Effects of Novel Photosynthetic Inhibitor [CuL(2)]Br(2) Complex on Photosystem II Activity in Spinach

The effects of the novel [CuL(2)]Br(2) complex (L = bis{4H-1,3,5-triazino [2,1-b]benzothiazole-2-amine,4-(2-imidazole)}copper(II) bromide complex) on the photosystem II (PSII) activity of PSII membranes isolated from spinach were studied. The absence of photosynthetic oxygen evolution by PSII membranes without artificial electron acceptors, but in the presence of [CuL(2)]Br(2), has shown that it is not able to act as a PSII electron acceptor. In the presence of artificial electron acceptors, [CuL(2)]Br(2) inhibits photosynthetic oxygen evolution. [CuL(2)]Br(2) also suppresses the photoinduced changes of the PSII chlorophyll fluorescence yield (F(V)) related to the photoreduction of the primary quinone electron acceptor, Q(A). The inhibition of both characteristic PSII reactions depends on [CuL(2)]Br(2) concentration. At all studied concentrations of [CuL(2)]Br(2), the decrease in the F(M) level occurs exclusively due to a decrease in Fv. [CuL(2)]Br(2) causes neither changes in the F(0) level nor the retardation of the photoinduced rise in F(M), which characterizes the efficiency of the electron supply from the donor-side components to Q(A) through the PSII reaction center (RC). Artificial electron donors (sodium ascorbate, DPC, Mn(2+)) do not cancel the inhibitory effect of [CuL(2)]Br(2). The dependences of the inhibitory efficiency of the studied reactions of PSII on [CuL(2)]Br(2) complex concentration practically coincide. The inhibition constant Ki is about 16 µM, and logKi is 4.8. As [CuL(2)]Br(2) does not change the aromatic amino acids’ intrinsic fluorescence of the PSII protein components, it can be proposed that [CuL(2)]Br(2) has no significant effect on the native state of PSII proteins. The results obtained in the present study are compared to the literature data concerning the inhibitory effects of PSII Cu(II) aqua ions and Cu(II)-organic complexes.