ANTENNA EXCITED-STATE DECAY KINETICS ESTABLISH PRIMARY ELECTRON-TRANSFER IN REACTION CENTERS AS HETEROGENEOUS

The decay of the excited primary electron donor P in bacterial photos ynthetic reaction centers (both membrane-bound and detergent-isolated) has been observed to be nonexponential on a time scale of some tens o f picoseconds. Although the multipicosecond nonexponentiality of P ha s been ascribed to heterogeneity in the rate of primary electron trans fer (PET), the decay kinetics can be interpreted equally well using ho mogeneous models. To address this ambiguity, we studied the decay of e xcited bacteriochlorophyll (Bchl) in the membrane-bound core antenna/r eaction center complexes of wild-type and mutant reaction center strai ns of Rhodobacter capsulatus. Reaction centers isolated from these sam e strains display a range of multiexponentiality in primary charge sep aration. The mutant strains carry substitutions of amino acids residin g near the monomeric Bchl on the active and/or inactive sides of the r eaction center. Transient absorption measurements monitoring the Q(y) bleach of antenna Bchls require at least two exponential components to fit all decays. The wild type was fitted with equal-amplitude compone nts whose lifetimes are 24 and 65 ps. The shortest-lived component is relatively insensitive to mutation, in contrast to the longer-lived co mponent(s) whose amplitude and magnitude were dramatically perturbed b y amino acid substitutions. Unlike the situation with isolated reactio n centers, here the only kinetic models consistent with the data are t hose in which the primary electron-transfer rate constant is heterogen eous 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 popu lation(s)-having longer-lived antenna decays-are limited by the rate o f 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 photosynthesi s.