Oxidation and Reduction Products of X Irradiation at 10 K in Sucrose Single Crystals: Radical Identification by EPR, ENDOR, and DFT Hendrik De Cooman, †,‡ Ewald Pauwels, ‡ Henk Vrielinck, † Einar Sagstuen, § Michel Waroquier, ‡ and Freddy Callens* ,† Department of Solid State Sciences, Ghent UniVersity, Krijgslaan 281-S1, B-9000 Gent, Belgium, Center for Molecular Modeling, Ghent UniVersity, Technologiepark 903, B-9052 Zwijnaarde, Belgium, and Department of Physics, UniVersity of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway ReceiVed: September 25, 2009; ReVised Manuscript ReceiVed: NoVember 4, 2009 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. Assisted by periodic density functional theory (DFT) calculations, R1 and R2 are identified as H-abstracted C1- and C5-centered radicals, R3 is tentatively assigned to an H-abstracted C6-centered radical, and R6 is identified as an alkoxy radical where the abstracted hydroxy proton has migrated to a neighboring OH group via intermolecular proton transfer. The latter radical had been characterized and identified in a previous study, but the present DFT calculations provide additional insight into its conformation and particular properties. This study provides the first step in unraveling the formation mechanism of the stable sucrose radicals detected after room-temperature irradiation and contributes to the understanding of the initial stages of radiation damage to solid-state carbohydrates. 1. Introduction Direct radiation effects, i.e., processes initiated by radiation energy deposited at the target molecule itself and not its environment, in the deoxyribose moiety of DNA have been shown to lead to cleavage of sugar-phosphate bonds in the DNA backbone (single and double strand breaks). Such lesions may in turn result in cell death, cancer, and mutagenesis. 1-3 The complex structure of DNA necessitates the study of smaller model systems exhibiting key features similar to the deoxyribose- (-phosphate) moiety. In this context, carbohydrate single crystals are suitable model systems because the crystalline state to some extent mimics the rigid, tightly packed structure of chromosomal DNA, and similar general principles may be expected to govern the formation mechanisms of primary radicals in these systems. As carbohydrates are fundamental constituents of several biological systems, understanding their radiation physics and chemistry is also of a more general importance. A prerequisite to obtain such fundamental insight is, however, detailed knowledge of the electronic and molecular structure of the primary radiation-induced radicals. In carbohydrate single crystals, X-ray irradiation mainly generates cations, i.e., one-electron-loss products, in the initial stage. The expelled electrons slow down through successive scattering events (producing secondary electrons) and are finally trapped either in a valence orbital of a molecule (yielding anions) or in a lattice void, between hydroxy groups of one or several neighboring molecules. 4-12 In general, the primary radicals then undergo complex multistep processes of subsequent chemical reactions, eventually yielding either stable (usually neutral) radical species or diamagnetic products (recombination). Primary and intermediate radicals often are metastable, in which case they can be studied experimentally by irradiating the sample in situ at low temperatures and performing electron magnetic resonance (EMR) experiments without annealing. As EMR experiments can provide a wealth of detailed structural information, such studies have been performed quite successfully for several carbohydrates and their derivatives (e.g., rham- nose, 7a,8-10,12-14 sucrose 7a,11,15 glucose-1-phosphate, 16,17 glucose, 18,19 methyl-glucose, 20-23 -D-fructose, 24,25 and trehalose 26,27 ). However, unambiguous identification of the major radical species based on EMR experimental data only remains very difficult, and often proves impossible. In recent years, highly accurate density functional theory (DFT) calculations on extended organic solid- state systems have become possible and practically available due to advances in computing power and density functionals, as well as to the development of new, more efficient codes for the calculation of EMR parameters. 28,29 Such calculations provide a powerful tool for the identification and detailed analysis of radical species. A combined approach extensively using different EMR methods and carefully designed DFT calculations has proven very successful for unambiguous identification and characterization of radiation-induced radicals as well as for the elucidation of their mechanisms of formation and decay. 24,30-35 Sucrose (Figure 1) offers an extra advantage as a model system for DNA because the glycosidic bond bears similarity to the sugar-phosphate bond in the DNA backbone. Recently, we identified the chemical structures of the three major stable radiation-induced radicals in sucrose single crystals, 34,36 and the glycosidic bond was shown to be ruptured in all three. Detailed * To whom all correspondence should be addressed. Phone: 003292644352. Fax: 003292644996. E-mail: freddy.callens@ugent.be. † Department of Solid State Sciences, Ghent University. ‡ Center for Molecular Modeling, Ghent University. § University of Oslo. J. Phys. Chem. B 2010, 114, 666–674 666 10.1021/jp909247z  2010 American Chemical Society Published on Web 11/19/2009