The light harvesting process in purple bacteria

We present and review the results of fluorescence upconversion and photon e cho experiments, and ab initio calculations performed in our group within t he last few years with respect to the light harvesting process in purple ba cteria. Carotenoids transfer energy to bacteriochlorophyll (BChl) mainly vi a the carotenoid S-2 --> BChl Q(x) pathway on a similar to 100 fs timescale . This transfer is reasonably reproduced by considering the Coulombic coupl ing calculated using the transition density cube method which is valid at a ll molecular separations. Carotenoids may also serve a role in mediating B8 00 --> 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 BChl. Further calculations on dimers of B850 BChl estimate the intra- and interpolypeptide coupling to be 315 and 2 45 cm(-1), respectively. These interactions are dominated by Coulombic coup ling, while the orbital overlap dependent coupling is similar to 20% of the total. Photon echo peak shift experiments (3PEPS) on LH1 and the B820 subu nit are quantitatively simulated with identical parameters aside from an en ergy transfer time of 90 fs in LR1 and infinity in B820, suggesting that ex citation 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 dis order is the dominant mechanism localizing excitation in LR1 and LH2. We su ggest that the competition between the delocalizing effects of strong elect ronic coupling and the localizing effects of disorder and nuclear motion re sults in excitation in the B850 and B875 rings being localized on 2-4 pigme nts within approximately 60 fs.