Photoprotective mechanisms in the core LH1 antenna pigment-protein complex from the purple photosynthetic bacterium, Rhodospirillum rubrum

Time-resolved absorption spectra from the femtoscecond to sub-millisecond time regimes were recorded in order to eluciate the photoprotective mechanisms of carotenoid (spirilloxanthin) in core LH1 light-harvesting pigment-protein complexes from a purple photosynthetic bacterium, Rhodospirillum (Rsp.) rubrum. Carotenoids can prevent the production of singlet oxygen by rapidly quenching the Bchl a triplet state. Through this triplet-triplet energy-transfer reaction the carotenoid is believed to protect the biological system from exposure to excess light. We set out to investigate this paricular function in the purple bacterium, Rsp. rubrum strain S1. We also investigated Rsp. rubrum strain G9+, which is the carotenoidless mutant of Rsp. rubrum, to make sure that the carotenoid is truly playing this important role. As a result, we could demonstrate that strain S1 realizes nearly 100% excitation energy transfer from triplet Bchl a to carotenoid. In addition, we found another major excited state deactivation channel of carotenoids, which takes away excess energy directly from the singlet excited state of Bchl a and so prevents the production of triplet excited Bchl a.