Electronic excitation transfer in the LH2 complex of Rhodobacter sphaeroides

Ultrafast fluorescence upconversion measurements were carried out on the peripheral (LH2) light harvesting antenna complex of Rhodobacter sphaeroides isolated in the detergents N-octyl-beta-D-glucopyranoside and lauryl dimethylamine oxide. The B800 and B850 bands were excited in separate experiments, and the B850 emission was detected in each case. We make use of the recently determined crystal structure of a purple bacterial LH2 complex to simulate our data and to calculate the exciton level structure of the B850 aggregate. The B800 to B850 excitation transfer occurs with a 650 fs time constant. The depolarization of B850 emission follows a wavelength dependent, biexponential decay with time constants 50-90 and 400-500 fs. We can reproduce the non-exponentiality of the depolarization by assuming incoherent hopping between dimeric sites with a 250 cm(-1) full width at half-maximum (fwhm) Gaussian distribution of site energies (inhomogeneity). We calculate the homogeneous hopping time between dimers in B850 to be similar to 100 fs. The exciton calculations including pigment energetic disorder demonstrate the validity of a hopping picture of excitation transfer within the B850 band at room temperature.