Molecular Mechanism of Spectral Tuning by Chloride Binding in Monkey Green Sensitive Visual Pigment

The visual pigments of the cones perceive red, green, and blue colors. The monkey green (MG) pigment possesses a unique Cl− binding site; however, its relationship to the spectral tuning in green pigments remains elusive. Recently, FTIR spectroscopy revealed the characteristic structural modifications of the retinal binding site by Cl− binding. Herein, we report the computational structural modeling of MG pigments and quantum-chemical simulation to investigate its spectral redshift and physicochemical relevance when Cl− is present. Our protein structures reflect the previously suggested structural changes. AlphaFold2 failed to predict these structural changes. Excited-state calculations successfully reproduced the experimental red-shifted absorption energies, corroborating our protein structures. Electrostatic energy decomposition revealed that the redshift results from the His197 protonation state and conformations of Glu129, Ser202, and Ala308; however, Cl− itself contributes to the blueshift. Site-directed mutagenesis supported our analysis. These modeled structures may provide a valuable foundation for studying cone pigments.