The Single Chlorophyll a Molecule in the Cytochrome b6f Complex: Unusual Optical Properties Protect the Complex against Singlet Oxygen

The cytochrome b6f complex of oxygenic photosynthesis mediates electron transfer between the reaction centers of photosystems I and II and facilitates coupled proton translocation across the membrane. High-resolution x-ray crystallographic structures (Kurisu et al., 2003; Stroebel et al., 2003) of the cytochrome b6f complex unambiguously show that a Chl a molecule is an intrinsic component of the cytochrome b6f complex. Although the functional role of this Chl a is presently unclear (Kühlbrandt, 2003), an excited Chl a molecule is known to produce toxic singlet oxygen as the result of energy transfer from the excited triplet state of the Chl a to oxygen molecules. To prevent singlet oxygen formation in light-harvesting complexes, a carotenoid is typically positioned within ∼4 Å of the Chl a molecule, effectively quenching the triplet excited state of the Chl a. However, in the cytochrome b6f complex, the β-carotene is too far (≥14 Å) from the Chl a for effective quenching of the Chl a triplet excited state. In this study, we propose that in this complex, the protection is at least partly realized through special arrangement of the local protein structure, which shortens the singlet excited state lifetime of the Chl a by a factor of 20–25 and thus significantly reduces the formation of the Chl a triplet state. Based on optical ultrafast absorption difference experiments and structure-based calculations, it is proposed that the Chl a singlet excited state lifetime is shortened due to electron exchange transfer with the nearby tyrosine residue. To our knowledge, this kind of protection mechanism against singlet oxygen has not yet been reported for any other chlorophyll-containing protein complex. It is also reported that the Chl a molecule in the cytochrome b6f complex does not change orientation in its excited state.