Inhibition of the photochemical activity of the isolated photosystem ii reaction centre by 2-cyanoacrylates

Certain derivatives of the 2-cyanoacrylates are potent inhibitors of photosystem II (PSII) mediated electron flow. These compounds block electron flow by binding to the D1 polypeptide as do many other types of photosystem II herbicides. In this paper we report the action of these species of 2-cyanoacrylates on the photochemical activity of the isolated PSII reaction centre consisting of the D1 and D2 polypeptides but free of other chlorophyll-binding proteins. The three species tested were 2-ethoxyethyl 2-cyano-3-(4-chlorobenzylamino)-3-isopropylacrylate (cyanoacrylate 1), isopropyl 2-cyano-3-(N-methylanilino)acrylate (cyanoacrylate II) and methoxymethyl 2-cyano-3-(4-chlorobenzylthio)-3-mercapto-acrylate sodium salt (cyanoacrylate III). Unlike the case with thylakoid membranes or PSII enriched membranes, cyanoacrylate III was a more potent inhibitor than cyanoacrylate I of the photoreduction of 2,6-dichlorophenol indophenol (DCPIP) in the isolated PSII reaction centre. This cyanoacrylate was also more effective than diuron (DCMU) in blocking DCPIP reduction in the reaction centre complex. Moreover, unlike DCMU and cyanoacrylates I and II, cyanoacrylate III was more effective in inhibiting the quinone-dependent photoreduction of cytochrome b559 in isolated PSII reaction centres. It is concluded that the hydrophilic nature of cyanoacrylate III gives it greater potency with the isolated reaction centre and that its site of action is likely to be closer to the histidine ligands involved in the binding of non-haem iron rather than in the Q_B binding niche. (See footnote on page 331 for nomenclature of cyanoacrylates).