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. 26,36,38,39 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 [H 2 O] 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.