Interactions of Halogenated Benzoquinones with the Non-Heme Iron (Q40o) in Photosystem II

Interactions o f halogenated benzoquinones with the acceptor side o f Photosystem (PS) II were studied by measuring fluorescence induction curves and flash-induced absorbance changes in PS II particles isolated from Synechococcus vulcanus. Following results were ob­ tained: 1) Addition o f some halogenated benzoquinones prior to 3-(3,4-dichlorophenyl)-l,ldimethylurea (D C M U ) in the dark increased the area above the fluorescence induction curve (work integral) by a factor o f two. 2) Based on the ability to increase the fluorescence work integral, halogenated benzoquinones could be divided into two groups. 3) 2,6-Dichlorobenzoquinone (2,6-DCBQ), trichlorobenzoquinone (TCBQ) and tetrahalogenated benzoquinones except tetrafluorobenzoquinone (fluoranil) (group A) increased the work integral, but 2,5-DCBQ and fluoranil (group B) did not. 4) Rapid reoxidation o f QA was observed in the presence o f quinones which belong to group A. These results were interpreted in terms o f dark oxidation o f by quinones belonging to group A. Possible mechanisms o f oxidation o f Q400 by these quinones in the dark are discussed.