Excitation energy transfer pathways in Lhca4

Abstract

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 S2 but, in addition, minor EET from the hot S1 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 ∼644 nm and ∼650 nm, respectively. About 25% of the excited Chls adecays very fast-within ∼15 ps. This decay isproposed tobe 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. © 2005 by the Biophysical Society.