QM/MM modeling of environmental effects on electronic transitions of the FMO complex

The Fenna-Matthews-Oslon (FMO) light harvesting pigment-protein complex in green sulfur bacteria transfers the excitation energy from absorbed sunlight to the reaction center with almost 100% quantum efficiency. The protein-pigment coupling (part of the environmental effects) is believed to play an important role in determining excitation energy transfer pathways. To study the effect of environment on the electronic transitions in the FMO complex, especially by taking into account the newly discovered eighth extra pigment, we have employed hybrid quantum-mechanics/molecular-mechanics (QM/MM) methods in combination with molecular dynamics (MD) simulations. The averaged site energies of individual pigments are calculated using the semiempirical ZINDO/S-CIS method considering the protein residues as atomic point charges along the MD trajectories. The exciton energies are calculated from the site energies and excitonic couplings based on MD simulations. The new eighth pigment displays the largest site energy and contributes mainly to the highest exciton level, which may facilitate transfer of excitation energies from the baseplate to the reaction center. Further, the multimode Brownian oscillator (MBO) model is used to fit the linear absorption spectra of the FMO complex, validating the exciton energies obtained from the QM/MM calculations. Our results indicate that the QM/MM method combined with MD simulations is a powerful tool to model the environmental effects on electronic transitions of light harvesting antenna complexes.