Ionic Liquids Tune The Catalytic Activity Of The Surface Active Enzymes

Enzymes are biological catalysts which are playing an important role in biosynthesis and biotransformation. Enzyme-catalyzed reaction efficiency depends on the catalytic activity of an enzyme, which depends on the medium and microenvironments, especially for the surface active enzymes.Room temperature ionic liquids (RTILs) are liquid organic salts and usually considered as green solvents due to the low vapor pressure. In addition, ILs constituted by long chain alkyl cation or anion have surface activity. Investigating the effect of ILs on the catalytic activity of an enzyme contributes to the development of medium or microenvironments based on ILs in which an enzyme has higher catalytic activity.1. Effect of head-group of surfactants based on ionic liquids on the catalytic activity of surface active enzymes in reverse micellesIn the present study, 1-hexadecyl-3-methylimidazolium bromide (C16mimBr) and hexadecylpyridinium bromide (CPB) were chosen and their phase diagrams in isooctane were studied. Then we determined their critical micelle concentration and calculated the size of their head-groups. The second order rate constant (κ2) of lipase and the kinetic parameters of HRP were investigated in cationic water-in-oil (W/O) microemulsions of the two different ionic liquids with varied head-group size and were compared with that in reverse microemulsion constituted by traditional quaternary ammonium surfactant having the same long chain alkyl. The enzyme activities were found to be markedly related to the size of surfactants. The larger the head-group of the surfactants, the higher the catalytic activity of an enzyme. The increase in head-group size presumably allows the enzyme to attain a flexible conformation as well as increase in the local concentration of enzyme and substrate, leading to the higher efficiency of enzyme.2. Effect of the physicochemical properties of binary ionic liquids on lipase activityIn the present study, the lipase-catalyzed hydrolysis of p-nitrophenyl butyrate is used as model reaction to determine the activity and stability of Candida rugosa lipase in binary ionic liquids (ILs). The binary ILs consist of hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6) and a small amount of hydrophilic 1-butyl-3-methylimidazolium nitrate ([Bmim]NO3) or 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([Bmim]CF3SO3) or 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4). The activity and stability of lipase are first correlated with the physicochemical properties of the binary ILs. In the three binary IL systems, both the hydrophilicity and polarity of systems increase with the increase of the content of hydrophilic ILs with a descending order of [Bmim]PF6/[Bmim]NO3> [Bmim]PF6/[Bmim]CF3SO3> [Bmim]PF6/[Bmim]BF4. This order is in contrast with the order of the lipase conformation stability, i.e., the higher the polarity of ILs, the more unstable the lipase conformation. However, the lipase activity and stability depend on the type and the content of the hydrophilic IL in binary ILs, showing a complex dependency. Analysis shows that the binary ILs not only affect the lipase activity via their influence on lipase conformation, but also affect it through their influence on the physicochemical properties of water. The present study helps to explore binary IL mixtures suitable for lipase-based biocatalysis.