Electronic pathway in reaction centers from Rhodobacter sphaeroides and Chloroflexus aurantiacus

Abstract

The reaction centers (RC) of Chloroflexus aurantiacus and Rhodobacter sphaeroides H(M182)L mutant were investigated. Prediction for electron transfer (ET) at very low temperatures was also performed. To describe the kinetics of the C. aurantiacus RCs, the incoherent model of electron transfer was used. It was shown that the asymmetry in electronic coupling parameters must be included to explain the experiments. For the description of R. sphaeroides H(M182)L mutant RCs, the coherent and incoherent models of electron transfer were used. These two models are discussed with regard to the observed electron transfer kinetics. It seems likely that the electron transfer asymmetry in R. sphaeroides RCs is caused mainly by the asymmetry in the free energy levels of L-and M-side cofactors. In the case of C. aurantiacus RCs, the unidirectionality of the charge separation can be caused mainly by the difference in the electronic coupling parameters in two branches.

Quantum Biological Switch Based on Superradiance Transitions

“…The photosystem II (PSII) RC of many bacteria, plants and algae, where the primary charge separation occurs, is arranged in two symmetric branches, even if only one of them is active for the ET. Different mechanisms which could be responsible for the asymmetry in the ET in the PSII RCs, and the related experiments, are discussed in [21][22][23][24][25][26][27][28][29][30] (see also references therein).…”

“…This transfer occurs in a very short time (a few picoseconds). On the other hand, the effective ET to the quinone occurs over a much longer time (a few microseconds) [26][27][28][29][30][31][32][33]. This allows one to consider a simplified model for the RC taking into account the short time dynamics corresponding to primary charge separation (disregarding longtime effects) by adding sinks through which the electron can escape the system.…”

“…Even though there is no general consensus on the role quantum coherence plays in the electron transfer (ET) efficiency ( 99%) [15][16][17][18][19][20], there is no doubt that the models for exciton transport in the LHCs and primary charge separation in the reaction centers (RCs) should utilize quantum coherent effects.The photosystem II (PSII) RC of many bacteria, plants and algae, where the primary charge separation occurs, is arranged in two symmetric branches, even if only one of them is active for the ET. Different mechanisms which could be responsible for the asymmetry in the ET in the PSII RCs, and the related experiments, are discussed in [21][22][23][24][25][26][27][28][29][30] (see also references therein).Here we do not address the question why only one branch is active, but we use the PSII RC as a prototype for an artificial biological switch, able to drive the ET to the left or the right symmetric branch, by controlling the couplings to the sinks.Primary charge separation in the RC can be modeled starting from a donor (a dimer, called the special pair where the excitation starts) and then including the ET through different protein subunits, (bacterio)chlorophylls and (bacterio)pheophytins, generally called chromophores. This transfer occurs in a very short time (a few picoseconds).…”

Noise-assisted quantum electron transfer in photosynthetic complexes

“…This approach is used for modeling the dynamics of the ET in photosynthetic complexes. (See, for example, [12,13], and references therein. )…”

“…At the same time, many efforts have been devoted to various modifications of the Markus theory. One of them is related to taking into account the sinks which represent the continuum electron energy reservoirs analogous to the HOMO and LUMO orbitals in the chemical systems [12,13,22] (see also references therein). These continuum manifolds are similar to those described by the Weisskopf-Wigner model [23,24].…”