Abstract
We present progress toward a first-principles parametrization of the Hamiltonian of the Fenna-Matthews-Olson pigment-protein complex, a molecule that has become key to understanding the role of quantum dynamics in photosynthetic exciton energy transfer. To this end, we have performed fully quantum mechanical calculations on each of the seven bacteriochlorophyll pigments that make up the complex, including a significant proportion of their protein environment (more than 2000 atoms), using linear-scaling density functional theory exploiting a recent development for the computation of excited states. Local pigment transition energies and interpigment coupling between optical transitions have been calculated and are in good agreement with the literature consensus. Comparisons between simulated and experimental optical spectra point toward future work that may help to elucidate important design principles in these nanoscale devices.
Highlights
The FMO complex is rare in being water-soluble and was the first PPC structure to be resolved by X-ray spectroscopy. (The most recent structure has a resolution of 1.3 Å.)[7] This revealed a relatively simple structure, consisting of a trimeric unit in which each monomer contains just seven bacteriochlorophyll (BChla) molecules
This has led to a number of new ideas concerning the role of multipigment delocalized states and their temporal coherences in the efficiency of photosynthetic energy funneling.[4,5,8−14] because funneling requires a source of dissipation to relax excitations down through the energy landscape,[2,3,15,16] these excited states are subject to significant external noise, most notably from the vibrational and phonon environment of the surrounding protein and water.[3,15,16]
Important issues that arise from such an approach, are that mean field charges may not accurately represent the electrostatic environment of the pigment[34] and, in the case of QM/MM simulations, neglect of Pauli repulsion between the pigment and surrounding point charges may lead to distortion of the electronic wave function
Summary
Calculations were based on the holo (8 BChla per monomer) form of the trimeric 1.3 Å X-ray crystal structure of Prosthecochloris aestuarii (PDB: 3EOJ).[7]. We have used a smeared ion representation under open-boundary conditions with a relative dielectric of 20.47,56 Optical spectra were calculated from Fermi’s golden rule in a joint valence and conduction basis.[44] Note that the current method of conduction NGWF optimization is suitable only for describing unoccupied states that are localized on the system and do not hybridize significantly with the continuum of higher-energy unbound states These are the states of interest for most properties because transitions to the continuum of unbound states do not have large optical matrix elements and are in general, optically dark. This material is available free of charge via the Internet at http://pubs.acs.org
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