Novel protection mechanisms against singlet oxygen formation by the Chl $a$ molecule in the cytochrome $b_{6}f$ complex of oxygenic photosynthesis

A Chl molecule is known to produce highly toxic singlet oxygen under light illumination as a result of energy transfer from its triplet excited state to oxygen. To prevent that, a carotenoid is typically positioned close to a Chl molecule ($\sim $4 $\AA$) in Chl containing proteins to ensure rapid triplet-triplet energy transfer from Chl to carotenoid. Surprisingly, the X-ray structures of the cytochrome $b_{6}f$ complex show that the $\beta$ -carotene is much too far from the only Chl $a$ found in this complex to provide effective protection by the usual triplet-triplet energy transfer mechanism. Our optical femtosecond time resolved experiments on diluted samples as well as on the single crystals of the $b_{6}f$ complex suggest that the Chl $a$ is protected by two novel mechanisms: (i) the yield of the Chl $a$ triplet state formation is reduced through electron-transfer exchange with the nearby amino acid residues, and (ii) a long distance triplet energy transfer to carotenoid mediated by a third mobile molecule (NSF MCB- 0516939, NIH GM-38323).