Low-Temperature Turnover Control of Photosystem II Using Novel Metal-Containing Redox-Active Herbicides

A novel approach of using metal-containing redox-active herbicides to prepare and study the light-induced intermediates of the photosystem II (PSII) photocycle is described. The redox-active herbicides feature an iron(III) ethylenediaminetetracetate [Fe III −(EDTA)] electron-acceptor group linked to a Q B -site binding dimethylphenylurea moiety by a hydrocarbon spacer. Like the nitroxyl-based redox-active herbicides previously described (Bocarsly, J. R.; Brudvig, G. W. J. Am. Chem. Soc . 1992, 114, 9762−9767), metal-containing herbicides accept electrons from the donor side of PSII while bound to the Q B site and restrict the S-state cycling to two stable charge separations. The use of Fe III −(EDTA) as an electron acceptor allows turnover at low temperatures. EPR studies of PSII upon continuous illumination at 225 K with 0.7 mM of redox-active herbicide, Fe III −(EDTA) linked by an ethane spacer to a dimethylphenyl urea group ( 4 ), produced a stable two-step S 1 to S 3 advance of the O 2 -evolving complex (OEC) and a stoichiometric reduction of the Fe III −(EDTA) moiety of the herbicide, while a control sample with 0.02 mM DCMU [3-(3,4-dichlorophenyl)-1,1-dimethyl-urea] and 0.7 mM of 4 exhibited only a one-step S 1 to S 2 advance of the OEC without significant reduction of the Fe III −(EDTA) moiety of the herbicide. Similar EPR results were obtained for 7, Fe III −(EDTA) linked to the dimethylphenylurea group by a pentane spacer. O 2 -evolution inhibition studies show that appending the Fe III −(EDTA) moiety to the phenylurea herbicide causes a significant decrease in the binding affinity compared to that of DCMU. On the basis of O 2 -evolution studies with various herbicide derivatives and different PSII sample types, the observed decrease in binding affinities is attributed to the degree of accessibility of the Q B -binding pocket to the herbicides and to electrostatic and hydrophilicity factors. The present study describes the use of novel metal-containing herbicides in studying long-range electron transfer in PSII and in trapping photogenerated two-electron oxidized intermediate states of the O 2 -evolving complex.