PARENT SESSION
Posters P4Aa Chlorophyll and bilin based antenna systems. Abstracts (239-271)

Excitation energy transfer in Lhca4. Krzysztof Gibasiewicz^{*,1, 4}, Roberta Croce², Tomas Morosinotto^{3, 5}, Janne Ihalainen⁴, Ivo van Stokkum⁴, Jan Dekker⁴, Roberto Bassi^{3, 5}, Rienk van Grondelle⁴, ¹ Department of Physics, Adam Mickiewicz University, Poland⁴ Faculty of Sciences, Division of Physics and Astronomy, Vrije Universiteit, The Netherlands² CNR-ITC Istituto di Biofisica, sezione di Trento, Italy³ Dipartimento Scientifico e Tecnologico, Università di Verona, Italy⁵ Université Aix-Marseille II, LGBP- Faculté des Sciences de Luminy - Département de Biologie, France

ABSTRACT- Excitation energy transfer (EET) in a reconstituted peripheral light-harvesting complex from Photosystem I (PSI), Lhca4, was studied at RT by femtosecond transient absorption spectroscopy. Two spectral forms of lutein were observed in L1 and L2 sites, characterized by significantly different interactions with nearby chlorophyll a (Chl a) molecules. A favorable interpretation of these differences is that efficiency of carotenoids (Car) to Chl EET is much lower in case of "blue" lutein in site L1 than "red" lutein in site L2. Major part of energy absorbed by the "red" lutein, 70%, is transferred to Chls on sub-100-ps time scale from the state S2 but, in addition, clear EET from hot S1 state within 400-500 fs is also observed. EET from relaxed S1 state occurs within 2-3 ps and is tentatively ascribed to violaxantin accommodated by site L2. Most of energy absorbed by the "blue" lutein is not transferred to Chls. No indication of EET from carotenoids to chlorophylls b (Chl b) was found. EET from Chl b to Chl a is biphasic and characterized by time constants of 300 fs and 3.3 ps. These rates are ascribed to EET from Chl b spectral forms absorbing at 644 nm and 650 nm, respectively. The studies on a WT preparation were compared to those on the NH-mutant missing red-most absorption band at 700 nm. Overall dynamics of EET in both preparations is similar except for the fast 15-ps decay of excitation occurring in WT only. This decay may be of biological significance: red Chls interacting with a carotenoid in site L2 offer a safe de-excitation channel. Thus, the red-most absorbing component of Photosystem I antenna, Lhca4, is suggested to play a photoprotective role in PSI super-complex.

KEY WORDS: carotenoids, excitation energy transfer, femtosecond transient absorption, excitonic coupling