Retinylidene Schiff bases in surfactant-solubilized water pools in heptane

Abstract

All-trans-retinal (1) was reacted with n-butylamine in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles in heptane to form all-trans-N-retinylidene-n-butylamine Schiff bases (2). The extent of protonation of 2 by 3-chloropropionic acid (CPA) to give 3 in AOT reverse micelles in heptane was found to depend on the ratio of [CPA] to [2], as well as on [H₂O]/[AOT] (i.e., on the ω value). At any given [2] and ω values, increasing amounts of CPA increased the protonation and at any given constant [2] and [CPA], increasing ω values also increased the protonation. Over a period of 24 hours, there was only 4% decomposition of 2 in AOT reverse micelles in heptane at ω = 24. Conversely, in three hours, 23% of 3 decomposed in the same system. The trans to cis photoisomerization of 2 in heptane occurred at a much faster rate in the presence of AOT reverse micelles than in their absence. The appearance of carboxylate peaks (FTIR, 1400-1500 cm⁻¹) indicated that the larger the AOT solubilized water pools, the greater the CPA dissociation. 1 also reacted with the α-NH₂ group of l-lysine (4) in AOT reverse micelles in heptane to give the corresponding Schiff base 6. Protonation of 6 occurred either intramolecularly or by reaction with unreacted 4. These results were discussed in terms of rhodopsin protonations.