The Effect of Various Substituted p‐Benzoquinones on Electron Transport in Spinach Chloroplasts

Four different substituted p‐benzoquinones resembling the natural chloroplast quinone, plastoquinone A, in structure [2,3‐dimethyl‐5‐bromo‐6‐n‐octadecylmercapto‐1,4‐benzoquinone (compound 6712); 2,3‐dimethyl‐5‐n‐propylamino‐1,4‐benzoquinone (compound 6711); 2.3‐dimethyl‐5‐oleylamino‐1,4‐bezoquinone (compound 6710); and 2.3‐dimethyl‐5‐n‐hexadecylamino‐1,4‐benzoquinone (compound 6709)] were tested for their effect on electron transport in spinach chloroplasts. 2,4‐Dimethyl‐p‐benzoquinone reduction by photosystem II was inhibited best by compound 6712 (I50= 27 μg/ml, or 54 μM conc.), followed by compounds 6709 and 6710 (I50= 63 μg/ml or 125 μM conc. of either); compound 6711, a short‐chain analog, was least effective (I50= 75 μg/ml or 0.3 mM conc.). (I50 is the dose which inhibits 50%.) Inhibition by compounds 6712 and 6710 could be reversed by the addition of exogenous plastoquinone A indicating that these two benzoquinones interfered with the reduction of plastoquinone A before the well‐known 2,5‐dibromo‐3‐methyl‐6‐isopropyl‐p‐benzoquine inhibition site between the two photosystems (Trebst and Reimer, 1973, Z. Naturforsch. 28c, 710). A negative synergistic effect between exogenous plastoquinone A and the four benzoquinones studied was observed on silicomolybdate reduction by photosystem II in the presence of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea. Another benzoquinone inhibition site, not reversed by exogenous plastoquinone A. occurred in photosystem I on the ascorbate plus N,N,N′,N′‐tetramethyl‐p‐phenylenediamine → methylviologen pathway. The relevance of these benzoquinone inhibition sites to photosynthetic electron transport is discussed.