Antenna excited state decay kinetics establish primary electron transfer in reaction centers as heterogeneous

Abstract

The decay of the excited primary electron donor P* in bacterial photosynthetic reaction centers (both membrane-bound and detergent-isolated) has been observed to be nonexponential on a time scale of some tens of picoseconds. Although the multipicosecond nonexponentiality of P* has been ascribed to heterogeneity in the rate of primary electron transfer (PET), the decay kinetics can be interpreted equally well using homogeneous models. To address this ambiguity, we studied the decay of excited bacteriochlorophyll (Bchl) in the membrane-bound core antenna/reaction center complexes of wild- type and mutant reaction center strains of Rhodobacter capsulatus. Reaction centers isolated from these same strains display a range of multiexponentiality in primary charge separation. The mutant strains carry substitutions of amino acids residing near the monomeric Bchl on the active and/or inactive sides of the reaction center. Transient absorption measurements monitoring the Qy bleach of antenna Bchls require at least two exponential components to fit all decays. The wild type was fitted with equal-amplitude components whose lifetimes are 24 and 65 ps. The shortest- lived component is relatively insensitive to mutation, in contrast to the longer-lived component(s) whose amplitude and magnitude were dramatically perturbed by amino acid substitutions. Unlike the situation with isolated reaction centers, here the only kinetic models consistent with the data are those in which the primary electron-transfer rate constant is heterogeneous, suggesting at least two structural populations of RCs. PET in the population with the shortest-lived antenna decay causes the kinetics to be transfer-to- trap-limited, whereas the kinetics in the other population(s)-having longer- lived antenna decays-are limited by the rate of PET. Observation of both types of kinetic limitation within a single light-harvesting system is unexpected and complicates any discussion of the rate-limiting step of light energy utilization in photosynthesis.