EPR study of radicals generated in starch by microwaves or by conventional heating

Irradiation of starch with microwaves or heating at relatively low temperature (483 K) generates radicals with anisotropic EPR signals (signal I and II) of lorentzian shape and similar g factors (g(av)=2.006 and 2.007 in X and Q band, respectively). Signal I, exhibiting a doublet hyperfine structure (HFS) with A(av),=1.19 mT (X band) and A(av)=1.24 mT (Q band), was ascribed to a carbon radical with an unpaired electron localized at C-(1) of the glucose unit, from which a hydrogen atom was abstracted. The electron interacts with the nuclear spin of the beta-hydrogen at C-(2). Signal II, with g factor values similar to that of signal I but without HFS, was assigned to a radical with unpaired electron localized also at C-(1) of the glucose unit from which, however, two hydrogens (alpha and beta) were abstracted from C-(1) and C-(2), respectively. Signals I and II show different saturation ability in the power range 0.3-30 mW. Radicals generated in the native starch at higher temperature (503 K) exhibit more intensive EPR spectra, with dominating signal II and lower HFS constant of the signal I. The same trend of weakening of the HF interactions is observed for oxidized starch, proving that the beta-hydrogen is abstracted more easily from C-(2) at higher temperature. The decay of the radicals generated by microwaves or by heating at 483 K, during storing at 293 K, occurs monotonously, according to a second order kinetics. On the other hand, the radicals formed at higher temperature (503 K) represent nonmonotonous changes with the time of storing at 293 K. Such behavior may be explained assuming nonuniform distribution of thermally generated radicals, which partially do not contribute to the signal intensity leading to the significant broadening of the EPR lines. During relaxation the radicals became better dispersed which makes them active in EPR.