EFFECTS OF HETEROGENEITY ON RELAXATION DYNAMICS AND ELECTRON-TRANSFER RATES IN PHOTOSYNTHETIC REACTION CENTERS

The excitation and relaxation of the nuclear medium modes which couple to the electronic states can significantly modulate the dynamics of electron transfer (ET) when the relaxation process is sufficiently slow. Recent molecular dynamics (MD) studies of reaction centers (RC) show that the dynamical response of the proteic environment has noticeable nonlinear behavior, and the observed slow decay of the time correlation functions is suggestive of glassy dynamics in the system. In this context, we consider two possible models for the RC relaxation times. The first model of a ''static disorder'' discusses relaxation dynamics in the presence of a ''rough'' free energy potential. A rough potential has a smooth background on which randomly fluctuating local perturbations are superimposed. Distribution of the perturbations determines the relaxation dynamics and may be responsible for the dynamical, temperature-dependent phase change in the medium. In particular, the mean sojourn time in such a free energy potential well is no longer exponentially distributed, and this effect may explain the complex decay dynamics of the primary donor in RCs. A similar behavior can be found in a complementary model of a ''dynamic disorder'' where spontaneous conformational isomerization of chromophores causes the transition from one configurational state to another and back. The importance of such a transition, induced by-a polar group isomerization in the neighboring amino acid side chains of bacteriochlorophylls, has been suggested in the MD studies of the time-variations of the electrostatic energy at the chromophores in RCs. With the assumption that the time the system spends in a particular conformation state is Poisson distributed, the decay time is evaluated by a stochastic analysis of the isomerization process and compared with kinetics predicted by other models of the primary ET in RCs.