TOWARD LEVEL-TO-LEVEL ENERGY TRANSFERS IN PHOTOSYNTHESIS - THE FENNA-MATTHEWS-OLSON PROTEIN

The trimeric Fenna-Matthews-Olson (FMO) protein is a bacteriochlorophy ll (BChl) a antenna complex, whose X-ray structure is now known at the atomic level for two green sulfur bacterial species (Prosthecochloris aestuarii and Chlorobium tepidum). Its steady-state Q(y) absorption s pectrum at low temperature exhibits considerable structure, with at le ast eight bands attributable to well-defined (groups of) BChl a excito n levels. The low-temperature absorption difference spectra of Cb, tep idum trimers excited at 789 nm are correspondingly multifeatured, and they show rich spectral evolution due to femtosecond and picosecond el ectronic energy transfers. Global analyses of these time-dependent Del ta A spectra lead to a phenomenological scenario for cascading and bra nching among exciton level groups responsible for specific steady-stat e absorption bands, The missing link in our understanding of structure -function causality in this protein stems from a lack of an ab initio theory for the effects of known protein environments on BChl a transit ion energies; this problem still limits our insights into the workings of spectrally heterogeneous antennas with multiple nonequivalent pigm ent sites. Optical anisotropy studies strongly suggest that FMO excita tions are typically localized to the 7 BChl a pigments within one prot ein subunit, rather than delocalized over the whole trimer, This local ization occurs because the resonance couplings between BChls belonging to different subunits (<20 cm(-1)) are several times smaller than the diagonal energy disorder (similar to 70 cm(-1)). Strong oscillations appear in the anisotropies (similar to 220 fs period) for pump-probe w avelengths that simultaneously overlap both of the exciton level group s responsible for the 825 and 815 nm bands in the a, spectrum, These a re not vibational coherences, but arise from quantum beating between l evels with nearly perpendicular transition moments.