Charge Separation in Photosynthetic Reaction Centers under Femtosecond Excitation

A. G. Yakovlev¹ and V. A. Shuvalov^1,2*

¹Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-3181

²Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow Region, 142292 Russia; fax: (277) 90532; E-mail: shuvalov@issp.serpukhov.su

^* To whom correspondence should be addressed.

Received April 27, 2000; Revision received September 29, 2000

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The process of electron transfer from the primary electron donor P* to the primary electron acceptor B_A in the reaction center of Rhodobacter sphaeroides R-26 under 30 fsec pulse excitation was studied in this work with the aim of establishing a relationship between the nuclear subsystem motion and charge transfer. For this purpose the fsec and psec oscillations in the bands of stimulated emission of P* and in the band of reaction product B_A^- at 1020 nm were investigated. It was established that the reversible formation of the P⁺B_A^- state is characterized by two vibration modes (130 and 320 cm^-1) and connected with an arrival of the wavepacket induced by fsec excitation to the intersection of potential surfaces P*B_A and P⁺B_A^-. The irreversible formation of the P⁺B_A^- state with the time constant of 3 psec is followed by oscillations with frequencies of 9 and 33 cm^-1. These results show that the irreversibility of electron transfer is determined by two factors: 1) by a difference between the energy width of the wavepacket and the gap between the named surfaces; 2) by a difference between the duration of wavepacket residence near the intersection of the surfaces and the relaxation time of the P⁺B_A^- state.

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KEY WORDS: photosynthesis, reaction center, electron transfer, wavepacket, femtosecond spectroscopy

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