Mechanolysis of Glucose-Based Polysaccharides As Studied by Electron Spin Resonance

Abstract

We report the detailed study of mechanically induced free radical (mechanoradical) formation of glucosebased polysaccharides such as cellulose and amylose based on electron spin resonance (ESR) on its comparison with plasma-induced radicals of polysaccharides. The observed ESR spectra of mechanically fractured samples by ball milling at room temperature have shown the multicomponent spectra, which differ in pattern from those of plasma-irradiated cellulose but are similar to those of plasma-irradiated amylose. The systematic computer simulations disclosed that the observed spectra of cellulose consist of three kinds of spectral components, an isotropic doublet (I) assigned to a hydroxylalkyl-type radical at C 1 , an anisotropic doublet of doublets (II) assigned to an acylalkyl-type radical at C 2 and/or C 3 as discrete components, and a singlet spectrum (III) assigned to dangling-bond sites (DBS), while those of amylose consist of two kinds of spectral components, I and III. One of the most intriguing facts is that the component radicals are all glucose-derived mid-chain alkyl-type radicals as in the case of plasma irradiation, although it is known that mechanoradicals are produced by the polymer main-chain scission. It can be reasonably assumed, therefore, that the mechanoradicals primarily formed by 1,4-glucosidic bond cleavage of polysaccharides at room temperature underwent a hydrogen abstraction from the glucose units to give rise to the glucose-derived mid-chain alkyl-type radicals. Furthermore, spectrum III was a major component in the simulated spectra of both cellulose and amylose, unlike those in the case of plasma irradiation, suggesting that cross-linking reactions simultaneously occur accompanied by a decrease in the molecular weight in the course of vibratory milling.

Nature of Mechanoradical Formation of Substituted Celluloses as Studied by Electron Spin Resonance

“…3A that the radical concentration more readily decreased after reaching the maximum value in each polysaccharide, although cellulose required much longer vibratory milling (120 min) to exhibit the maximum value in the mechanoradical concentrations. 23) In the case of plasmolysis, however, the radical concentrations tended to level off in all plasma-irradiated polysaccharides (Fig. 3B).…”

“…23) Consequently, the timecourse changes in mechanoradical concentration shown in Fig. 3A indicate that the ensuing reactions of the mechanoradical formed exceed the radical generation, as the molecular weights of polysaccharides come close to the limiting value, and that substituted celluloses are more prone than cellulose to undergo the ensuing radical reactions such as a recombination reaction due to mechanical force.…”

“…5 that the ratio of change in each spectral intensity does not vary much with component spectra, accounting for rather small spectral changes in pattern in the course of mechanolysis and plasmolysis in each case. Although the broad singleline spectrum was the major spectral component in the mechanolysis of both cellulose and amylose, 23) this was not the case for substituted celluloses.…”

“…We reported detailed analyses of room-temperature ESR spectra of the mechanoradicals formed in synthetic polymers 21,22) and polysaccharides such as cellulose and amylose 23) under strictly anaerobic conditions, and discussed the novel and significant features of mechanoradical formation and changes in physicochemical properties caused by the mechanolysis.…”

Determination of free radicals and flavan-3-ols content in fermented and unfermented teas and properties of their infusions

“…The values of its HFS splitting were significantly higher than those observed for signals of semiquinone radicals [26]; hence, we ascribed tentatively signal III to carbohydrate radical. The formation of such centre could occur under the influence of solar, mechanical, irradiation or thermal energy and was observed in living plants, as well as in other biological materials subjected to c-irradiation, heating or grinding [27][28][29][30][31] Fig. 1 The experimental (exp) and simulated (sim) EPR spectra of different kinds of teas, registered at 293 K, at 3 mW, in the range 5 mT and their particular signal components (I, II, III, IV) used to simulation of the spectra: a Teekane black, b Lipton black, c Lipton green, d Bio-Active green, e Yunnan white I, II-signals of semiquinone radicals, III-carbohydrate radical localised at C(1) atom of glucose, IV-carbohydrate radical localised at C(1) atom of glucose unit in oligosaccharide structure created after dehydration b probably mono or disaccharides [31][32][33].…”