Radicals In Single Crystals Research Articles

Electron paramagnetic resonance study of the paramagnetic center in gamma-irradiated tetrahydrophthalic anhydride single crystal

Electron Paramagnetic Resonance (EPR) analysis of gamma-irradiated tetrahydrophthalic anhydride (C8H8O3) single crystals was performed at 120 K. The single crystals were exposed to 60Co-gamma source at room temperature. The EPR spectra of gamma-irradiated single crystals of tetrahydrophthalic anhydride (THPA) were recorded at 10-degree intervals for different orientations of crystals in a magnetic field. Detailed examination of THPA single crystals by EPR spectroscopy method showed the presence of two carbon-centered THPA anion radicals. Both radicals were determined to have the same structure. The anion radicals observed in gamma-irradiated THPA single crystal were created by the scission of the carbon–oxygen double bond. The g values of the radiation damage centers observed in the THPA single crystal and the hyperfine structure constants of the free electron with nearby protons were obtained. Highlights Single crystals of tetrahydrophthalic anhydride were exposed to gamma radiation. Radiation damage centers in single crystals of tetrahydrophthalic anhydride were determined by EPR spectroscopy method. The presence of two anion radicals in single crystals of tetrahydrophthalic anhydride was determined. The formation mechanism of the paramagnetic centers in the tetrahydrophthalic anhydride is similar to that of succinic anhydride.

Properties of gamma induced radicals in single crystal of iminodiacetic acid hydrochloride

The aim of this study is to investigate the paramagnetic species induced by gamma irradiation in single crystals of iminodiacetic acid hydrochloride, HN(CH2COOH)2·HCl, at room temperature by electron paramagnetic resonance technique, and to determine their magnetic properties. The observed species in iminodiacetic acid hydrochloride single crystals were attributed to the H2NĊHCH2(COOH)2·Cl radical. The g values and hyperfine coupling constants of the unpaired electron with the environmental α-proton, methylene protons, the NH proton, and 14N nucleus were determined. The EPR spectra of radical found to be remained unchanged for more than two months. The results were observed as being in good conformity with the existing literature data.

Phonon interactions with methyl radicals in single crystals

The high temperature ESR spectra’s anomalous appearance at very low temperatures for the methyl radical created in single crystals is explained by magnetic dipole interactions with neighboring protons. These protons acting via phonon vibrations induce resonant oscillations with the methyl group to establish a very temperature sensitive ‘‘relaxation’’ mode that allows the higher energy ‘‘E’’ state electrons with spin 12 to ‘‘decay’’ into ‘‘A’’ spin 12 states. Because of the amplitude amplification with temperature, the ‘‘E’’ state population is depleted and the ‘‘A’’ state population augmented to produce the high temperature ESR spectrum. This phenomenon is found to be valid for all but the very highest barriers to methyl group tunneling. In support, a time dependent spin population study shows this temperature evolution in the state populations under this perturbation.

Thermal stability of radiation-induced free radicals in [formula omitted]-irradiated l-alanine single crystals

Decay of the radiation-induced stable free radicals in l-alanine single crystals and powders at the temperatures from 379 to 476 K was examined by electron paramagnetic resonance. For single crystals, the calculated activation energy of the radical decay is 104.3±1.7 kJ/mol (i.e. 12 538±202 K) and the frequency factor ln ν 0 is 24.1±0.4 min −1. The lifetime of the radical in single crystals at 296 K is 162 years. The results confirm the long-term stability of the radicals, but the decay was found to be faster in large crystals than in powders.

Displacement of the tyrosyl radical cofactor in ribonucleotide reductase obtained by single-crystal high-field EPR and 1.4-A x-ray data.

The R2 protein of class I ribonucleotide reductase generates and stores a tyrosyl radical essential for ribonucleotide reduction and, thus, DNA synthesis. X-ray structures of the protein have enabled detailed mechanistic suggestions, but no structural information has been available for the active radical-containing state of the protein. Here we report on methods to generate the functional tyrosyl radical in single crystals of R2 from Escherichia coli (Y122(*)). We further report on subsequent high-field EPR experiments on the radical-containing crystals. A full rotational pattern of the spectra was collected and the orientation of the g-tensor axes were determined, which directly reflect the orientation of the radical in the crystal frame. The EPR data are discussed in comparison with a 1.42-A x-ray structure of the met (oxidized) form of the protein, also presented in this paper. Comparison of the orientation of the radical Y122(*) obtained from high-field EPR with that of the reduced tyrosine Y122-OH reveals a significant rotation of the tyrosyl side chain, away from the diiron center, in the active radical state. Implications for the radical transfer connecting the diiron site in R2 with the substrate-binding site in R1 are discussed. In addition, the present study demonstrates that structural and functional information about active radical states can be obtained by combined x-ray and high-field EPR crystallography.

An electron spin-resonance study of radicals in single crystals

An ESR study of centres in x-irradiated ammonium perchlorate single crystals is carried out that is aimed at resolving the controversy that has recently arisen over the possible occurrence of low-temperature phase transitions in the compound (room temperature structure: orthorhombic; space group Pnma). The ESR spectra are recorded as a function of the orientation of the magnetic field H in the three orthogonal crystallographic planes at 300 K, 77 K, and 4.2 K, and also as a function of temperature for H parallel to the principal z-component of the hyperfine coupling tensor. The analysis of the spectra shows that while the -tensor remained axial over the entire temperature range 2.0052(3), 2.0055(3), 2.0057(3) and 2.0090(3), 2.0096(3), 2.0105(3) at 300 K, 77 K, and 4.2 K respectively), , which is rhombic at room temperature , , and at 300 K) becomes more axial as the temperature is lowered , , and at 77 K, and , , and at 4.2 K). No changes in the spectra attributable to any phase transition were observed, and we conclude that does not undergo any phase transition below room temperature. Furthermore, it is found that during radiative decomposition the mirror-related Cl - O bonds, even though weaker than the other Cl - O bonds, do not break, thus confirming our earlier conclusion that mirror symmetry is preserved during radiation damage.

Simulation of ENDOR spectra of radicals with anisotropic hyperfine and nuclear quadrupolar interactions in disordered solids

Abstract A general theory to calculate ENDOR spectra of S = 1 2 radicals in single crystal or disordered solids is presented. The theory is suitable for paramagnetic systems with n interacting nuclei which can be described by the spin-Hamiltonian H = β B g S + Σ i=1 n I i A i S + I i Q i I i − g i β N BI i . There are restrictions on the relative magnitude of the hyperfine, quadrupolar and nuclear Zeeman interactions, nor on the relative orientation of principal axes of the tensors. ENDOR transition frequencies and intensities are calculated with perturbation theory under the assumption that the electron Zeeman interaction is dominating. A simple formula for first order transition moments, including the effect of the quadrupolar coupling, is presented. The impact on the transition moment of the orientational distribution of the radio-frequency field in single crystal and disordered systems is taken into account. The method is tested against experimental proton- and 14N-ENDOR data of neutral and ionic radicals in disordered solids.

High-field EPR on the primary electron donor cation radical in single crystals of heterodimer mutant reaction centers of photosynthetic bacteria — first characterization of the G-tensor

First high-field/high-frequency (95 GHz) EPR experiments on the cation radical of the primary electron donor (D) in single crystals of the HL (M202) heterodimer mutant of the photosynthetic bacterium Rhodobacter sphaeroides are reported. The measurements have been performed in order to determine the electronic G-tensor of D+ in the mutant and to compare it to the G-tensor of D+ in wild type (R-26). In the heterodimer mutant of the two bacteriochlorophyll (BCHl) molecules constituting D in R-26 is replaced by a bacteriopheophytin. In D2+ of the mutant the unpaired electron is localized on the BChl, thus, its G-tensor should reflect monomer properties, whereas in D+ of R-26 the unpaired electron is delocalized over the two BChl molecules. The preliminary results reported here show that directions of the principal axes of the G-tensor of D+ in HL (M202) are similar to those of R-26 investigated previously. Possible origins of this result are discussed.

Determination of the g tensor of the primary donor cation radical in single crystals of Rhodobacter sphaeroides R-26 reaction centers by 3-mm high-field EPR

Three millimeter (W band, 95 GHz) EPR meassurements were performed on single crystals of Rb. sphaeroides R-26 reaction centers at room temperature to determine the g tensor of the primary donor cation radical P 865 .+ . Due to the applied high magnetic field (3.4 T), the magnetically inequivalent sites in the unit cell of the crystals could be resolved. Some of the results are corroborated by additional measurements on P 865 .+ in perdeuterated reaction centers from fb. sphaeroides wild type 2.4. 1, which could be crystallized for the first time

3 mm EPR investigation of the primary donor cation radical in single crystals of Rhodobacter sphaeroides R-26 reaction centers

The EPR g-factor rotation pattern of the primary donor cation radical ▪ in the x y plane of illuminated reaction center single crystals of Rb. sphaeroides R-26 near room temperature was measured at 3 mm wavelength (W-band, 95 GHz). In contrast to earlier X-band (9.5 GHz) EPR experiments, the two magnetically inequivalent sites of the four reaction centers per unit cell were well resolved. ▪ in frozen reaction center solution was also measured by 3 mm EPR. The analysis of both the frozen-solution spectrum and the single-crystal rotation pattern yields the principal values of the g-tensor and the phase angle between the two magnetically inequivalent sites in the x y plane.

ESR Study of the Thymine Anion Radical in a Single Crystal of Thymine Monohydrate

ESR spectroscopy was used to study free radicals in an irradiated single crystal of thymine monohydrate at 77 K. At low microwave power (20 microW), a broad doublet is found to be super-imposed on the well known resonance patterns of the 5-yl and 7-yl radicals. The doublet spectrum has been analysed as a difference spectrum. Its spectroscopic properties and the observed transformation into the 5-yl, H-addition radical on warming the crystal are consistent with its anion nature, T(-).

Radiation damage in androst-4-en-3,17-dione molecules: an ESR study of free radicals in single crystals

Electron spin resonance spectroscopy has been used to investigate γ-radiation damage of androst-4-en-3,17-dione molecules in single crystals. The spectra found are attributed to hydrogen abstraction radicals. It is possible to assign a definite structure to the specimen by analyzing the orientational variation of the spectra. The unpaired electron of the radical is delocalized in the 2 p z-orbitals of the C(6), C(4) and O(3) atoms giving rise to a hyperfine spectrum from interaction with two equivalent α-protons in positions 4 and 6 and with two non-equivalent β-protons attached to C(7). The theoretical splittings and the spin density distribution of the unpaired electron are calculated by the INDO method.

Free Radical Formation in Crystals of 2′-Deoxyguanosine 5′-Monophosphate Irradiated at 15 K: An ESR Study

Radiation-induced radicals in single crystals of 2'-deoxyguanosine 5'-monophosphate (5'-dGMP) at 15 K have been studied by electron spin resonance (ESR) spectroscopy. At low temperatures three radicals were analyzed in detail. The negatively charged pi anion of the guanine base completely dominated the spectra. Weaker resonances were due to an alkoxy radical with the spin density in the C3'-O3' region of the sugar moiety as well as another sugar-centered radical. The anion rapidly decayed upon exposure to uv light at 15 K or by annealing above 25 K. In both cases no successor radical was observed. The second sugar-centered radical decays at 200 K with a concomitant appearance of the resonance from the C8 H-addition radical. By annealing at 295 K the latter resonance was the only one observed. After irradiation at 295 K, however, an additional resonance from a sugar-centered radical, which has been analyzed previously by B. Rakvin and J. N. Herak (Radiat. Res. 88, 240-250 (1981)) was observed. A reinvestigation of this resonance was performed.

ESR characterization of the hydroxyalkyl radical in single crystals of 1,6-hexanediol and 1,8-octanediol and crystal structure of 1,6-hexanediol

ESR measurements have been performed at Q-band on X-irradiated single crystals of 1,6-hexanediol and 1,8-octanediol at room temperature. The stable radical is of the type −CH 2−CH·−OH. The g-value is anisotropic in the range 2.0025–2.0041. Four anisotropic hyperfine interactions could be resolved and the corresponding tensors have been calculated with high accuracy to give well-fitted simulations of experimental spectra. The crystal structure of 1,6-hexanediol was determined from X-ray diffractometer data collected at 295 K.

ENDOR analysis of H-addition radicals and their conversion in irradiated crystals of tryptamine hydrochloride

In a previous study it was shown that the dominant radiation-induced room-temperature radical in single crystals of tryptamine⋅HC1 is formed by a net H addition to C7 of the indole ring. In the present work, it is demonstrated that this species is the precursor for two other indole H-addition radicals. By storage at room temperature, the C7 adduct slowly converts into the C4 adduct. The formation of the C4 adduct could easily be followed by ENDOR. The ENDOR spectra are characterized by hyperfine coupling to six protons bonded to the indole moiety. Upon annealing the crystal above 70 °C, the C7 adduct is more rapidly converted into the C4 adduct, but then the annealing also induces a simultaneous conversion into the C2 adduct. This species also is characterized by six proton hyperfine interactions in the strong-coupling region. The C7 add→C2 add process could be reversed by exposure to UV light at room temperature, whereas the C7 add→C4 add process is irreversible upon this procedure. Semiempirical molecular orbital calculations support the interpretations made. The mechanisms of the different processes is discussed in light of the crystal and molecular structure of the compound, and related to similar processes in other compounds, e.g., the purine and pyrimidine bases.

The indole H-adduct radical in single crystals of tryptamine⋅HCL: An ESR–ENDOR study

Single crystals of tryptamine hydrochloride have been irradiated with 4.0 MeV electrons at 295 K and studied by ESR and ENDOR techniques. The main species trapped at 295 K was identified to be a radical formed by hydrogen addition to the C7 position of the indole moiety. The ENDOR analysis of five proton interactions in the strong coupling region (ν≳17 MHz) yielded the supporting evidence for this interpretation. Two of the couplings are due to the methylene β protons, with principal values (corrected for second-order contributions) of 100.9, 96.4, and 94.2 MHz, and 125.6, 121.3, and 119.2 MHz. The other three are are due to α protons, two of them of the ordinary type with principal values of −43.9, −27.5, and −13.0 MHz, and −57.4, −34.8, and −18.2 MHz. The third α-proton interaction is due to the central proton in an allyl fragment, exhibiting a coupling tensor with principal values of 11.72, 7.74, and 3.86 MHz. The g value is nearly isotropic at 2.0022. Spectral simulations show that these five interactions are the only ones resolved by ESR at any orientation of the crystal in the applied magnetic field.

Free radicals in dicarboxylic acids: an e.s.r. study of radical conversions in gamma-irradiated single crystals of glutaric acid and glutaric-2,2,4,4-d4 acid.

The gamma-radiation-induced free radicals in single crystals of glutaric acid and glutaric-2,2,4,4-d4 acid were studied in the temperature range 77-300 K by e.s.r. techniques. At 77 K the decarboxylation radical and the anion are stabilized. At higher temperatures the decarboxylation radical is found to be converted into a hydrogen abstraction radical with an activation energy of 6.3 +/- 0.5 kcal/mole for the non-deuterated crystal. This radical is stable at room temperature. The anion seems be be converted to an unidentified intermediate radical which in turn is converted to the gamma-acyl radical. An analysis of the g-value anisotropy and of the 13C hyperfine splitting variation for this radical in the deuterated crystal is consistent with the assigned radical structure. By heat treatment the alpha-acyl radical is converted to another form of the hydrogen abstraction radical with an activation energy of 9.6 +/- 0.6 kcal/mole in the deuterated crystal. U.V.-light (gamma = 254 nm) transforms one of the room temperature radicals into the other.

Radiation Damage in Solid 5-halouracils: Free Radicals in Single Crystals of 5-fluorouracil

X-irradiation at 300 K of single crystals of 5-fluorouracil results in the formation of two different radical species observable by e.s.r. and ENDOR-spectroscopy. One is an alpha-fluoro radical RCF(CH2)R' formed by saturation of the 5,6-double bond of the pyrimidine ring. The principal values of its alpha-fluorine interaction are 170, -9 and -18 G; the isotropic part of the methylene beta-proton couplings are 46.3 and 28 G, respectively; the g-tensor has principal values of 2.0029, 2.0066, and 2.0052. The other radical species is formed by enolization of the C4-carbonyl function. The resulting spin-density distribution gives rise to three observable interactions, an alpha-proton (-5.2, -14.9, -11.2 G) at C6, an OH-proton from the C4--OH group (-1.7, -5.0, -4.1 G) and a residual alpha-fluorine interaction (0,0,11.2G). Irradiation at 77 K yields an e.s.r.-pattern which is tentatively assigned to the molecular anion radical. These findings are related to the radiation chemistry of solid 5-halouracils.

Free radicals in dicarboxylic acids: an e.s.r. study of gamma-irradiated single crystals of glutaric acid at 77 K.

Electron spin resonance techniques were used to study the gamma-radiation-induced free radicals in single crystals of glutaric acid in the temperature range from 77 K to 300 K. Three different radicals are stabilized at 77 K. The decarboxylation radical is the dominant species and the other two radicals are assigned to the anion and to the substituted acetyl sigma-radical. When the temperature of the crystal is raised, these radicals disappear and the previously studied room temperature radicals appear. E.S.R.-data and the results from semi-empirical INDO-MO calculations were compared in order to elucidate the structures of the various radicals.

X-ray Induced Free Radicals in Single Crystals of N-acetyl-L-leucine

X-ray Induced Free Radicals in Single Crystals of N-acetyl-L-leucine