Does the Chromophore’s Ring Move after Photoexcitation of Rhodopsin?

By comparing the shift of the absorption maxima when a visual pigment is converted to its lumirhodopsin photointermediate for two classes of pigments, we can infer whether or not the pigment’s β-ionone ring has left its binding site. We compare this shift for the long-wavelength sensitive visual pigment of chicken iodopsin (λmax=571nm), which has polar residues in the ring binding site that interact with the ring, with that for three pigments, which do not. We conclude that by the time the Lumi product of the pigment is formed, the ring has moved away from the ring binding site.