Lithium formate for EPR dosimetry: radiation-induced radical trapping at low temperatures.

Radiation-induced primary radicals in lithium formate. A material used in EPR dosimetry have been studied using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-Induced EPR (EIE) techniques. In this study, single crystals were X irradiated at 6-8 K and radical formation at these and higher temperatures were investigated.

Solved? The reductive radiation chemistry of alanine.

The structural changes throughout the entire reductive radiation-induced pathway of l-α-alanine are solved on an atomistic level with the aid of periodic DFT and nudged elastic band (NEB) simulations. This yields unprecedented information on the conformational changes taking place, including the protonation state of the carboxyl group in the "unstable" and "stable" alanine radicals and the internal transformation converting these two radical variants at temperatures above 220 K. The structures of all stable radicals were verified by calculating EPR properties and comparing those with experimental data.

Dominant stable radicals in irradiated sucrose: g tensors and contribution to the powder electron paramagnetic resonance spectrum.

Ionizing radiation induces a composite, multiline electron paramagnetic resonance (EPR) spectrum in sucrose, that is stable at room temperature and whose intensity is indicative of the radiation dose. Recently, the three radicals which dominate this spectrum were identified and their proton hyperfine tensors were accurately determined. Understanding the powder EPR spectrum of irradiated sucrose, however, also requires an accurate knowledge of the g tensors of these radicals.

Structural specificity of alkoxy radical formation in crystalline carbohydrates.

A DFT study of radiation induced alkoxy radical formation in crystalline α-l-rhamnose has been performed to better understand the processes leading to selective radical formation in carbohydrates upon exposure to ionizing radiation at low temperatures. The apparent specificity of radiation damage to carbohydrates is of great interest for understanding radiation damage processes in the ribose backbone of the DNA molecule. Alkoxy radicals are formed by deprotonation from hydroxyl groups in oxidized sugar molecules.

Room temperature radiation products in trehalose single crystals: EMR and DFT analysis.

Radicals generated in trehalose single crystals by X radiation at room temperature were investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR measurements, together with periodic density functional theory calculations. In the first days after irradiation, three radical species (I1, I2 and I3) were detected, two of which (I1 and I2) dominate the EPR spectrum and could be identified as H-abstracted species centered at C3' (I1) and C2 (I2), the latter with additional formation of a carbonyl group at C3. Annealing the sample at 40 °C for 3 days or storing it in ambient conditions for three months resulted in another, more stable EPR spectrum.

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.

Identification of primary free radicals in trehalose dihydrate single crystals X-irradiated at 10 K.

Primary free radical formation in trehalose dihydrate single crystals X-irradiated at 10 K was investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques. The ENDOR results allowed the unambiguous determination of six proton hyperfine coupling (HFC) tensors. Using the EIE technique, these HF interactions were assigned to three different radicals, labeled R1, R2 and R3.

On the identity of the radiation-induced stable alanine radical.

Using periodic DFT calculations, it is concluded that the stable radiation-induced alanine radical most probably is the result of reductive deamination and protonation of the detached amino group, yielding an NH(4)(+) ammonium ion and a negatively charged radical.

Oxidation and reduction products of X irradiation at 10 K in sucrose single crystals: radical identification by EPR, ENDOR, and DFT.

Electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) measurements on sucrose single crystals at 10 K after in situ X irradiation at this temperature reveal the presence of at least nine different radical species. Nine proton hyperfine coupling tensors were determined from ENDOR angular variations and assigned to six of these species (R1-R6) using EIE. Spectral simulations indicate that four of those (R1-R3 and R6) dominate the EPR absorption.

Early-stage evolution of the EPR spectrum of crystalline sucrose at room temperature after high-dose X irradiation.

Abstract X irradiation of sucrose single crystals at room temperature leads to the production of stable radicals, which give rise to the dosimetric electron paramagnetic resonance (EPR) signal. In the first few hours after irradiation, however, the shape of the EPR spectrum changes drastically. Based on two-dimensional field-frequency electron nuclear double resonance (FF-ENDOR) measurements, we demonstrate that, after high-dose ( approximately 5 kGy) and high-dose-rate irradiation, several species with limited stability at room temperature are produced next to the stable radicals.

Determination of the g tensors for the dominant stable radicals in X-irradiated beta-D-fructose single crystals.

In spite of recent successful identifications of radicals produced after X-ray irradiation at 10 and 77 K in beta-D-fructose, the structure of the two stable radicals dominating the electron paramagnetic resonance (EPR) spectrum after room temperature irradiation is still unclear. Based on the agreement between proton hyperfine (HF) tensors obtained in electron nuclear double resonance (ENDOR) experiments and the results of single molecule density functional calculations, a model for these radicals, involving OH abstraction at the C2 ring position, had previously been proposed, but this assignment could not be confirmed when the radical was embedded in a crystal environment. In this paper, we therefore provide additional experimental information for these radicals.

ENDOR and HYSCORE analysis and DFT-assisted identification of the third major stable radical in sucrose single crystals X-irradiated at room temperature.

Recently, the chemical structure of two of the three major stable radicals (T2 and T3) produced in sucrose single crystals by X-irradiation at room temperature was identified by comparing Density Functional Theory (DFT) calculations of Electron Magnetic Resonance parameters with experimental results [H. De Cooman, E. Pauwels, H.

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.

Schonland ambiguity in the electron nuclear double resonance analysis of hyperfine interactions: principles and practice.

For the analysis of the angular dependence of electron paramagnetic resonance (EPR) spectra of low-symmetry centres with S=1/2 in three independent planes, it is well-established-but often overlooked-that an ambiguity may arise in the best-fit g<--> tensor result. We investigate here whether a corresponding ambiguity also arises when determining the hyperfine coupling (HFC) A<--> tensor for nuclei with I=1/2 from angular dependent electron nuclear double resonance (ENDOR) measurements. It is shown via a perturbation treatment that for each set of M(S) ENDOR branches two best-fit A<--> tensors can be derived, but in general only one unique solution simultaneously fits both.

Temperature study of a glycine radical in the solid state adopting a DFT periodic approach: vibrational analysis and comparison with EPR experiments.

The major radiation-induced radical in crystalline glycine is examined using DFT calculations, in which both molecular environment and temperature are accounted for. This is achieved by molecular dynamics simulations of the radical embedded in a supercell under periodic boundary conditions. At 100 and 300 K, a vibrational analysis is performed based on Fourier transformation of the atomic velocity autocorrelation functions.

Identification and conformational study of stable radiation-induced defects in sucrose single crystals using density functional theory calculations of electron magnetic resonance parameters.

One of the major stable radiation-induced radicals in sucrose single crystals (radical T2) has been identified by means of density functional theory (DFT) calculations of electron magnetic resonance parameters. The radical is formed by a net glycosidic bond cleavage, giving rise to a glucose-centered radical with the major part of the spin density residing at the C 1 carbon atom. A concerted formation of a carbonyl group at the C 2 carbon accounts for the relatively small spin density at C 1 and the enhanced g factor anisotropy of the radical, both well-known properties of this radical from several previous experimental investigations.

Radiation-induced defects in sucrose single crystals, revisited: a combined electron magnetic resonance and density functional theory study.

The results are presented of an electron magnetic resonance analysis at 110 K of radiation-induced defects in sucrose single crystals X-irradiated at room temperature, yielding a total of nine (1)H hyperfine coupling tensors assigned to three different radical species. Comparisons are made with results previously reported in the literature. By means of electron paramagnetic resonance and electron nuclear double resonance temperature variation scans, most of the discrepancies between the present 110 K study and a previous 295 K study by Sagstuen and co-workers are shown to originate from the temperature dependence of proton relaxation times and hyperfine coupling constants.

EPR and ENDOR analysis of Fe3+ impurity centers in fluoroelpasolite lattices.

Fe(3+) ions in hexagonal and cubic fluoroelpasolite crystals (A(1)(2)B(I)M(III)F(6)) have been investigated in a combined Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) study. A detailed analysis of the ENDOR spectra for the nearest (19)F and (23)Na shells in X (9.5 GHz) and Q band (34 GHz) allowed the complex EPR spectra to be disentangled and to determine the spin Hamiltonian parameters for the various S = 5/2 Fe(3+) centres.