R. Jimenez et al., ELECTRONIC EXCITATION TRANSFER IN THE LH2 COMPLEX OF RHODOBACTER-SPHAEROIDES, Journal of physical chemistry, 100(16), 1996, pp. 6825-6834
Ultrafast fluorescence upconversion measurements were carried out on t
he peripheral (LH2) light harvesting antenna complex of Rhodobacter sp
haeroides isolated in the detergents N-octyl-beta-D-glucopyranoside an
d 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 excito
n level structure of the B850 aggregate. The B800 to B850 excitation t
ransfer occurs with a 650 fs time constant. The depolarization of B850
emission follows a wavelength dependent, biexponential decay with tim
e constants 50-90 and 400-500 fs. We can reproduce the non-exponential
ity of the depolarization by assuming incoherent hopping between dimer
ic sites with a 250 cm(-1) full width at half-maximum (fwhm) Gaussian
distribution of site energies (inhomogeneity). We calculate the homoge
neous 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 B
850 band at room temperature.