Synthesis and Use of Radiolabelled Cyanoacrylate Probes of the Photosystem II Herbicide Binding Site

Cyanoacrylates are potent inhibitors o f photosynthetic electron transport (PET) and are po­ tentially useful probes o f the photosystem II herbicide binding site. A series o f cyanoacrylates was synthesized and the Hill inhibition activities evaluated in order to select compounds suita­ ble for radioactive synthesis. A cyanoacrylate, 2-(2-nitrophenoxy)ethyl 3-benzylamino-2cyano-2-pentenoate, was found to displace diuron from the photosystem II herbicide binding site. For this compound the dissociation constant o f the inhibitor/binding site complex was found to be 2 x 10“ 8 M with an active site concentration o f 2 nmol/mg chlorophyll. In a similar system the corresponding figures for diuron were 1.2 x 10“ 8 m and 1.3 nmol/mg chlorophyll. Photoaffinity labelling o f 1% II thylakoid proteins with 2-(2-azidophenoxy)ethyl 3-[7-l4C]benzylamino-2-cyano-2-pentenoate showed weak binding in the 32 and 28 kD mass regions, consistent with binding to the D, peptide.