Retinal has a highly dipolar vertically excited singlet state: implications for vision.

We have measured the effect of an intense electric field on the absorption spectrum of solutions of all-trans retinal, its unprotonated Schiff base with n-butylamine, and the Cl- salt of this protonated Schiff base. The field-induced change in extinction coefficient as a function of wavelength was analyzed to determine the ground-state dipole moment (mug), the change in dipole moment on excitation (deltamu), and the direction of mug and deltamu). These experiments have shown that all three species become highly dipolar upon excitation to the first allowed excited singlet state (deltamu = 15.6, 9.9, 12D, respectively). The ground-state and excited-state dipole moments are nearly parallel to the long axis of these molecules. Excitation is accompanied by a shift of negative charge toward the carbonyl or Schiff base terminus, making the ionone end of these molecules positively charged. The large excited state dipole moment of all-trans retinal indicates that the vertically excited state, which is of 1Bu parentage (C2h), has become significantly mixed with even-parity states. On the basis of previous theoretical calculations, this mixing is expected to facilitate isomerization in the singlet manifold. We have also found that 11-cis retinal has a large deltamu (12.7 +/- 1.4 D) on excitation. In the visual pigments, the interaction of the excited-state dipole moment of retinal with a suitably located charged group could control the position of the absorption maximum. Also, the large shift in charge density upon excitation of retinal may lead to new electrostatic interactions between the chromophore and the protein that would act as a driving force for the initial conformational changes in visual excitation.