Excited states of peridinin in PCP: 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 PCP complex. Here we focus specifically on the energy ordering of the bright excited states of eight peridinin chromophores in PCP. We provide a detailed analysis of the effects of the electrostatic interactions of peridinins with the protein matrix on excitation energies. Our calculations point to peridinins 611 and 621 as the lowest energy excitation sites among the eight carotenoid chromophores. Despite the substantial transition dipole moment, our results point to marginal effects of the environment polarization on the computed excitation energy. This conclusion is supported by both the zero-order polarization approximation to TDDFT/EFP and linear-response TDDFT/EFP, polarizable embedding QM/MM calculations. Our simulations also indicate that methods beyond molecular mechanics should be used to sample the peridinins’ conformational space.