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 approximately 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 b, 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.

Institute of Physical Biology University of South Bohemia 373 33 Nové Hrady Czech Republic

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