ИСТИНА

Photosynthesis is the most efficient energy conversion process existing on Earth. Light-harvesting antennae and a reaction center (RC) are the pigment protein complexes, which responsible for the sunlight absorption, energy transport and primary chemical reactions. Paradoxically, RCs of higher plants, algae and many bacteria share the same physical and chemical principles of photosynthetic machinery. Depending on type of photosynthetic species, the RC consists of two branches of protein-bound cofactors: chlorophylls or bacteriochlorophylls (BChl), pheophytins or bacteriopheophytins (BPheo) and quinons.The first step of the electron transport chain is a formation of the P+B- charge separation state during 3 ps after excitation of a dimer of BChls known as the special pair (SP). Then the P+H- state populates within a short time about 0.5 ps [1,3]. Transient absorption spectroscopy allows us to investigate the exciton and the charge separation states dynamics and observe relaxation processes in RCs. The existence of coherent modulations in stimulated emission from the SP excited state (characteristic energies <300 cm-1) is a profound feature of bacterial RC [1]. The nature of such modulations still under debate but two water molecules located near the special pair in the RC of purple bacteria gave rise to a possible explanation of the excited state absorption at 935 nm and the stimulated emission at 1020 nm [2,3,4].According to the X-ray spectroscopy, water molecules are sited in immediate proximity of the SP, accessory BChl and histidine. The distances between water molecule and the closest neighbors are in a range of 3-7 Å. They are notably larger than the hydrogen-bond length (2.98 Å) in (H2O)2. With regard to this fact, we consider two molecules in the bacterial RC as a mediator of the electron transport chain. We have been proposed that the influence of water molecules during the first radical pair state formation is strong enough to cause the coherent modulations. The time evolution of the system has been modeled by modern physical approach based on the hierarchy coupled equations for the density matrix [5] which is, actually, the nonperturbative theory (comparing with modifications of Redfield relaxation theories where dynamics of the system usually calculated on the assumption of a weak interaction of the electronic degrees of freedom with the bath). The results of modeling of the stimulated emission and the excited-state absorption are in qualitative correspondence with the experimental data. [1] A.G. Yakovlev, A.Y. Shkuropatov, V.A. Shuvalov, “Nuclear wavepacket motion producing a reversible charge separation in bacterial reaction centers,” FEBS Letters, 466, 209-212, January 2000 [2] R.Yu. Pishchalnikov, S.M. Pershin, A.F. Bunkin, “H2O and D2O spin-isomers as a mediator of the electon transfer in the reaction center of purple bacteria,” Physics of wave phenomena, vol. 20, pp. 184-192, July 2012. [3] Darius Abramavicius and Shaul Mukamel, “Quantum oscillatory exciton migration in photosynthetic reaction centers,” J. Chem Phys., vol. 133, p. 064510, August 2010 [4] N. Ivashin, S. Larsson, “Trapped water molecule in the charge separation of a bacterial reaction center,” J. Chem. Phys. B, vol. 112, 12124-12133, September 2008 [5] Vladimir Novoderezhkin and Rienk van Grondelle, “Spectra and Dynamics in the B800 Antenna: Comparing Hierarchical Equations, Redfield and Förster Theories”, J Chem Phys B, 117, 11076-11090, 2013