Duroquinol as an electron donor for chloroplast electron transfer reactions

Duroquinol (tetramethylhydroquinone) was found to function as an electron donor in chloroplasts. Non-cyclic electron transfer from duroquinol to electron acceptors such as oxygen proceeded at high rates, was insensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) but was sensitive to the plastoquinone antagonist 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone (DBMIB). The electron transport from duroquinol was coupled to the synthesis of ATP. Spectroscopic studies of chloroplast electron carriers in the dark indicated the high-potential "Rieske" iron-sulfur center, cytochrome f, plastocyanin and P-700 were all reduced by duroquinol. The dark reduction of the "Rieske" iron-sulfur center and cytochrome f were inhibited by DBMIB but not by DCMU. These results have been interpreted in terms of a linear sequence of electron carriers in the non-cyclic electron transport chain which includes plastoquinone, the "Rieske" iron-sulfur center, cytochrome f, plastocyanin and P-700.