Radiation chemistry of carbohydrates. Part XVII. Radical processes following γ-irradiation of frozen concentrated solutions of sucrose and D-glucose at 77 K

Abstract

The radical species trapped in D-glucose and sucrose ices following γ-irradiation at 77 K have been examined by electron spin resonance. In addition to the doublet assigned to the perpendicular features of randomly oriented OH radicals, trapped electrons (e_t^–) were produced which could be removed with visible light (540–580 nm.) and addition of electron scavengers. The e_t^– had a singlet spectrum with g-value 2·0014 ± 0·0004 and line-width 13·5 ± 0·5 G in H₂O ices and 6·3 ± 0·3 G in D₂O ices. The proton coupling constant, on the assumption of interaction of e_t^– with 8 equivalent nuclei from 4 surrounding water molecules, is 5·26 ± 0·3 G, which is in reasonable agreement with the proton constants in alkaline and neutral ices.

Spectra due to D-glucose and sucrose radicals, after thermal annealing to 190 K, can be resolved into a 1:1 doublet (g= 2·0030 ± 0·0004) with hyperfine splitting of 20·5 G and a 1 : 2 : 1 (triplet (g= 2·0025 ± 0·0004)) with hyperfine splitting of 29·5 G. Possible structures of the radicals are considered.

G(carbohydrate radicals) and G(e_t^–) increase linearly with solute concentration. Radical yields extrapolated from the concentrated carbohydrate ices to the solid state are in good agreement with experimental values determined for the polycrystalline state at 77 K, which for D-glucose and sucrose are G= 3·2 and 2·7 respectively. The results indicate that direct radiation action yields the carbohydrate radicals and there is no evidence that electronic excitation transfer from associated water molecules contributes to radical formation. Maximum G(e_t^–) values, obtained by extrapolation are 0·9 for D-glucose and 0·7 for sucrose ices. Consideration is given to the nature of the electron trapping site.