PLASMA-INDUCED FREE-RADICALS OF POLYCRYSTALLINE MYOINOSITOL STUDIED BY ELECTRON-SPIN-RESONANCE - ORBITAL REHYBRIDIZATION-INDUCED EFFECT OF HYDROXYLALKYL RADICALS ON THEIR REACTIVITIES IN CRYSTALLINE STATE

We report the first detailed ESR study of plasma-induced carbohydrate radicals in powdered myo-inositol formed by plasma irradiation followed by temperature annealing. On the basis of the ESR kinetics coupled with the systematic computer simulations, it was found that plasma irradiation produced the hydroxylalkyl radicals at all four possible sites by hydrogen elimination with a preferential formation at C2 and C5. However, a striking difference in the reactivity (stability) of the hydroxylalkyl radicals was observed. The radical at C1(=C3) (1) was extremely stable even at 120-degrees-C for 2 h, while the radical at C4(=C6) was very unstable so that it underwent spontaneously the dehydration to give the acylalkyl radical exclusively at C3(=C1) (4). We rationalize the observed anomalies in terms of the special conformation of O-H bond at C1(=C3) in crystalline myo-inositol, which is the only O-H bond s-trans to the C-H bond with respect to the C-O bond axis. This specific conformation leads to the different reactivity of the hydroxyl group at C1(=C3) induced by an orbital rehybridization from tetrahedral sp3 carbon to the planar sp2 carbon on the radical formation. Although this specific interpretation is valid only in a solid when the hydrogen of hydroxyl group is fixed by the hydrogen bonding network, the difference in radical reactivity is a general phenomenon with broad implications for carbohydrate radicals, providing a pioneering concept to facilitate assignment of the carbohydrate radical structure in the crystalline state.