Noise breaking the twofold symmetry of photosynthetic reaction centers: electron transfer.

In this work we present a stochastic model to elucidate the unidirectionality of the primary charge separation process in the bacterial reaction centers where two symmetric ways of electron transfer (ET), starting from the common electron donor, are possible. We have used a model of three sites/molecules with ET beginning at site 1 with the option to proceed to site 2 or site 3. If the direct ET between sites 2 and 3 is not allowed and electron cannot escape from the system then it is shown that the different stochastic fluctuations in the energy of sites and the interaction between sites on these two ways are sufficient to cause the transient asymmetric electron distribution at site 2 and 3 during relaxation to the steady state. This means that overall asymmetric ET can be caused by the transient asymmetric electron distribution if there is a possibility for an electron to escape from the three-site system. To explore this possibility we have introduced a sink into the model at the end of each of the sites 2 and 3. The dependence of the asymmetry in electron transfer on the value of the sink parameter, introduced through an additional imaginary diagonal matrix element of the Hamiltonian, was investigated. Results show indeed that the unidirectionality of the electron transfer generated in the system of three molecules depends strongly on the sink parameter value.