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 prepa re and study the light-induced intermediates of the photosystern II (PSII) photocycle is described. The redox-active herbicides feature an iron(III) e thylenediuninetetracetate [Fe-III-(EDTA)] electron-acceptor group linked to a Q(B)-site binding dimethylphenylurea moiety by a hydrocarbon spacer. Lik e the nitroxyl-based redox-active herbicides previously described (Bocarsly , J. R.; Brudvig, G. W. J. Am. Chem. Sec. 1992, 114, 9762-9767), metal-cont aining 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 sepa rations. The use of Fe-III-(EDTA) as an electron acceptor allows turnover a t 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 (2), produced a stable two-step S-1 to S-3 advance of the O-2-evolving complex (OEC) and a stoichiometric reduc tion of the Fe-III-(EDTA) moiety of the herbicide, while a control sample w ith 0.02 InM DCMU [3-(3,4-dichlorophenyl)-1,1-dimethyl-urea] and 0.7 mM of 4 exhibited only a one-step SI to St advance of the OEC without significant reduction of the Fe-III-(EDTA) moiety of the herbicide. Similar EPR result s were obtained for 7, Fe-III-(EDTA) linked Ca 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 d ecrease 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 an d to electrostatic and hydrophilicity factors. The present study describes the use of novel metal-containing herbicides in studying long-range electro n transfer in PSII and in trapping photogenerated two-electron oxidized int ermediate states of the O-2-evolving complex.