Energy transfer dynamics of the B800—B850 antenna complex of Rhodobacter sphaeroides: a hole burning study

Hole burned spectra for the B800—B850 bacteriochlorophyll a antenna complex of Rb. sphaeroides chromatophores have been obtained for burn wavelengths ranging from the high-energy side of the B800 absorption profile to the low-energy side of B850 profile. The results indicate that B800—B800 energy transfer at 1.6 K is slow relative to B800 to B850 transfer which occurs in 2.4 ps. Whereas B800 is largely site inhomogeneously broadened, B850 is found to exhibit a homogeneous linewidth of about 220 cm−1 at low temperatures. The latter results is used to explain the temperature independence of the B800 to B850 energy transfer rate. The homogeneous linewidth of 220 cm−1 is argued to be a measure of the exciton bandwidth of B850. It is concluded that B800 and B850 are weakly exciton-coupled and that the mechanism for B800 to B850 energy transfer is most likely of the Förstertype. The B850 vibronic hole structure allows for an identification of bacteriochlorophyll a modes that participate in Förster transfer.