Radiation Chemistry of Solid-State Carbohydrates Using EMR

Abstract

We review our research of the past decade towards identification of radiation-induced radicals in solid state sugars and sugar phosphates. Detailed models of the radical structures are obtained by combining EPR and ENDOR experiments with DFT calculations of g and proton HF tensors, with agreement in their anisotropy serving as most important criterion. Symmetry-related and Schonland ambiguities, which may hamper such identification, are reviewed. Thermally induced transformations of initial radiation damage into more stable radicals can also be monitored in the EPR (and ENDOR) experiments and in principle provide information on stable radical formation mechanisms. Thermal annealing experiments reveal, however, that radical recombination and/or diamagnetic radiation damage is also quite important. Analysis strategies are illustrated with research on sucrose. Results on dipotassium glucose-1-phosphate and trehalose dihydrate, fructose and sorbose are also briefly discussed. Our study demonstrates that radiation damage is strongly regio-selective and that certain general principles govern the stable radical formation.

EMR Study and DFT-Assisted Identification of Transient Radicals in X-Irradiated Crystalline Sucrose

“…A comparable mismatch between experimental results and calculated HF tensors was obtained and attributed to the more complex structure of the radical than a simple H abstraction. This radical, which is unstable above 50 K, 8 is clearly different from U1. Another argument to favor C6 over C6′ as the location for U1 comes from the relative magnitude of calculated g values in the ⟨a*bc⟩ frame.…”

“…In view of the fact that radical ions in carbohydrates are rare, centers were assumed to be neutral. 8 Fourteen radicals fall under these constraints, many of which can be eliminated by using a more careful inspection of the potential radical sites, the point dipole approximation, and semiempirical rules, 41,42 before a final confrontation with high-level DFT calculations. The smallest deviation angles between crystallographic directions in the pristine molecule and the directions of the most positive anisotropic components of experimental tensors are presented in Table 4 and discussed further for U1 and U2 separately.…”

“…Due to the complex nature of macrobiomolecules, studying sugar radicals in them directly is far from trivial; therefore, good model systems are desirable. Sucrose (Figure ), the main component of table sugar, is not only considered to be such a model system but also presents interest as a retrospective radiation dosimeter , and enables characterization of irradiated foodstuffs. , For a thorough discussion on the topic of radiation-induced radicals in carbohydrates and sugars, in particular, the reader is referred to ref , while a concise review on the dosimetric studies of sucrose has recently been made by Karakirova and Yordanov …”

On the identity of the last known stable radical in X-irradiated sucrose

“…It is possible that the the unpaired electron is first found on a neighboring site (C4' or C6') and then gets transferred to C5'. These matters should be further explored experimentally with annealing experiments after low temperature irradiation [14] and computationally with nudged elastic band simulations [36,37,38].…”

“…Periodic DFT calculations were employed to test radical models, devised from the experimental data [13] and solid-state radiation chemistry principles. The calculation methodology has been developed and validated in other studies on various carbohydrates and amino acids [14,15,16]. The model validation procedures rely on a comparison of experimental g and HF tensors to calculated ones.…”