BIMANES—26. AN ELECTRON TRANSFER REACTION BETWEEN PHOTOSYSTEM I1 AND MONOBROMOBIMANE INDUCES STATIC CHLOROPHYLL a QUENCHING IN SPINACH CHLOROPLASTS

Abstract— Monobromobimane in chloroplasts lowers both the quantum yield of system II photochemistry and the yield of chlorophyll a fluorescence. Illumination of the chloroplasts in the presence of monohromobimane is an absolute prerequisite to the manifestation of this phenomenon, which proceeds via the Photosystem II intermediate, the semiquinone radical anion, Q_A-. The latter transfers an electron to monobromobimane to yield an anion radical (mBBr^·), which may either lose bromide ion to yield a reactive radical (mB^·), or acquire a proton and undergo further reduction, eventually forming syn-(methyl, methyl) bimane. In turn, mB reacts with the protein of the light-harvesting complex, to form a product which acts as static excitation energy quencher in the chlorophyll pigment bed of photosystem 11. Polarographic reduction of monobromobimane shows an adsorption wave at O V and two reduction waves. Prolonged reduction in water at -0.5 V yields syn-(methyl, methyl) bimane (which is further reduced at more negative potentials) and bromide ion. Thus, both electrochemical and chloroplast-induced reduction produce syn-(methyl, methyl) bimane. Monobromobimane may then serve as a Photosystem II activated probe in elucidating the conformation of intrinsic thylakoid membrane polypeptides.