Role of the cosurfactant in water-in-oil microemulsion: interfacial properties tune the enzymatic activity of lipase

The enzymatic activity of Lipase VII from Candida rugosa was studied in the quaternary water-in-oil microemulsion cetyltrimethylammonium bromide (CTAB)/water/pentanol (POH)/hexane (C6). The enzyme-catalyzed hydrolysis of p-nitrophenylbutirate (p-NPB) was found essentially independent from the water/surfactant ratio. On the other hand, the enzyme kinetics was strongly affected by the cosurfactant/surfactant ratio. The interfacial composition and the reverse micelles size, as well as the substrate partition between organic bulk and surfactant aggregates, have been determined through pulsed gradient spin-echo NMR measurements. The correlation of the microemulsion structure with enzyme activity has therefore been possible. The lipase affinity for p-nitrophenylbutirate is lower in microemulsion compared to water because the hydrophobic effect is weaker in the former system. The turnover number is unaffected from the reverse micelles size and from the interfacial bromide concentration, while is strongly dependent on the interfacial composition. The data presented indicate a marked influence of the extent of interfacial surface on the catalytic efficiency of lipase.