Formation of singlet oxygen by decomposition of protein hydroperoxide in photosystem II

Singlet oxygen ((1)O(2)) is formed by triplet-triplet energy transfer from triplet chlorophyll to O(2) via Type II photosensitization reaction in photosystem II (PSII). Formation of triplet chlorophyll is associated with the change in spin state of the excited electron and recombination of triplet radical pair in the PSII antenna complex and reaction center, respectively. Here, we have provided evidence for the formation of (1)O(2) by decomposition of protein hydroperoxide in PSII membranes deprived of Mn(4)O(5)Ca complex. Protein hydroperoxide is formed by protein oxidation initiated by highly oxidizing chlorophyll cation radical and hydroxyl radical formed by Type I photosensitization reaction. Under highly oxidizing conditions, protein hydroperoxide is oxidized to protein peroxyl radical which either cyclizes to dioxetane or recombines with another protein peroxyl radical to tetroxide. These highly unstable intermediates decompose to triplet carbonyls which transfer energy to O(2) forming (1)O(2). Data presented in this study show for the first time that (1)O(2) is formed by decomposition of protein hydroperoxide in PSII membranes deprived of Mn(4)O(5)Ca complex.