How do proteins regulate the driving force of excitation transfer in a photosynthetic marine algae?

Abstract

This contribution presents a detalied theoretical study on how proteins regulate the electronic couplings responsible for exciton delocalization and electronic energy transfer (EET) in photosynthetic pigmentprotein complexes. Understanding environment effects, which cause line broadening and screen electronic interactions, is fundamentally important because of its central role in the control of EET dynamics. Recent work has furthermore shown that simple models for solvation may not be sufficient to explain effects that go beyond Förster theory, such as coherent contribution to energy transfer. Here, we focus on the phycobiliprotein PE545 from the unicellular photosynthetic cryptophyte algae Rhodomonas CS24,3 and apply a novel combined quantum mechanics/molecular mechanics (QM/MM) method4 that explicitly incorporates environment polarization (protein and solvent) at the atomic level on the calculation of site energies and electronic couplings, thus going beyond the continuum dielectric approximation. In addition, we run molecular dynamics (MD) simulations of the PE545 complex in order to explore the effect of protein structural motions on the predicted properties. Our results unveil strong variations in the effective dielectric properties experienced by the different pigmet pairs in the PE545 system. In addition, our results provide insights into the limitations of structure-based methods based on the crystal structure, as opposed to the averaged-structure picture obtained from MD simulations