Ab Initio Excitation Energy Transfer in Peridinin-Chlorophyll a-Protein Complex

The study of excitation energy transfer in photosynthetic light-harvesting antenna complexes is important because of their high quantum efficiencies, which can approach 100% between pigments in some systems. Peridinin-Chlorophyll a-Protein (PCP) is a light-harvesting complex which has been isolated from the dinoflagellate Amphidinium carterae and characterized by x-ray crystallography. PCP is a trimeric light-harvesting protein complex which contains eight peridinin carotenoids and two chlorophyll a pigment molecules per protein monomer, and the excitation energy transfer efficiency within this system is estimated at close to 90%. To study the excitation energy transfer in PCP, we employ the Förster Resonance Energy Transfer (FRET) model, which assumes that energy transfer between pigments occurs by an incoherent dipole-dipole interaction. We calculate the FRET coulombic coupling and spectral overlap terms using ab initio multireference configuration interaction (MR-CI) with semi-empirical basis sets. The two dominant excitation energy transfer pathways within PCP are from the S₂ state of peridinin to the Q_x band of chlorophyll a, and from the S₁/ICT (intramolecular charge transfer) state of peridinin to the Q_y band of chlorophyll a. In peridinin, absorption in the visible spectrum is due to the strongly allowed S₀ → S₂ transition, while the S₀ → S₁ transition is optically forbidden as well as having significant double excitation behavior. While the other transitions in PCP are single excitations, we must model our system using MR-CI to accurately describe the double excitation behavior of the S₁/ICT state in peridinin. Our excitation energy transfer study of PCP using FRET with parameters derived from ab initio MR-CI reveals the interplay of pigment geometry and environment on the relative contributions of energy transfer rates and efficiencies within the complex as a whole.