Photosynthesis is one of the most important and thus most widely studied biological processes. Its exciton transfer pathway involves a number of energetically coupled chromophore molecules that form an ‘energy funnel’ directed to the reaction center. The overall transfer process can exhibit near-unity efficiency in some situations, and recent studies suggest that such a high efficiency may result from orchestration between the quantum interactions among nearby chromophores and the noise exerted by their bath environment. Theoretical studies of the process with the chromophores in the condensed phase can be conducted at various levels of models. Of those, methods with atomistic details of the system can be of particular interest. Sometimes, molecular dynamics simulations are adopted for such studies. In this case, molecular models that can accurately describe the behaviors of the chromophores are definitely needed. Here, we focus on the modeling of the bacteriochlorophyll-a molecule in the prototypical Fenna-Matthews-Olson complex. First, we investigate how reliable a model can be when it is based on harmonic approximations, with the force parameters fitted to quantum chemical calculation results in the electronic ground and the excited states. In this case, significant overestimation in the chromophore reorganization energy is observed especially when a separate set of quantum chemical calculations are performed to generate excited state energies. This implies that the protein can force the chromophore to drift away from its equilibrium conformation on its energy landscape, and the harmonic approximation may not be appropriate for describing such a modification. An improved model is also tried and the subsequent observations will be discussed in a comparing manner.

Development of bacteriochlorophyll a potential energy surface by quantum chemical calculations

249th National Meeting (2015)

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