Carotenoid mediated B800–B850 coupling in LH2

Ab initio calculations of the excited state properties of a portion of the light harvesting 2 complex consisting of a supermolecule of two B850 bacteriochlorophyll (BChl), one B800 BChl, and one rhodopin glucoside carotenoid show significant mixing between the BChl and carotenoid states. Calculation of the B800–B850 coulombic coupling, through the transition density cube method, both with and without the carotenoid reveal that the carotenoid affects an increase in the B800–B850 coupling via an indirect (superexchange) mechanism. This carotenoid-mediated coupling explains a portion of the difference between the B800–B850 energy transfer rate determined experimentally and through traditional (Forster) calculations.