Excited State Energy Transfer Pathways in Photosynthetic Reaction Centers. 3. Ultrafast Emission from the Monomeric Bacteriochlorophylls

Abstract

Ultrafast singlet excited state energy transfer occurs from the monomeric bacteriopheophytin (H) and bacteriochlorophyll (B) chromophores to the primary electron donor or special pair (P) in bacterial photosynthetic reaction centers. Because of rapid quenching of the singlet excited state of B by energy transfer to P, 1B emission has not previously been observed in functional reaction centers. Using fluorescence upconversion, spontaneous fluorescence associated with the monomeric bacteriochlorophylls is observed for excitation of the monomeric bacteriochlorophylls and bacteriopheophytins at 85 K. The decay kinetics of the fluorescence match the kinetics of the rise of emission from 1P, the ultimate acceptor of singlet energy. Together with measurements of the time-resolved fluorescence anisotropy, the data suggest that 1B is populated in the energy transfer pathway from 1H to P. By exciting H in wild-type and in the reaction center mutant M182HL, where contributions from the chromophores in the B sites on...