Bathochromicity of retinal schiff bases in cellulose matrix

Abstract

Schiff bases of all-trans-retinal (formed with n-butylamine, tryptamine and β-naphthylamine) and of benzaldehyde, trans-cinnamaldehyde and all-trans-retinal with aniline exhibit an appreciable red shift in their UV-visible maxima on intercalation in cellulose matrix relative to their absorption in solution in the absence of acid. Treatment of these model compounds with trichloroacetic acid in solution gives the corresponding protonated salts. The red shift due to the cellulose environment is, however, less than the red shift in acid solutions. However, an exception is all-trans-N-retinylidenetryptamine for which the red shift in cellulose is quite close to the corresponding value for the protonated salt in heptane and methanol. N-Benzalideneaniline and trans-N-cinnamalideneaniline, with shorter polyenic moieties, tend to show a greater bathochromic shift in cellulose. all-trans-N-Retinylidene-n-butylamine, all-trans-N-retinylidenetryptamine and all-trans-N-retinylidene-β-naphthylamine show a reduced bathochromic shift when intercalated in cellulose pretreated with a base such as n-butylamine. The chromophore of all-trans-N-retinylidenetryptamine is stabilized by the presence of the indole moiety. These results indicate the importance of hydrogen-bond interactions at the chromophore sites of rhodopsins. A mechanistic proposal for explaining protonation, stability and wavelength regulation in the opsin family of proteins is discussed.