Implications of modelling the chromophore of rhodopsin and bacteriorhodopsin with resonance Raman spectra of retinal Schiff bases

Resonance Raman spectra of protonated and unprotonated Schiff bases of all‐trans retinal with propylamine, hexylamine, butylamine, deuterated butylamine (d₉), methylamine and N¹⁵ methylamine have been obtained. The spectra of methylated and deuterated butylamine Schiff bases of retinal have also been recorded. It was found that the carbon–nitrogen stretching vibration associated with the Schiff base is insensitive to changes in the mass and/or length of the saturated hydrocarbon group attached to the Schiff base except for the case of methylamine. The fingerprint region, on the other hand, is sensitive to alterations in the Schiff base substituent. Deuterating the Schiff base has a larger effect on the carbon–nitrogen stretching frequency than replacing the Schiff base nitrogen N¹⁴ with N¹⁵. Furthermore, the data on deuterated Schiff bases indicate that the CCH bending vibrational mode is altered by deuteration. Implications concerning use of these compounds for modelling visual pigments and bacteriorhodopsin are discussed.