Energy and electron transfer in photosynthetic proteins

Adrien Alexis Paul Chauvet, Purdue University

Abstract

We spectroscopically resolved the reduced state of the primary electron acceptor of Photosystem I, A0, in its lowest energy Qy region for the first time without the addition of chemical reducing agents and without extensive data manipulation. To carry this out, we used the menB mutant of Synechocystis sp. PCC 6803 in which phylloquinone is partially replaced by plastoquinone-9 in the A1 sites of Photosystem I. The subsequent long lived A0- state is most probably due to dysfunctional A1 sites. This conclusion is inferred based on the monitored A0- lifetime of ∼22 ns that is typical of charge recombination between A0 - and P700+. The maximum bleaching (A 0- - A0) was found to occur at 684 nm with a corresponding extinction coefficient of 43 mM-1 cm -1. The data show evidence of an electrochromic shift of an accessory chlorophyll pigment, suggesting that the latter Qy absorption band is centered around 682 nm. The study of electrochromic shift in the P798+ of Heliobacterium modesticaldum also shed lights on its RC core composition: we propose the first spectral evidence that, while the A 0 pigment is Chl a, the accessory pigment is BChl g. Femtosecond spectroscopy reveals that energy equilibration within the BChl g antenna pigments occurs in 0.67 ps, trapping of the energy by the special pair takes place in ∼5 and ∼20 ps and that the resulting A0- has a lifetime of ∼450 ps. With similar kinetics for energy and electron transfers, the green sulfur bacteria Chlorobium tepidum has the advantage that its special pair, P840, can be clearly differentiated from the main antenna absorption band at low temperature. Such unique feature permitted us to observe for the first time the rapid electron transfer from the directly excited special pair (BChl a) to the primary electron acceptor (Chl a) and infer that the intrinsic charge separation in the RC occurs in <200 fs. We also confirm that the slow heme reduction in the dimeric cytochrome b6f>complex, occurring in ∼150 s, involves both, the heme bn and bp, simultaneously. Under complete reduction of the hemes, the b6f complex most probably undergoes conformational changes. This conclusion is supported by the consequent electrochromic shift of the embedded Chl a that is opposite, in direction, to the predictions based on the current oxidized b6f complex structure.

Degree

Ph.D.

Advisors

Savikhin, Purdue University.

Subject Area

Biochemistry|Physics|Biophysics

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