Influence of ionic liquids on lipase activity and stability in alcoholysis reactions

Abstract

Lipase activity and stability in ionic liquids containing N,N-dialkylimidazolium cations and different anions were investigated in alcoholysis reactions. Anions were the main factor affecting enzyme activity and stability, while hydrophobic and hydrophilic ionic liquids showed quite different phenomena. The initial enzyme activity in [Tf₂N]⁻ and [PF₆]⁻ based ionic liquids was much higher than that in ionic liquids containing anions such as [BF₄]⁻, [N(CN)₂]⁻ and [OAc]⁻. Novozyme 435 retained approximately 5%, 40% and 100% of the blank sample activity after being immersed in [BF₄]⁻, [PF₆]⁻ and [Tf₂N]⁻ based ionic liquids for 24 h. To reveal the influence of the ionic liquid properties, the initial enzyme activities were correlated with hydrophobicity (log P), polarity (E^N_T), hydrogen bond basicity (β) and viscosity. The initial enzyme activity increased with log P value and decreased with E^N_T value. In addition, the initial enzyme activity increased with β value in a narrow range (0.24–0.26) and then decreased continuously with the increase of β value. For [Tf₂N]⁻ based ionic liquids, the initial enzyme activity increased with viscosity, while the adverse relationship was found in [BF₄]⁻ and [PF₆]⁻ based ionic liquids. The enzyme secondary structure changes in ionic liquids were also investigated by FT-IR, which showed that the α-helix content decreased and the β-sheet content increased with the increase of immersion time.