Effects of Static Heterogeneity on the Parallel-Sequential-Superexchange Mechanism for the Primary Charge Separation in Bacterial Photosynthesis

Abstract

Until the early 1990’s, the experimental psec and subpsec data for the kinetics of the primary charge separation process in the photosynthetic RC were interpreted in terms of a single lifetime for each of the electronically excited singlet states of the bacteriochlorophyll dimer (1P*) and the ion pairs [1–[4]. This approach seemed to indicate that heterogeneity effects on the kinetics are minor. This physical situation pointed towards the possibility that the dynamics in membrane proteins is drastically different from that in globular proteins [5–[7], where low-temperature chemical dynamics, e.g., the recombination of CO to the heme site of myglobin, is dominated by a nonexponential heterogeneous kinetics [5–[7]. Of course, heterogeneous kinetics, involving static inhomogeneity effects, indicates the existence of distinct microenvironments (for the relevant prosthetic groups involved in the reaction), which do not interconvert on the relevant time scale of the dynamic process. The existence of such static distribution of microenvironments, which is ubiquitous in globular proteins, results in a static distribution of the energetics, nuclear Franck-Condon factors or/and the electronic couplings which determine the rates. The apparent absence of inhomogeneous kinetics [1–[4] for ET in the photsynthetic RC was surprising