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Electron paramagnetic resonance (EPR) spectroscopy, with Cu 21 ions as a paramagnetic probe, was applied to study the process of radical generation in starch and changes occurring in its structure upon thermal treatment and oxidation. Thermally generated radicals in starch containing Cu 21 ions are localized on carbon atoms different from those in the pure starch. EPR signal parameters of the radical species (g av = 2.0047 -2.0049; A av = 0.9 -1.0 mT) suggest a lower number of oxygen atoms in the vicinity of radical centres than that observed in pure starch and weakening of the interaction of the unpaired electron with the nuclear magnetic moment of the b hydrogen (for pure starch g av = 2.0059 -2.0065; A av = 1.2 -1.3 mT). A mechanism of thermal generation of radicals in the native and oxidized starch containing Cu 21 ions was proposed. It consists of breaking of RCO-Cu 21 or RCOO-Cu 21 bonds in the native and oxidized starch, respectively, reduction of Cu 21 to Cu 1 ions and formation of transient RCO. or RCOO. species, active in abstraction of hydrogen from the neighbouring C atom of the glucose unit, at which the unpaired electron became localized.

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Fluorescence and electron paramagnetic resonance measurements were used to study selenium influence on photosystem activity in rape seedlings affected by Cd stress. Water cultures containing Hoagland nutrients were supplemented with 400 microM of CdCl(2), 2 microM of Na(2)SeO(4) and a mixture of both CdCl(2) and Na(2)SeO(4). The seedlings were cultured till the first leaf reached about 1 cm in length. Cadmium-induced changes in the activity of both photosystems were partly diminished by Se presence in the nutrient medium. Electron microscopy photographs confirmed less degradation in chloroplasts of plants cultured on media containing Se. It is suggested that sucrose groups of starch, which is deposited in greater amounts in Cd-stressed plants, may act as traps for free radicals produced under those conditions.

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 (gav=2.006 and 2.007 in X and Q band, respectively). Signal I, exhibiting a doublet hyperfine structure (HFS) with Aav=1.19 mT (X band) and Aav=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 β‐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 (α and β) 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 β‐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.