Excitation energy transfer pathways in Lhca4

EET in reconstituted Lhca4, a peripheral light-harvesting complex from Photosystem I of Arabidopsis thaliana, containing 10 chlorophylls and 2 carotenoids, was studied at room temperature by femtosecond transient absorption spectroscopy. Two spectral forms of Lut were observed in the sites L1 and L2, characterized by significantly different interactions with nearby chlorophyll a molecules. A favorable interpretation of these differences is that the efficiency of EET to Chls is about two times lower from the blue'' Lut in the site L1 than from the red'' Lut in the site L2 due to fast IC in the former case. A major part of the energy absorbed by the red'' Lut,; 60% - 70%, is transferred to Chls on a sub-100-fs timescale from the state S-2 but, in addition, minorEET from the hot S-1 state within 400 - 500 fs is also observed. EET from the S1 state to chlorophylls occurs also within 2 - 3 ps and is ascribed to Vio and/or blue'' Lut. EET from Chl b to Chl a is biphasic and characterized by time constants of; 300 fs and 3.0 ps. These rates are ascribed to EET from Chl b spectral forms absorbing at similar to644 nm and similar to650 nm, respectively. About 25% of the excited Chls a decays very fast - within similar to15 ps. This decay is proposed to be related to the presence of the interacting Chls A5 and B5 located next to the carotenoid in the site L2 and may imply some photoprotective role for Lhca4 in the photosystem I super-complex.