Geometric model of trapped electrons in trehalose single crystals x-ray irradiated at 3 K. An EPR studya)

Abstract

Three different radicals were identified by EPR in x-ray irradiated single crystals of trehalose at 3 K. The species are the trapped electron, a hydroxy alkyl radical, and an alkoxy radical. The electron is trapped in an intermolecular site formed by two hydroxyl groups, one on the carbohydrate and the other on a water molecule as evidenced by the anisotropic proton hyperfine couplings. A geometric model for the trapping site is presented. The trapped electron decays by cleavage of an OH bond and the liberated hydrogen atom abstracts another hydrogen atom from an adjacent carbon atom forming a hydroxy alkyl radical. The site of the alkoxy radical has been identified. The primary reaction mechanism is discussed.

Radiation Chemistry of Solid-State Carbohydrates Using EMR

“…It is situated close to the C5 and C6 atoms in the glucose unit. Considering this site assignment for the IMTE, the following scheme for the production of the C5(-H) radical can be devised [19], in agreement with mechanisms proposed by Samskog et al for trehalose [111] and rhamnose [112] R OH e R…”

“…The situation after LT irradiation appears rather clear, with the formation of an IMTE center (not seen by Tarpan et al [20]), two alkoxy radicals (probably at O2 and O4', Samskog and Kispert report only the latter [111]) and two hydroxyalkyl radicals (identified as C5(-H) and C5'(-H) , Samskog and Kispert report only the latter [111]). Furthermore, it was noted [111] that the intensity of C-centered radical(s) increased after thermal annealing to 50 K and was stable at 77 K. The fact that Tarpan et al did not observe the IMTE at slightly higher irradiation temperature, but did detect an extra C5-centered radical, might imply a thermal transformation from the former to the latter radical at a relatively low temperature.…”

“…Furthermore, it was noted [111] that the intensity of C-centered radical(s) increased after thermal annealing to 50 K and was stable at 77 K. The fact that Tarpan et al did not observe the IMTE at slightly higher irradiation temperature, but did detect an extra C5-centered radical, might imply a thermal transformation from the former to the latter radical at a relatively low temperature. On the other hand, the reasons for not detecting the IMTE at X-band [20] may be similar as for sucrose (see Section 6.4.2).…”

“…Consequently, it has so far not been possible to determine the location of the IMTE by direct comparison of calculated and experimental HF tensors. It may however be anticipated that the electron will be trapped in the dipolar field between at least two OH-dipoles in carbohydrate crystals [108][109][110][111][112]. Plausible sites for inter-molecular electron trapping in carbohydrates have been calculated applying the semi-continuum potential model [110], and considerable rearrangements of OH-dipoles, even at 4 K, have been suggested.…”

“…Table 6.11 reveals a very complex and in part confusing array of radicals formed by irradiation at various temperatures. Based on our recent EMR/DFT work [12,20,101] many early radical assignments [4,111,120] needed to be corrected. However, it was not always evident to establish the correspondence with these earlier publications unambiguously.…”