Simultaneous Solvent and Counterion Effects on the
Absorption Properties of a Model of the Rhodopsin Chromophore
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Abstract
The ASEP/MD (averaged solvent electrostatic
potential from molecular dynamics) method was employed in studying
the environment effects (solvent and counterion) on the absorption
spectrum of a model of the 11-<i>cis</i>-retinal protonated
Schiff base. Experimental studies of the absorption spectra of the
rhodopsin chromophore show anomalously large solvent shifts in apolar
solvents. In order to clarify their origin, we study the role of the
counterion and of the solute–solvent interactions. We compare
the absorption spectra in the gas phase, cyclohexane, dichloromethane,
and methanol. The counterion effect was described from both a classical
and quantum point of view. In the latter case, the contribution of
the chromophore-counterion charge transfer to the solvent shift could
be analyzed. To the best of our knowledge, this is the first time
that counterion and solvent effects on the absorption properties of
the 11-<i>cis</i>-retinal chromophore have been simultaneously
examined. We conclude that the counterion–solute ionic pair
in the gas phase is not a good model to represent the solvent shift
in nonpolar solvents, as it does not account for the effect that the
thermal agitation of the solvent has on the geometry of the ionic
pair. In contrast to nonpolar solvents, the experimental solvent shift
values in methanol can be exclusively explained by the polarity of
the medium. In dichloromethane, the presence of the counterion does
not modify the solvent shift of the first absorption band, but it
affects the position of the second excited state. In the three solvents
considered, the first two excited states become almost degenerate