Diquat (1,1′-ethylene-2,2′-dipyridylium dibromide) in photoreactions of isolated chloroplasts

Abstract

Abstract Diquat (1,1′-ethylene-2,2′-dipyridylium dibromide) has been shown to support three types of photophosphorylation by illuminated, isolated chloroplasts from Swiss chard: non-cyclic, pseudo-cyclic, and cyclic photophosphorylation. Diquat also competitively inhibits photophosphorylation catalyzed by phenazine methosulfate. This, together with the interpretation of results from the interaction of Diquat and inhibitors and cofactors of photophosphorylation, places Diquat at the same site as phenazine methosulfate in the electron-transfer path of chloroplasts. Phenazine methosulfate and Diquat have been photoreduced almost quantitatively by illuminated chloroplasts under strictly anaerobic conditions. An experimental E0′ for Diquat has been obtained by an enzymic reduction, which is in close agreement with the value from the potentiometric titration reported in the literature.