Theory of the Optical Property of Retinal in Visual Pigments

Abstract

Previous calculations on the π-electron system in retinal are refined in a more rigorous way. Firstly, the σ-π interaction indispensable to derive the bond alternation is semiempirically determined from the first principle. Secondly, the effect of a sufficient number of singly- and doubly-excited configurations are examined in CI calculation.
It is thus confirmed again that the wavelength of optical absorption maximum of retinal forming protonated Schiff-base binding is about 200 nm longer than that of retinal forming normal Schiff-base binding. The concept of partially-protonated Schiff-base binding of retinal, which is a phenomenological description of the previous theory for the interaction of retinal with opsin, is also formulated by introducing the degree (x) of protonation, and the x-dependences of the wavelengths and oscillator strengths of optical absorption are calculated.