Spectral Tuning of Chlorophylls in Proteins – Electrostatics vs. Ring Deformation

In photosynthetic complexes, tuning of chlorophyll light-absorption spectra by the protein environment is crucial to their efficiency and robustness. Water Soluble Chlorophyll-binding Proteins from Brassicaceae (WSCPs) are useful for studying spectral tuning mechanisms due to their symmetric homotetramer structure, the ability to rigorously modify the chlorophyll’s protein surroundings, and the availability of crystal structures. Here, we present a rigorous analysis based on hybrid Quantum Mechanics and Molecular Mechanics simulations with conformational sampling to quantify the relative contributions of steric and electrostatic factors to the absorption spectra of WSCP-chlorophyll complexes. We show that when considering conformational dynamics, chlorophyll ring deformation accounts for about one-third of the spectral shift, whereas protein electrostatics accounts for the remaining two-thirds. From a practical perspective, protein electrostatics is easier to manipulate than chlorophyll conformations, thus, it may be more readily implemented in designing artificial protein-chlorophyll complexes with desired spectral shift.