Computational models for coupled electronic-vibrational energy transfer in biological photosynthetic complexes
Lee, Mi Kyung
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The specialized pigment-protein complexes involved in the first process of photosynthesis are light-harvesting structures that are composed of networks of chromophores in protein scaffolds. Though light-harvesting complexes vary in chromophore composition and protein structure, they are capable of transferring the absorbed energy as molecular excitation energy from chromophore to chromophore with maximal efficiency. Thus, numerous interdisciplinary studies focus on elucidating energy transfer mechanisms in these biological complexes and how the same principles can be applied to artificial photosynthetic and photovoltaic machines. From advanced spectroscopic measurements and theoretical models, the interaction between the excited electronic states and the nuclear vibrational degrees of freedom is now established to be crucial for efficient energy transfer. In light-harvesting complexes of plants and bacteria, it is now understood that the classical-like vibrational modes of the protein and solvent environment drive energy transfer between the energetically close electronic states of the chromophores. On the other hand, recent spectroscopic measurements on algae light-harvesting complexes discovered signatures of quantized, high frequency vibrational modes of the chromophore. Unfortunately, a deterministic interpretation of the data and the underlying Hamiltonian is hindered due to significant inhomogeneous spectral line-broadening. Though numerous model Hamiltonians have been proposed from theoretical work, various computational approximations employed in these studies necessitate empirical parameter tuning in order to obtain agreement with benchmark linear optical spectra. Thus in this work, we present a simple, but improved, computational prescription to compute the ensemble of Hamiltonians for four closely-related algae light-harvesting complexes. We verify the reliability of our proposed models by comparing simulated optical spectra with experimental measurements. We show that static disorder and inhomogeneous broadening are significant for phycobiliproteins due to large site energy fluctuations. We also show that the nuclear environment plays an important role in defining the trapping state, or the final energy acceptor. Finally, our work for the first time suggests that EET dynamics can be tuned by varying the titration states of the chromophores.