Glutamic acid 181 is uncharged in dark-adapted visual rhodopsin.

We have performed a high-level quantum chemical analysis to study the chromophore-protein interaction involving the charged (B) and/or uncharged (C) form of E181 and also a mutant E181Q model in the presence of the primary counterion E113 (A). As the magnitude of the calculated spectral shifts on either side remains within +/-10 nm, we show that the orientation of the dipole moment vector is the key to unlocking the puzzle on this contentious issue. We find that E181 is present in the uncharged (or) protonated form in the dark-adapted visual Rhodopsin, and therefore an electrostatically neutral environment is envisaged.