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

We report the first in-depth ESR study of plasma-induced polycarbohydr ate radicals in powdered cellulose and amylose. On the basis of the ES R kinetics coupled with the systematic computer simulations, it was fo und that plasma irradiation produced preferentially the alkoxylalkyl r adical 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 wa s present only in celullose, which is assignable to the hydroxylalkyl radical at C-2 and/or C-3 of the glucose units of cellulose. Presumabl y, the hydroxylalkyl radical at C-3 of the glucose units of amylose wh ich should likewise exhibit a triplet was not formed due to the suppre ssion of hydrogen abstraction at C-3 in the helical tertiary structure of amylose. However, the acylalkyl radicals which are formed by dehyd ration of the hydroxylalkyl radicals were observed in both polycarbohy drates. This anomaly in amylose was rationalized in terms of spontaneo us dehydration of the hydroxylalkyl radical formed at C-2 of the gluco se units due to a special rehybridization-induced effect of the carbon -radical center on such dehydration reactions valid only in a solid st ate. It was also shown that the polycarbohydrate radicals were rapidly reacted with oxygen in air, unlike monocarbohydrate radicals which ar e stable even after prolonged standing in air at room temperature. Thi s difference could be ascribed to the difference in the polymorphic fo rms between polycarbohydrates and monocarbohydrates.