All-trans-retinal (1) intercalated in lecithin liposomes reacts with n-butylamine yielding the all-trans-N-retinylidene-n-butylamine (2). The rate of formation of 2 in liposomes is approximately 5-6 times slower as compared to the rate in aqueous buffer (phosphate, berate and NaCl-NaOH) depending on pH of the medium. As compared to organic homogeneous media of n-hexane or aprotic acetonitrile, in liposome the rate of formation of 2 is faster.:tn protic methanol, however, the schiff base 2 is formed with almost the same efficiency as it is formed in liposome (phosphate buffer, pH 12). While the schiff base 2 undergoes rapid hydrolysis in aqueous buffer, it remains stable in liposomal matrix in the dark. Rapid chromophoric destruction of liposomal-2 is observed under UV irradiation. Addition of organic acids to liposome constituted 2 causes its protonation yielding the corresponding all-trans-N-retinylidene-n-butylammonium salt 3. The stability of 3 in liposomal environment is influenced by the type of counter anion. Among all the anions (trifluoroacetate, trichloroacetate, dichloroacetate, chloroacetate, chloropropionate and propionate) studied, trifluoroacetate anion is most effective in stabilising 3. It has been found that liposomes can provide microenvironment necessary for solubilisation and stabilisation of retinal schiff bases as desired in biological applications. The formation and stability of 2 have been discussed in terms of general mechanism involving a common tetrahedral intermediate.
