Influence of novel CU(II) complex on the photosystem II photochemical activity of spinach

The mechanism of action of a large group of modern agents used for weed control inhibits electron transfer in the light stage of photosynthesis at one or another site of the electron-transport chain. Impact of the novel [CuL₂]Br₂ complex (L = bis4H-1,3,5-triazino [2,1-b] benzothiazole-2-amine, 4-(2-imidazole)copper(II) bromide complex) on the activity of PSII membranes isolated from spinach were studied by of photosynthetic oxygen evolution as well as changes in the PSII chlorophyll fluorescence yield (F_V) related to photoreduction of the Q_A plastoquinone, the PSII primary quinone electron acceptor. We show that [CuL₂]Br₂ effectively inhibits PSII photochemical activity as revealed by suppression of both the characteristic PSII photoreactions with the same similar effectiveness. [CuL₂]Br₂ decreases F_M level exclusively at the expense of F_V; causes neither changes in the F₀ level nor the retardation of the photoinduced rise in F_M. Artificial electron donors do not cancel the inhibitory effect of [CuL₂]Br₂. It is likely that [CuL₂]Br₂, which is included somewhere on the donor side or between its components, inhibits and/or slows down natural electron donation and/or serves as an exogenous electron donor for RC. Thus, the PSII reaction center's components are most likely the major target of [CuL₂]Br₂ inhibitory activity. The data obtained are compared to the literature information concerning the inhibitory effects of Cu(II) aqua ions and Cu(II)-organic complexes on PSII.