Improving the lipase activity profile in cationic water-in-oil microemulsions of hydroxylated surfactants

The activity of Chromobacterium viscosum lipase (CV-lipase) in cationic water-in-oil (w/o) microemulsions is significantly lower compared to that in bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT)-based (anionic) systems.(26.36,3s,39) In the present study, we estimated the second-order rate constants k(2) in lipase-catalyzed hydrolysis of p-nitrophenylcaproate, in newly developed cationic w/o microemulsions of synthesized surfactants (2-6, Chart 1) containing hydroxyethyl moieties at the polar headgroup. The kinetic studies at pH = 6.0 (pH refers to the pH of the aqueous buffer solutions used in preparing the w/o microemulsions) show that the catalytic efficiency of CV-lipase was systematically increased with the sequential increment of hydroxyethyl groups at the polar heads of surfactants (1-4, Chart 1), possibly due to the increase in the interfacial concentration of water [H2O](i) in consequence of the added hydrogen bonding ability of hydroxyl groups. To this end, we found that in a 0.05 M 4/water/32.3:1 (v/v) isooctane/n-hexanol w/o microemulsion, the activity of lipase is almost 4-fold higher than that in 0.05 M cetyltrimethylammonium bromide (CTAB)/water/9:1 (v/v) isooctane/n-hexanol(37) systems and is enhanced by an order of magnitude compared to that in the case of CTAB/water/heptane-chloroform (1:1, v/v) w/o microemulsions.(26) Moreover, to our knowledge, the observed k(2) is the highest-ever lipase activity, hitherto unattainable in any cationic w/o microemulsion. Simultaneously, for the first time, the estimated activity of lipase in the former cationic w/o microemulsion is akin to the best-ever activity of lipase found in a w/o microemulsion of AOT.(26) In addition, the pH profile and the stability of CV-lipase were investigated in newly developed w/o microemulsions.