Excited States of Peridinin in PCP: A Polarizable Embedding QM/MM Study

A complete mechanistic understanding of the excitation energy transfer in the Peridinin Chlorophyll Protein (PCP) complex has been a long sought after goal due to the protein's high light-harvesting efficacy. Multiple factors including the relative energies, electronic couplings, and the chromophores' interactions with the protein environment collectively contribute to the overall efficiency of the energy transfer in the PCP complex. Here we focus specifically on the energy ordering of the bright excited states of the eight peridinin chromophores in monomeric PCP. We provide a detailed analysis of the effects of the electrostatic interactions of peridinins with the protein matrix on excitation energies. Our results point to peridinins 611 and 621 as the lowest-energy excitation sites among the eight carotenoid chromophores. Despite the substantial transition dipole moment, the effects of the environment polarization on the computed excitation energies are found to be minor as compared to the effects of environment electrostatics. This conclusion is supported by both the perturbative state-specific and linear-response TDDFT/EFP polarizable embedding QM/MM calculations. Our simulations also indicate that methods beyond molecular dynamics with conventional molecular mechanics force fields should be used to sample the configurational space of the peridinins.