SPECTROCHEMISTRY OF POLYCARBOHYDRATE FREE-RADICALS GENERATED BY ARGON PLASMOLYSIS - EFFECT OF TERTIARY STRUCTURE ON FREE-RADICAL FORMATION

We report the first in-depth ESR study of plasma-induced polycarbohydrate radicals in powdered cellulose and amylose. On the basis of the ESR kinetics coupled with the systematic computer simulations, it was found that plasma irradiation produced preferentially the alkoxylalkyl radical at C-1 of the glucose units in both polycarbohydrates. However, the observed ESR spectra are largely different in pattern between the two anomeric polymers: The triplet spectrum with ca. 3.0 mT of HSC was present only in celullose, which is assignable to the hydroxylalkyl radical at C-2 and/or C-3 of the glucose units of cellulose. Presumably, the hydroxylalkyl radical at C-3 of the glucose units of amylose which should likewise exhibit a triplet was not formed due to the suppression of hydrogen abstraction at C-3 in the helical tertiary structure of amylose. However, the acylalkyl radicals which are formed by dehydration of the hydroxylalkyl radicals were observed in both polycarbohydrates. This anomaly in amylose was rationalized in terms of spontaneous dehydration of the hydroxylalkyl radical formed at C-2 of the glucose units due to a special rehybridization-induced effect of the carbon-radical center on such dehydration reactions valid only in a solid state. It was also shown that the polycarbohydrate radicals were rapidly reacted with oxygen in air, unlike monocarbohydrate radicals which are stable even after prolonged standing in air at room temperature. This difference could be ascribed to the difference in the polymorphic forms between polycarbohydrates and monocarbohydrates.