Dielectric effects due to the environment on the structure and proton affinity of retinal Schiff base models

We performed DFT calculations, using the DMOL implementation of the COSMO solvent model, to investigate the effect of different dielectric responses of the environment on the structure and electronic configuration of two unsubstituted Schiff base models of retinal, including three and four conjugated double bonds, as well as a monomethylated model. The results show that the application of different dielectric constants in the calculations significantly influences the proton affinity of the molecule. Applying a continuum model as a model for the protein environment, we may conclude that the protein can efficiently adjust the pK_a of the chromophore by modifying local screening effects in the vicinity of the retinal Schiff base and, in this way, control the proton transfer.