An ionic liquid (IL) is a salt in which the ions are poorly coordinated, resulting in these solvents being liquid below 100 °C or even at room temperature. ILs generally consist of large sized anions and cations, have certain unique advantageous properties and hence are considered as 'green solvents'. Thermal stability of the α/β-serine hydrolase (SH) domain in PE1 and PE2 proteins of Mycobacterium tuberculosis (M.tb) possessing esterase activity was studied in the presence of aprotic ILs consisting of imidazolium cations and anions. Addition of ILs to an aqueous solution of proteins prevented their unfolding and aggregation at higher temperatures. The thermal denaturation curve of proteins with ILs shifted to higher temperatures compared to the absence of ILs from CD spectra. The remaining activities of PE1/PE2 proteins with 1.4 M [EMIM][BF4], [EMIM][Cl], [BMIM][BF4] and [BMIM][Cl] exhibited 100%/100%, 58.96%/58.84%, 78.92%/78.94% and 54.63%/54.92% greater activities, respectively after the heat treatment at 30 °C for 35 min. We conclude that the remaining activities of both proteins are sufficiently maintained after the heat treatment and this depends upon the nature, concentration of ILs, and the thermal incubation time. Specifically, [EMIM][BF4] and [BMIM][BF4] exhibit higher thermal stabilization compared to [EMIM][Cl] and [BMIM][Cl].
Keywords: Ionic liquids; Mycobacterium tuberculosis; PE1 and PE2 proteins; Remaining activity and esterase activity; α/β-serine hydrolase domain.
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