The Light Harvesting Process in Purple Bacteria

We present and review the results of fluorescence upconversion and photon echo experiments, and ab initio calculations performed in our group within the last few years with respect to the light harvesting process in purple bacteria.Carotenoids transfer energy to bacteriochlorophyll (BChl) mainly via the carotenoid S2 ->BChl Q pathway on a ~ 100 fs timescale.This transfer is reasonably reproduced by considering the Coulombic coupling calculated using the transition density cube method which is valid at all molecular separations.Carotenoids may also serve a role in mediating B800 ->B850 energy transfer in LH2 by perturbing the transition density of the B850 as shown by ab initio calculations on a supermolecule of two B850 BChls, one carotenoid and one B800 BCh1.Further calculations on dimers of B850 BChl estimate the intra-and interpolypeptide coupling to be 315 and 245 cm -", respectively.These interactions are dominated by Coulombic coupling, while the orbital overlap dependent coupling is 20% of the total.Photon echo peak shift experiments (3PEPS) on LH1 and the B820 subunit are quantitatively simulated with identical parameters aside from an energy transfer time of 90 fs in LH1 and oo in B820, suggesting that excitation is delocalized over roughly two pigments in LH1.3PEPS data taken at room and low temperature (34 K) on the B800-B820 suggest that static disorder is the dominant mechanism localizing excitation in LH1 and LH2.We suggest that the competition between the delocalizing effects of strong electronic coupling and the localizing effects of disorder and nuclear motion results in excitation in the B850 and B875 rings being localized on 2-4 pigments within approximately 60 fs.