Carotenoids in Energy Transfer and Quenching Processes in Pcb and Pcb-PS I Complexes from Prochlorothrix hollandica

Chlorophyll (Chl) a/b-binding proteins from Prochlorothrix hollandica known as Pcb antennae were studied by femtosecond transient absorption technique to identify energy transfer rates and pathways in Pcb and Pcb PS I complexes. Carotenoids transfer energy to Chl with low efficiency of similar to 25% in Pcb complexes. Interestingly, analysis of transient absorption spectra identified a pathway from the hot S-1 state of zeaxanthin and/or beta-carotene as the major energy transfer channel between carotenoids and chlorophylls in Pcb whereas the S-2 state contributes only marginally to energy transfer. Due to energetic reasons, no energy transfer is possible via the relaxed S-1 state of carotenoids. The low overall energy transfer efficiency of carotenoids recognizes chlorophylls as the main light-harvesting pigments. Besides Chl a, presence of Chl h, which transfers energy to Chl a with nearly 100% efficiency, significantly broadens the spectral range accessible for light-harvesting and improves cross section of Pcb complexes. The major role of carotenoids in Pcb is photoprotection.