Identification and Conformational Study of Stable Radiation-Induced Defects in Sucrose Single Crystals using Density Functional Theory Calculations of Electron Magnetic Resonance Parameters

Abstract

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. The experimentally determined and DFT calculated proton hyperfine coupling tensors agree very well on all accounts. The influence of the exact geometrical configuration of the radical and its environment on the tensors is explored in an attempt to explain the occurrence and characteristics of radical T3, another major species that is most likely another conformation of T2. No definitive conclusions with regard to the actual structure of T3 could be arrived at from this study. However, the results indicate that, most likely, T3 is identical in chemical structure to T2 and that changes in the orientation of neighboring hydroxy groups or changes in the configuration of the neighboring fructose ring can probably not account for the type and size of the discrepancies between T2 and T3.

Determination of the g Tensors for the Dominant Stable Radicals in X-Irradiated β-d-Fructose Single Crystals

“…The molecular structure and geometry of the latter may differ considerably from those of the intact molecules, even in the solid state (crystals). This was recently convincingly established for sucrose single crystals, where all three dominant stable radicals were shown to involve glycosidic bond rupture and carbonyl formation on a carbon atom adjacent to the main site of unpaired electron density 7,14,15 . A prominent radical in glucose-1-phosphate, detected at 77K after in situ Xirradiation at 77 K, was also found to display these characteristics 16,17 .…”

Radiation Chemistry of Solid-State Carbohydrates Using EMR

“…It was verified that periodic cell sizes larger than about 1 nm along each crystal axis do no longer demonstrate influences of interactions between radicals in periodic images (see e.g. [8]). As opposed to single molecule and cluster calculations, no constraints on atomic relaxations have to be imposed in this type of calculations.…”

“…In these first attempts to properly take into account the full lattice environment of the radical, SH parameter calculations were still performed on clusters cut out of the periodically optimized structure, because g and HF tensors reflect local properties, which are known not to be well reproduced by plane wave basis sets [8,9,14,[17][18][19]55]. A further major improvement in the calculations came from the implementation of HF [98] and g tensor calculations [99,100] in periodic codes (CP2K) using Gaussian and augmented plane wave basis sets.…”

“…Also these two radicals have two more pronounced HF interactions, which are strongly overlapping at this particular field orientation and for this reason are not indicated in the FF-ENDOR spectrum. The T1-T3 radicals have been thoroughly characterized through angular dependent EPR and ENDOR measurements [55] and detailed molecular models were found for them, based on DFT calculations (see Section 6.3.4) [8,9]. They constitute the dominant contribution to the central part of the powder EPR spectrum of irradiated sucrose (see Section 6.4.5).…”

“…However, only some five years ago the identity of three radicals, dominating the room temperature (RT) stable electron paramagnetic resonance (EPR) spectrum, was convincingly established [8,9]. This resulted primarily from a detailed comparison of proton hyperfine (HF) interactions, determined from single crystal electron nuclear double resonance (ENDOR) experiments, with the results of highlevel density functional theory (DFT) calculations.…”