Calculation of Couplings and Energy-Transfer Pathways between the Pigments of LH2 by the ab Initio Transition Density Cube Method

A practical method for accurate evaluation of the Coulombic contribution to the electronic coupling for energy transfer at any donor-acceptor separation is reported. The method involves the exact interaction between transition densities of each chromophore which are calculated ab initio and may include electron correlation. The method is used to calculate coupling strengths between the pigments of the bacterial light-harvesting complex, LH2, and to compare with results using the ideal dipole approximation (IDA). The results suggest that the relatively symmetric transitions of bacteriochlorophyll a (Bchla) pigments are reasonably well described by the IDA for separations >15 Å, although deviations are significant at smaller separations. The less symmetric transition of the twisted carotenoid pigment is rather poorly described by the IDA and shows significant deviation even at separations of well over 20 Å. The calculated coupling strengths are combined with estimates of the spectral overlap integral to estimate energy-transfer rates and time scales. The total depopulation time scale of the carotenoid S2 state is estimated to be 85 fs, in reasonable agreement with experiment. The B800-B850 transfer time is estimated to be 1.3 ps (a factor of 2 slower than experiment). Rapid (<400 fs) B800-B800 energy transfer is also predicted. Moreover, the calculations suggest that energy flows both from the carotenoid and the B800 Bchla into pigments of several different protomer units, indicating that interaction between protomer units is important in the LH2 function.