Excited state energy transfer pathways in photosynthetic reaction centers.3. Ultrafast emission from the monomeric bacteriochlorophylls

Ultrafast singlet excited state energy transfer occurs from the monomeric b acteriopheophytin (H) and bacteriochlorophyll (B) chromophores to the prima ry electron donor or special pair (P) in bacterial photosynthetic reaction centers. Because of rapid quenching of the singlet excited state of B by en ergy transfer to P, B-1 emission has not previously been observed in functi onal reaction centers. Using fluorescence upconversion, spontaneous fluores cence associated with the monomeric bacteriochlorophylls is observed for ex citation of the monomeric bacteriochlorophylls and bacteriopheophytins at 8 5 K. The decay kinetics of the fluorescence match the kinetics of the rise of emission from P-1, the ultimate acceptor of singlet energy. Together wit h measurements of the time-resolved fluorescence anisotropy, the data sugge st that B-1 is populated in the energy transfer pathway from H-1 to P. By e xciting H in wild-type and in the reaction center mutant M182HL, where cont ributions from the chromophores in the B sites on the L and M sides are spe ctrally resolved, the amplitudes of the kinetic traces at several wavelengt hs between 790 and 825 nm can be used to construct the time-resolved emissi on spectrum of B-1. The Stokes shift of the accessory bacteriochlorophylls in wild-type on the femtosecond time scale is dose to zero, while for the m onomeric bacteriopheophytin in the B-M binding site in the M182HL mutant, t he Stokes shift is less than 100 cm(-1). These results have significant imp lications fur the mechanism of ultrafast energy transfer in the reaction ce nter.