Ab initio molecular orbital calculations of electronic couplings in the LH2 bacterial light-harvesting complex of Rps-acidophila

The results of ab initio molecular orbital calculations of excited states a nd electronic couplings (for energy transfer) between the B800 and B850 bac teriochlorophyll a (Bchl) chromophores in the peripheral light-harvesting c omplex (LH2) of the purple photosynthetic bacterium Rhodopseudomonas acidop hila are reported. Electronic couplings are estimated from "supermolecule" calculations of Bchl dimers using the CI-singles methodology and 3-21G" or 6-31G" basis sets. A scheme for dissecting the coupling into contributions from the Coulombic coupling and the short-range coupling (i.e., dependent o n interchromophore orbital overlap) is reported. B850 couplings are calcula ted to be [total (Coulombic + short)]: intrapolypeptide dimer 320 (265 + 55 ) cm(-1) and interpolypeptide dimer 255 (195 + 60) cm(-1) at the CIS/6-31G* level. These results differ significantly from those estimated using the p oint dipole approximation. The effect of including Mg ligands (His residues ) and H-bonding residues (Trp and Tyr) is also investigated. The consequenc es for superradiance and energy transfer dynamics and mechanism are discuss ed.