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submitted on 27.05.2020 and posted on 29.05.2020by Yigal Lahav, Dror Noy, Igor Schapiro
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.