Radiation-induced radical formation in solid state sugars: a review of recent EMR and DFT results

Carbohydrates are important constituents of several biological systems, including DNA, and elucidating their radiation chemistry is therefore of general importance. In particular, sugar radicals play a crucial role in radiation-induced single and double strand breaks in DNA, which can lead to mutations and, finally, cancer. Certain sugars such as sucrose (table sugar) are also promising dosimetric materials. An advanced knowledge of the radiation-induced processes in carbohydrates may therefore provide better insight into the DNA radiation chemistry and aid in establishing reliable sugar dosimetry protocols. The first step in acquiring such knowledge is identification of the radical structures. Electron Magnetic Resonance (EMR) experiments on irradiated sugar single crystals allow a very detailed characterisation of the radicals via the g-tensor and the hyperfine interactions between the unpaired electron spin and the nuclear spins in the lattice. Single crystals also offer the advantage of mimicking to some extent the compact structure of chromosomal DNA. Numerous EMR studies on single crystals of sugars and sugar derivatives have been performed the last decades, but radical identification by EMR experiments alone is often ambiguous and sometimes not feasible. The last few years, highly accurate Density Functional Theory (DFT) calculations on extended organic solid state systems have become possible. These provide a powerful tool to help clarify and interpret experimental results and enable unambiguous structural identifications that were not possible before. In this talk, an overview will be given of recently identified radiation-induced radicals in single crystals of sugars (e.g. sucrose,1,2,3 fructose4) and sugar derivatives (e.g. glucose 1-phosphate5,6). The results pertain to primary as well as intermediate and stable species and the identifications are mainly based on the agreement, both in principal values and directions, between experimentally determined and DFT calculated proton hyperfine tensors. Common structural features are highlighted and possible commonly operative formation mechanisms are discussed

Modeling Radiation-Damage Processes in Organic Solids via DFT Calculations of EMR Parameters

High-energy radiation induces radicals in organic materials. When created in biological macromolecules such as DNA, these can cause harm to living organisms. This detrimental effect is also exploited for the sterilisation of e.g. foodstuffs, and radiation-induced radicals are used for dosimetry purposes. Knowledge of the structure of the radicals and their formation mechanisms is therefore of fundamental importance. In particular, radiation-induced radicals in solid sugars are studied (i) as model systems to gain insight into the precise role of the deoxyribose unit in the radiation chemistry of DNA and (ii) because of their potential as (emergency) dosimeters. X-irradiation typically gives rise to a variety of primary radicals in these systems, which then transform into stable radicals or diamagnetic species via one or more radical reactions. A prerequisite for unraveling the formation mechanisms is the identification of the different intermediate (semistable) radicals. Experimentally, solid-state sugar radicals can be characterised in detail by electron magnetic resonance (EMR) experiments. These allow determination of EMR parameters which describe the interaction of the unpaired-electron spin with its lattice environment, e.g. with (nearby) nuclear spins. Theoretical calculations of EMR parameters with DFT codes are increasingly being used to help clarify, interpret and explain experimental results. Recently we have managed, in a combined experimental and theoretical approach, to identify the structure of the major radiation-induced stable radicals in solid sucrose [1,2,3] (see Figure). We currently are investigating their formation mechanism, also via both EMR experiments and DFT modeling. A summary of the results obtained so far are presented

Noise in Electron Devices

Contains reports on one research project.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 36-039-AMC-03200(E

Combined Electron Magnetic Resonance and Density Functional Theory Study of Thermally Induced Free Radical Reactions in Fructose and Trehalose Single Crystals

Both as models for studying the effects of radiation on the DNA sugar unit and for applications in dosimetry, radiation-induced defects in sugars have in the past few decades been intensively studied with electron magnetic resonance (EMR) techniques, often with considerable success. However, irradiation generally gives rise to a large variety of free radicals, resulting in strongly composite Electron Paramagnetic Resonance (EPR) spectra. This complexity makes studying them quite a challenge. Despite considerable efforts, little is still known about the identity of the radicals and even less about the radical formation and transformation processes and mechanisms. At room temperature (RT) the primary radiation products, which may be stabilized upon low temperature (LT) irradiation, transform into stable radicals via multistep reaction mechanisms. While the species formed at LT are expected to be formed by simple processes, the molecular structure and geometry of the stable radicals may differ considerably from that of the intact molecule even in the solid state (crystals). Studying the intermediate radicals in the reactions occurring after LT irradiation helps elucidating the formation and identity of the stable radicals. The structural identification of these radicals is in most cases the result of a combination of EPR, Electron Nuclear Double Resonance (ENDOR) and ENDOR Induced EPR (EIE) experiments and advanced quantum chemistry calculations based on Density Functional Theory (DFT). In the present study a summary is given of the experimental EMR results obtained so far on radiation-induced radicals at different temperatures in fructose and trehalose single crystals and powders. “In situ” X-irradiation at LT (10 K) without annealing, leads to spectra strongly different from those observed after RT irradiation and offers the possibility to study and characterize the primary radiation products [1]. Performing EMR measurements on samples irradiated and/or annealed at various temperatures between LT (10 K or 77 K) and RT allows us to study the intermediate products, and such studies therefore have the potential to devise mechanistic links between the primary radicals and the stable radicals. In the present work, our own measurements are compared with results reported in the EMR literature. An outline at future experimental (EMR) and theoretical (DFT) research will also be given

Which role do excited states play in radiation damage to organic solid-state compounds?

Ionizing radiation induces radicals in organic materials. When such species are created in biological macromolecules like DNA, they harm living organisms. This detrimental effect is explicitly exploited for the sterilisation of e.g. foodstuffs, and radiation-induced radicals are quantitatively used for radiation dosimetry purposes. For understanding radiation actions at different levels of molecular and cellular organisation, knowledge of the radical structures and their formation mechanisms is of fundamental importance. In this context, radiation-induced processes in solid sugars are studied, among others, to gain insight into the role of the deoxyribose unit in the radiation chemistry of DNA. X-irradiation typically gives rise to a variety of primary radicals in these systems, which transform into stable radicals or diamagnetic species via one or more radical reactions. By combining electron magnetic resonance experiments and density functional theory (DFT) calculations, we recently identified the major stable [1,2], as well as the major primary [3] radiation-induced radicals in solid sucrose (see figure). We are currently investigating how the primary radicals transform into the stable ones. A general but important observation is that in sucrose and similar carbohydrates, e.g. rhamnose, the primary radical formation (typically by way of net H-abstraction) is selective: it preferentially takes place at particular carbons and oxygens. This selectivity apparently cannot be explained simply on thermodynamical grounds. It may be hypothesised that, after the initial oxidation event (leaving the radical cation in an excited state), the hole ‘migrates’ to a particular carbon or oxygen, after which de-excitation and deprotonation processes yield a neutral radical. It is our goal to examine factors possibly explaining the experimentally observed selectivity. So far we have made some preliminary ground-state calculations on energy profiles of deprotonation reactions in rhamnose single crystals, as well as time-dependent DFT calculations of excited states in this system

Direct-effect radiation chemistry of solid-state carbohydrates using EMR and DFT

To contribute to a mechanistic understanding of radical reaction pathways in the sugar-phosphate backbone of DNA, we are investigating primary radicals induced by X-rays, as well as their transformation into stable radicals or diamagnetic products, in crystalline sugar and sugar derivatives. Radicals are identified and characterized mainly via the hyperfine interactions of the electron spin with protons in the molecular environment. These interactions are determined experimentally with electron magnetic resonance (EMR) techniques and compared to theoretical ab initio calculations based on density functional theory in a periodic approach. Different stages of the radiation-induced processes are investigated by irradiating in situ at various temperatures and controlled annealing experiments. Here, results obtained in single crystals of the dipotassium salt of glucose 1-phosphate (K2G1P) and the disaccharides sucrose and trehalose are presented. The dominant radical in K2G1P after irradiation at 77 K exhibits a broken phospho-ester bond and is chemically identical to one of the major stable sucrose radicals, the latter all being characterized by a broken glycosidic bond. This suggests that the ester bond is radiation sensitive and that the phosphate group is not essential for the reaction pathway leading to this scission. Surprisingly, however, no evidence for glycosidic bond scission has so far been observed in trehalose. Rather, a simple H-abstraction alkyl radical is remarkably stable in this system. In all three compounds, dominant radicals are formed with one or several concerted carbonyl group formations. Extended studies are necessary to establish how and to which extent structural or geometrical factors determine the radiation chemistry, but certain general principles are starting to emerge