after warming the crystals to room temperature, was identified as the well-known product of net hydrogenation of C8 of the (N1-deprotonated) guanine component. Radical R1, evidently formed by protonation of the primary electron addition product, was present as roughly 60% of the total radicals detected at 10 K. Radical R2 was present as roughly 27% of the total yield, and the concentration of R3 contributed the remaining 13%. R3 is evidently the product of one-electron oxidation followed by deprotonation; thus, the balance of oxidation and reduction products is approximately equal within experimental uncertainty.     

Radiation products at 77 K in trehalose single crystals: EMR and DFT analysis

The radicals obtained in trehalose dihydrate single crystals after 77 K X-irradiation have been investigated at the same temperature using X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) techniques. Five proton hyperfine coupling tensors were unambiguously determined from the ENDOR measurements and assigned to three carbon-centered radical species (T1, T1*, and T2) based on the EIE spectra. EPR angular variations revealed the presence of four additional alkoxy radical species (T3 to T6) and allowed determination of their g tensors. Using periodic density functional theory (DFT) calculations, T1/T1*, T2, and T3 were identified as H-loss species centered at C4, C1', and O2', respectively. The T4 radical is proposed to have the unpaired electron at O4, but considerable discrepancies between experimental and calculated HFC values indicate it is not simply the (net) H-loss species. No suitable models were found for T5 and T6. These exhibit a markedly larger g anisotropy than T3 and T4, which were not reproduced by any of our DFT calculations.     

Combined electron magnetic resonance and density functional theory study of 10 K X-irradiated beta-D-fructose single crystals

Primary free radical formations in fructose single crystals X-irradiated at 10 K were investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR induced EPR (EIE) techniques. ENDOR angular variations in the three principal crystallographic planes and a fourth skewed plane allowed the unambiguous determination of five proton hyperfine coupling tensors. From the EIE studies, these hyperfine interactions were assigned to three different radicals, labeled T1, T1* and T2. For the T1 and T1* radicals, the close similarity in hyperfine coupling tensors suggests that they are due to the same type of radical stabilized in two slightly different geometrical conformations. Periodic density functional theory calculations were used to aid the identification of the structure of the radiation-induced radicals. For the T1/T1* radicals a C3 centered hydroxyalkyl radical model formed by a net H abstraction is proposed. The T2 radical is proposed to be a C5 centered hydroxyalkyl radical, formed by a net hydrogen abstraction. For both radicals, a very good agreement between calculated and experimental hyperfine coupling tensors was obtained.     

EPR and ENDOR study of radiation-induced radical formation in purines: sodium inosine crystals X-irradiated at 10 K

X-irradiated single crystals of sodium inosine (Na(+)*Inosine(-)*2.5H(2)O), in which the hypoxanthine base is present as the N1-deprotonated anion, were investigated using K-band (24 GHz) electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and ENDOR induced EPR (EIE) techniques at 10 K. At least five different radicals were present immediately after irradiation at 10 K. R1, which decayed upon warming the crystals to 50 K, was identified as the electron-loss product of the parent N1-deprotonated hypoxanthine base. Hyperfine couplings to HC8 and HC2 were fully characterized with ENDOR spectroscopy, and the identification was supported by DFT calculations. R2, which also decayed on warming to 50 K, exhibited nearly equal couplings to HC2 and HC8. Taken in combination with an extensive set of DFT calculations, the experimental results indicate that R2 is the (doubly negative) product of electron-gain by the initially anionic N1-deprotonated hypoxanthine parent. R3, which exhibited hyperfine coupling only to HC8 could not be identified. R4, which persisted on annealing to 260 K, exhibited one large alpha-proton hyperfine coupling which was fully characterized by ENDOR. Based on DFT calculations and the experimental data, R4 was identified as the product of net H-abstraction from C5'. The remaining HC5' was the source of the measured alpha-proton coupling. R5, present at low temperature and the only observable radical after warming the crystals to room temperature, was identified as the C8-H addition radical. The alpha-coupling to HC2 and beta-couplings to the pair of C8 methlyene protons were fully characterized by ENDOR.     

Radiation-induced radicals in glucose-1-phosphate. I. Electron paramagnetic resonance and electron nuclear double resonance analysis of in situ X-irradiated single crystals at 77 K

Electron magnetic resonance analysis of radiation-induced defects in dipotassium glucose-1-phosphate dihydrate single crystals in situ X-irradiated and measured at 77 K shows that at least seven different carbon-centered radical species are trapped. Four of these (R1-R4) can be fully or partly characterized in terms of proton hyperfine coupling tensors. The dominant radical (R2) is identified as a C1-centered species, assumedly formed by a scission of the sugar-phosphate junction and the concerted formation of a carbonyl group at the neighboring C2 carbon. This structure is chemically identical to a radical recently identified in irradiated sucrose single crystals. Radical species R1 and R4 most likely are C3- and C6-centered species, respectively, both formed by a net hydrogen abstraction. R3 is suggested to be chemically similar to but geometrically different from R4. Knowledge of the identity of the sugar radicals present at 77 K provides a first step in elucidating the formation mechanism of the phosphoryl radicals previously detected after X-irradiation at 280 K. In paper II, the chemical identity, precise conformation, and possible formation mechanisms of these radical species are investigated by means of DFT calculations and elementary insight into the radiation chemistry of sugar and sugar derivatives is obtained.     

EPR and ENDOR study of radiation-induced radical formation in purines: hypoxanthine hydrochloride monohydrate crystals X-irradiated at 10 K

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) study of hypoxanthine.HCl.H(2)O crystals irradiated at low temperatures (10 K) identified three radical species. In these crystals, the parent molecules exist in a cationic form with a proton at N7. R1 was the product of net hydrogen addition to N3 and exhibited alpha-proton hyperfine couplings to HC2, HN1, HC8, and HN3. The coupling to HC2 has an isotropic component smaller than usual, evidently an indication that the bonds to C2 are nonplanar. R2 was the product of net hydrogen loss from N7, equivalent to the one-electron oxidation product of neutral hypoxanthine, and exhibited alpha-proton hyperfine couplings to HC2 and HC8. Both couplings are characteristic of planar bonding arrangements at the centers of spin. R3 was provisionally identified as the product of net hydrogen addition to O6 and exhibited hyperfine alpha-proton couplings to HC8 and NH1. To identify the set of radicals, the experiments employed four crystal types: normal, deuterated only at NH positions, deuterated at HC8 and NH positions, and deuterated at HC8 only. The low-temperature data also showed clear evidence for H/D isotope effects in formation and/or stabilization of all radicals. To aid and support the identifications, the experimental results were compared to DFT calculations performed on a variety of radical structures plausible for the parent molecule and molecular packing within the crystal.     

Structure of radicals from X-irradiated guanine derivatives. 2. An experimental and computational study of 9-ethylguanine single crystals

An EPR (electron paramagnetic resonance) and ENDOR (electron-nuclear double resonance) study of 9-ethylguanine crystals X-irradiated at 10 K detected evidence for three radical forms. Radical R1, characterized by three proton and two nitrogen hyperfine interactions, was identified as the product of net hydrogenation at N7 of the guanine unit. R1, which evidently formed by protonation of the primary electron addition product, exhibited an unusually distorted structure leading to net positive isotropic components of the alpha-coupling to the hydrogen attached to C8 of the guanine unit. Radical R2, characterized by two nitrogen and three proton hyperfine couplings, was identified as the primary electron loss product, *G+. Distinguishing between *G+ and its N1-deprotonated product is difficult because their couplings are so similar, and density functional theory (DFT) calculations were indispensable for doing so. The results for R2 provide the most complete ENDOR characterization of *G+ presented so far. Radical R3 exhibited a narrow EPR pattern but could not be identified. The identification of radicals R1 and R2 was supported by DFT calculations using the B3LYP/6-311+G(2df,p)//6-31+G(d,p) approach. Radical R4, detected after irradiation of the crystals at room temperature, was identified as the well-known product of net hydrogenation at C8 of the guanine component. Spectra from the room temperature irradiation contained evidence for R5, an additional radical that could not be identified. Radical concentrations from the low temperature irradiation were estimated as follows: R1, 20%; R2, 65%; R3, 15%.     

Q-band EPR and ENDOR of low temperature X-irradiated beta-D-fructose single crystals

Beta-D-fructose single crystals were in situ X-irradiated at 80 K and measured using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques at Q-band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon-centered radicals stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only beta-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single alpha-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical structures. The radical exhibiting only beta-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2-C3 bond and a double C2-O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster density functional theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found.     

Raman spectroscopy of nacrite single crystals at 298 and 77 K

A Raman microscope in conjunction with a thermal stage has been used to determine the Raman spectra of single crystals of nacrite at 298 and 77 K. The spectra obtained are a function of the physics of the spectrometer and were orientation dependent. Bands are observed at 3710, 3646, 3630 and 3623 cm(-1). Upon obtaining the Raman spectra at liquid nitrogen temperature, the band at 3648 cm(-1) was not observed but an additional band at 3603 cm(-1) appeared. This latter band may be attributed to the hydroxyl stretching of non-hydrogen bonded interlayer hydroxyls in the nacrite. The bands attributed to both the inner and inner surface hydroxyls moved to lower frequencies upon cooling to liquid nitrogen temperatures. Low frequency bands also showed orientation dependence.     

A study of thermal and dielectric behavior of manganese malonate dihydrate single crystals

Thermal decomposition of manganese malonate dihydrate single crystals grown by gel method has been studied using the TG-DTA and DSC techniques. The presence of water molecules and the dehydration stages are discussed. Dielectric constant, dielectric loss, and AC conductivity have been estimated as a function of temperature in the range of 40–120 °C for four different frequencies. Thermal studies reveal that the material is thermally stable up to123 °C. The dielectric measurements indicate that the dielectric parameters increase with the increase in temperature. Also, the dielectric constant and dielectric loss factor values decrease whereas the electrical conductivities increase with the increase in frequency of the AC applied.     

Observation of muonium substituted free radicals in a durene single crystal

Two chemically inequivalent 2,3,5,6-tetramethyl cyclohexadienyl radicals are observed in a durene single crystal by means of the muon spin rotation technique. Orientations and anisotropies of the muon hyperfine interactions are determined.     

Identification of free radicals induced by UV irradiation in collagen water solutions

Electron paramagnetic resonance (EPR) method has shown that hydrogen atoms and acetic acid free radicals appear in surrounding acetic acid-water solution of collagen under ultraviolet (UV) irradiation. These free radicals interact with the collagen molecule; consequently, seven superfine components of EPR spectrum with the split of aH = 11.3G and g-factor 2.001 appear. It is assumed that this spectrum is related to the free radical occurred on the proline residue in collagen molecule. In order to discover .OH hydroxyl radicals even in minor concentration, spin trap 5.5-dimethyl-1-pyrroline N-oxide (DMPO) has been applied. During the irradiation of collagen water solution in the presence of spin trap, EPR spectrum of the DMPO/.OH adduct has not been identified, while the above mentioned spectrum has been observed once the hydrogen peroxide H2O2 and FeSO4 were added to the sample. That means that water photolysis does not take place in collagen water-solution due to UV irradiation. It was suggested that occurrence of hydrogen radical is connected with the electron transmission to the hydrogen ion. The possible source of free electrons can be aromatic residues, photo ionization of which takes place in collagen molecule due to UV irradiation.     

An electron spin resonance study of X-irradiated single crystals of the triphenylphosphine-borontrichloride adduct: the triphenylchlorophosphorane radical

Certain lines in the ESR spectrum of X-irradiated Ph₃P·BCl₃ are attributed to Ph₃PCl and this radical is shown to have local C_3v symmetry with the unpaired electron in a P-Cl σ^* orbital.     

Selective formation of radicals in irradiated single crystals: 5,5′-dihydroxybarbituric acid trihydrate

A single crystal ESR study of the title compound shows that the oxy radical RO· is selectively formed in one of the two OH groups attached to the same carbon atom. In relation to this observation, selective formation of radicals in some symmetrical molecules is discussed. It is suggested that a slight deviation from exact symmetry causes highly selective reactions in the crystalline media. Irradiated single crystals offer us a well defined chemical reaction field in which selective reactions take place. Studies from such a view may be of considerable interest in relation to enzymatic activity.     

Behavior of vanadium dioxide single crystals synthesized under the various oxygen partial pressures at 1500 K

The effects of nonstoichiometry upon the behavior of vanadium dioxide single crystals in the vicinity of the semiconductor/metal transition temperature (T_c) were experimentally investigated. According to the electrical and thermal measurements, more stoichiometric vanadium dioxide exhibited the less electrical conductivity gap, the larger thermal and electrical hysteresis, and the lower transition temperature than the increased nonstoichiometric one near transition. Infrared absorptions and X-ray observations indicated the local and overall lattice distortion in the nonstoichiometric crystal due to the existence of V⁵⁺ ions. Furthermore, an intermediate phase between the low-temperature monoclinic and the high-temperature tetragonal phases was found in the nonstoichiometric VO₂. On the other hand, no evidence for this intermediate phase was found in the stoichiometric one. Finally, some comparisons and discussions of our present data with the previously published ones were made.