Dark production of reactive oxygen species in photosystem II membrane particles at elevated temperature:: EPR spin-trapping study

In our study, EPR spin-trapping technique was employed to study dark production of two reactive oxygen species, hydroxyl radicals (OH center dot) and singlet oxygen (O-1(2)), in spinach photosystem II (PSII) membrane particles exposed to elevated temperature (47 degrees C). Production of OH center dot, evaluated as EMPO-OH adduct EPR signal, was suppressed by the enzymatic removal of hydrogen peroxide and by the addition of iron chelator desferal, whereas externally added hydrogen peroxide enhanced OH center dot production. These observations reveal that OH center dot is presumably produced by metal-mediated reduction of hydrogen peroxide in a Fenton-type reaction. Increase in pH above physiological values significantly stimulated the formation of OH center dot, whereas the presence of chloride and calcium ions had the opposite effect. Based on our results it is proposed that the formation of OH center dot is linked to the thermal disassembly of water-splitting manganese complex on PSII donor side. Singlet oxygen production, followed as the formation of nitroxyl radical TEMPO, was not affected by OH center dot scavengers. This finding indicates that the production of these two species was independent and that the production of O-1(2) is not closely linked to PSII donor side. (c) 2007 Elsevier B.V. All rights reserved.