origin of 115-130 degrees C thermoluminescence bands in chlorophyll-containing material

High-temperature thermoluminescence (TL) emitted in the temperature region from +50 to +150 degrees C has been studied in a variety of chlorophyll-containing samples that were allowed to dry during the TL measurement. Analysis of the recorded traces by a multicomponent-fitting procedure revealed the existence of up to three bands of nonphotosynthetic origin with peak positions at 62-75, 114-128 and 151-157 degrees C and apparent activation energies of 27.0-28.8, 14.1-15.4 and 22.1-23.3 kcal/mol (the bands are denoted as HT1, HT2 and HT3, respectively). Low-temperature treatment of leaves, incubation of algae in the presence of paraquat, exposure of algae or isolated thylakoids to a strong light, all conditions known to stimulate oxidative damage to membrane lipids, caused appearance of a small HT1 band and significant rise in the intensity of the HT2 band. The increase in the HT2 component correlated positively with accumulation of conjugated dienes and malondialdehyde in thylakoids illuminated with a strong light. Different quenchers of active oxygen species and scavengers of free radicals added to preilluminated thylakoids or thylakoid lipid extracts before the TL measurements, as well as injection of argon into the TL measuring chamber, caused no changes in the intensity of the HT2 emission. The HT2 band in the thylakoids increased strongly upon addition of linoleate peroxidized by hydroxyradicals generated in the Fenton reaction but remained unchanged if the linoleate was oxidized with the use of lipoxygenase. We suggest that the HT2 band arises due to thermal decomposition of lipid cyclic peroxides present in the samples. In turn, the decomposition reaction leads to formation of carbonyls in triplet state with following migration of excitation energy toward chlorophyll. Contrary to the HT1 and HT2 bands, the HT3 band of TL cannot be associated with the thermolysis of lipid peroxidation products already present in the samples before starting the TL gradient.