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

Abstract

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. The anisotropy of the EPR and EIE spectra indicated that R1 and R2 are alkyl radicals (i.e.carbon-centered) and R3 is an alkoxy radical (i.e. oxygen-centered). The EPR data also revealed the presence of an additional alkoxy radical species, labeled R4. Molecular modeling using periodic Density Functional Theory (DFT)calculations for simulating experimental data suggest that R1 and R2 are the hydrogenabstracted alkyl species centered at C5" and C5, respectively, while the alkoxy radicals R3 and R4 have the unpaired electron localized mainly at O2 and O4. Interestingly, the DFT study on R4 demonstrates that the trapping of a transferred proton can significantly influence the conformation of a deprotonated cation.Comparison of these results with those obtained from sucrose single crystals Xirradiated at 10 K indicates that the carbon situated next to the ring oxygen and connected to the CH 2 OH hydroxymethyl group is a better radical trapping site than other positions.

Radiation Chemistry of Solid-State Carbohydrates Using EMR

“…Irradiation at 3K led to three (close to) primary radicals, proposed to be an IMTE (radical I), an O4' alkoxy radical (radical III) and again a 1:2:1 triplet (radical II), also found after irradiation at 77 K and assumed to be the same as found previously [4]. After irradiation at 10 K (Tarpan et al [20]), the trapped electron could no longer be detected at variance with the radicals II and III, relabeled as R1 and R4 respectively. A thorough ENDOR and DFT analysis convincingly modified the identification of the 1:2:1 triplet II/R1 to a C5'-centered radical, whereas the III/R4 assignment to the O4' alkoxy radical appeared plausible but still not conclusive.…”

“…A thorough ENDOR and DFT analysis convincingly modified the identification of the 1:2:1 triplet II/R1 to a C5'-centered radical, whereas the III/R4 assignment to the O4' alkoxy radical appeared plausible but still not conclusive. Two more radicals, R2 and R3, were convincingly identified as C5 and O2-centered radicals, respectively [20]. RT, long term stable S1 Suggested [4,12] S2 Suggested [4,12] Tarpan et al [101] also extended the EPR investigations of Samskog and Kispert after irradiation at 77K [120] with ENDOR and DFT.…”

“…This allowed to perform the SH parameter calculations directly on the periodically optimized structure. Our latest computational results for neutral carbohydrate radicals are all performed in this way [12,20,21,57,101]. In the case of charged radicals, the periodic boundary conditions induce an infinite charge on the lattice, which would have to be balanced by a background countercharge density.…”

“…Under favorable conditions a proton will be transferred from an OH + group of the cation to a neighboring molecule, leaving behind an O-centered neutral alkoxy radical. As the hydrogen bonding scheme often is a continuous chain of O-bonded protons (C-OH and/or H 2 O) throughout the lattice, the deprotonation has in some cases been shown to initiate a proton shuffle covering several molecules in the lattice [20,21]. The driving force for the deprotonation may partly result from a change in the acid-base equilibrium for the cation as compared with the neutral molecule [141] and the excitation energy available by vibrational relaxation of the initial oxidation product.…”

“…A first important reason for this is that stable radical structures in organic solids very often differ substantially from the pristine molecule (crystal) structures and are the result of a complex reaction chain after the initial ionization or electron capture process [15]. The radicals stabilized after irradiation at low temperatures (~ liquid He or N 2 ) are, however, more directly structurally related to those of the pristine molecules, and although such studies are experimentally far more challenging, the identification of the radical species is therefore in principle simpler [16][17][18][19][20][21]. Following the evolution of the EPR and ENDOR spectra towards the RT stable stage via thermal annealing is one approach to elucidate the structures of the stable species and to unveil their formation mechanisms.…”