EXCITATION MIGRATION AND TRAPPING IN HOMOGENEOUS AND HETEROGENEOUS LATTICES - APPLICATION TO THE LIGHT-HARVESTING ANTENNA COMPLEX OF PHOTOSYNTHETIC BACTERIA

Energy transfer in arrays of photosynthetic antenna pigments containing a photochemical trap is studied combining the transition probability matrix method with random walk theory and experimental data on fluorescence lifetimes and overall trapping efficiencies in systems with open and closed photochemical trap. New exact expressions for the fluorescence decay time and the trapping at infinite time are derived that contain the probability to escape from the trap as a single parameter. Two approximate relations for the fluorescence decay time, tau(e), and T, the probability that trapping has occurred at infinite time, are derived that can be applied to homogeneous and heterogeneous antenna arrays: tau(e) = tau(t)tau(cl)/(tau(t) + tau(cl)pf) T = tau(cl)/(tau(cl) + tau(t)[(1/p - 1)/f + 1]), where tau(t) is the characteristic time of photochemical trapping, tau(cl) the fluorescence decay time for closed traps, f the probability to escape from the trap, and p the equilibrium probability to find the excitation on the trap. p is independent of the initial conditions (selective or random excitation); p = 1/N for homogeneous lattices, while for heterogeneous symmetric lattices p can be easily calculated with the transition probability matrix method. For homogeneous lattices the above relations are accurate to within a few percent over the entire range 0 < f < 1; for the intermediate values of f they are a considerable improvement over published expressions for the fast and slow hopping limits (f = 1 and f = 0, respectively). The relations derived here are used for determining the escape probability f and the mean residence or hopping time h for the core antenna system of the photosynthetic bacterium Rhodobacter sphaeroides.