Probing the Influence of Novel Organometallic Copper(II) Complexes on Spinach PSII Photochemistry Using OJIP Fluorescence Transient Measurements

Modern agricultural cultivation relies heavily on genetically modified plants that survive after exposure to herbicides that kill weeds. Despite this biotechnology, there is a growing need for new sustainable, environmentally friendly, and biodegradable herbicides. We developed a novel [CuL(2)]Br(2) complex (L = bis{4H-1,3,5-triazino[2,1-b]benzothiazole-2-amine,4-(2-imidazole) that is active on PSII by inhibiting photosynthetic oxygen evolution on the micromolar level. [CuL(2)]Br(2) reduces the F(V) of PSII fluorescence. Artificial electron donors do not rescind the effect of [CuL(2)]Br(2). The inhibitory mechanism of [CuL(2)]Br(2) remains unclear. To explore this mechanism, we investigated the effect of [CuL(2)]Br(2) in the presence/absence of the well-studied inhibitor DCMU on PSII-containing membranes by OJIP Chl fluorescence transient measurements. [CuL(2)]Br(2) has two effects on Chl fluorescence transients: (1) a substantial decrease of the Chl fluorescence intensity throughout the entire kinetics, and (2) an auxiliary “diuron-like” effect. The initial decrease dominates and is observed both with and without DCMU. In contrast, the “diuron-like” effect is small and is observed only without DCMU. We propose that [CuL(2)]Br(2) has two binding sites for PSII with different affinities. At the high-affinity site, [CuL(2)]Br(2) produces effects similar to PSII reaction center inhibition, while at the low-affinity site, [CuL(2)]Br(2) produces effects identical to those of DCMU. These results are compared with other PSII-specific classes of herbicides.