On the origin of unchanged lipase activity profile in cationic reverse micelles

The results of the present investigation demonstrate that the interfacial r egions of cetyltrimethyl-ammonium bromide (CTAB)/isooctane/1-hexanol/water cationic reverse micelles are densely populated by bromide counterions (3.7 -2.5 M) and cosurfactant (8.1-6.4 M) across W-0 (the mole ratio of water to surfactant) = 12-44 and the interfacial concentrations remain grossly unch anged in the presence of a catalytic amount of interfacially solubilized li pase at both pH 6.0 and 7.0 (pH refers to the pH of the aqueous buffer solu tions used in preparing the reverse micellar solutions). Both the second-or der rate constants k(2) and the equilibrium yields for the lipase catalyzed hydrolysis of p-nitrophenylcaproate were also found to be essentially unch anged across the entire range of W-0. Our results throw new insight into th e origin of such constant second-order rate constants and equilibrium yield s for lipase-catalyzed interfacial hydrolysis reactions in cationic reverse micelles across W-0 = 12-44. To our knowledge, the work delineated herein provides the first experimental evidence consistent with grossly unchanged interfacial compositions as the origin for the unchanged activity profiles and the equilibrium yields for lipase-catalyzed reactions in cationic rever se micelles across a wide range of W-0.