EXCITON MIGRATION AND TRAPPING IN PHOTOSYNTHESIS

Abstract— Mobile electronic excited states, excitons, undergo random walks through the antenna chlorophyll arrays of photosynthetic organisms. The time interval from exciton creation, by photon absorption, until its first arrival at a reaction center (RC) is called the "first passage time" (FPT) of the random walk. A theory of exciton migration and trapping presented here predicts that the exciton lifetime, as measured from chlorophyll fluorescence decay in chromatophores or P700 complexes, is a linear function of the fractional number of quanta absorbed directly by the antenna, not by the RC. The slope of this line is the FPT, and its intercept is the exciton's lifetime as limited only by photoconversion at the RC. This photoconversion-limited lifetime is simply related to the in situ photoconversion rate constant via two parameters, each of which is experimentally accessible. It is also possible to obtain values of individual FoUrster rate constants, at least approximately, from measurements of exciton lifetime as functions of temperature and excitation wavelength. This new theory, based on lattice random walk models, receives some support from fluorescence measurements done on Rhodopseudomonas sphaeroides R26 chromatophores. In its present form the theory is only applicable to one-antenna-component systems, like Rp. sphaeroides R26 or Rhodospirillum rubrum chromatophores or P700 complexes, but should be readily extendible to multi-antenna-component systems including whole chloroplasts.