Dynamical confinement effects on chromophore photoexcitations in a light-harvesting protein

Naturally evolved light-harvesting complexes are assembled from highly evolutionarily-conserved sequences of amino acids that form functional chemical pockets for chromophores. The protein environment surrounding the chromophore influences its dynamical structure, which can have consequences for light harvesting efficiency. Here, we explore the impact of the C-phycocyanin protein scaffold on the dynamical structure and photoexcited states of the phycocyanobilin (PCB) chromophore using a combination of molecular dynamics and the ab initio Bethe Salpeter equation approach. We compute low-lying excited states of the PCB chromophore for an ensemble of snapshots from molecular dynamics trajectories, and use a normal mode analysis to understand the nature of thermal displacements of PCB in the confined protein environment. We analyze the excited state energies in terms of one electron and electron-hole contributions, identifying displacement patterns with the strongest exciton-vibrational coupling. Our results provide insight into how thermal motion of the chromophore is altered in protein environments and the resulting impact on photo-excited states, with implications for other natural light harvesting materials.