PARENT SESSION
Posters P3A Bacteriochlorophyll based antenna systems. Abstracts (219-238)

Charge and protein dynamics in the reaction center of purple bacteria studied by ultrafast midinfrared spectroscopy. Natalia Pawlowicz-Wereszczynska^*,1, Jacques Breton², Luuk van Wilderen¹, Rienk van Grondelle¹, Marie Louise Groot¹, ¹ Faculty of Sciences, Amsterdam, The Netherlands² Service de Bioenergetique, Bat. 532, France

ABSTRACT- The reaction center (RC) of purple bacteria Rb. sphaeroides R26 is a transmembrane pigment-protein complex responsible for charge separation in photosynthesis. Excitation of the RC by light initiates electron transfer from the primary electron donor P, a pair of strongly coupled bacteriochlorophyll a (BChl a) molecules. Time-resolved Visible pump/MidInfraRed probe spectroscopy in the region between 1600 and 1800 cm^-1 was applied to investigate electron transfer and radical pair/protein relaxation in the Rb. sphaeroides RC at room temperature. In this work we have studied samples with and without quinone Q_A. The RC ′s were excited with two different wavelengths: at 600 nm (unselectively) and 800 nm (direct excitation of Bacteriochlorophyll a (BChl a)). The region between 1600 and 1800 cm^-1 probes the absorption changes in the C=O stretches of the chromophore (and protein). After photoexcitation of the RC the P^* state decays on a time scale of 3.7 ps to state P⁺B_A^-(accessory BChl a). This is the first report of the midIR absorption spectrum of B_A; we find that the keto C=O stretch of B_A is around 1675 cm^-1, indicating a relatively strong H-bond with the protein. After subsequent electron transfer to H_A in ∼0.5 ps, the state P⁺H_A^- is formed. Radical pair relaxation on the 20-30 ps time scale is accompanied by a minor change in a product band at 1664 cm^-1, which we tentatively ascribe to amide C=O.

KEY WORDS: Energy/electron transfer, Photosynthetic reaction center, Visible/MidInfraRed pump-probe spectroscopy, Reaction center of purple bacteria