Reaction rates in aqueous solutions of cationic colloidal surfactants and calixarenes: Acceleration and resolution of two steps of fluorescein diesters hydrolysis

The hydrolysis of fluorogenic substrates of fluorescein diester series is a popular and well-documented indicative reaction used in enzyme biochemistry for cholinesterase systems and related fields. The overwhelming majority of studies have been performed using the fluorescence spectroscopy and conventionally considering this two-step reaction as a single-step one. We developed a spectrophotometry-based approach for the separate determination of the rate constants of the first and second hydrolysis steps of fluorescein diesters, such as diacetyl- and dilaurylfluorescein. This approach was applied to micellar solutions of cationic colloidal surfactants and calixarenes in water. Cationic surfactants cetyltrimethylammonium bromide, cetyl-N-pyridinium chloride, cationic Gemini surfactant 16–4–16 bromide, di-n-tetradecyldimethylammonium bromide and a zwitterionic surfactant cetyldimethylammonium propanesulfonate were used. As amphiphilic cationic calixarenes, the wide rim choline or imidazolium derivatives of calix[4,6]arenes decorated at the narrow rim with methyl, propyl, hexyl, octyl, and dodecyl tails in form of tetra- and hexachlorides were used. These systems display marked acceleration of the hydrolysis reaction of diacetylfluorescein in water. The results were compared with the kinetic data in 50 mas. % ethanol. Non-ionic surfactant Triton X-100 displays no influence on kinetics, whereas an anionic surfactant sodium laurylsulfate strongly reduces the reaction rate. In the presence of calixarene aggregates, an expressed catalytic effect was observed for diacetylfluorescein, but not for dilaurylfluorescein. For instance, the rate constants of stepwise hydrolysis of diacetylfluorescein increase by ca. 200–750 times on going from water to aqueous solutions of 5,11,17,23-tetra(N,N-dimethyl-N-hydroxyethylammonium)methylene-25,26,27,28-tetradodecyl-oxycalix[4]arene tetrachloride. At the same time, a distinct macrocyclic effect of calixarene was also observed: the micelles of the “quarter” compound N,N-dimethyl-N-hydroxyethyl-4-dodecyloxybenzylammonium chloride also accelerate the reaction, but in a much less expressed manner.