Efficient resolution of (R,S)-1-(1-naphthyl)ethylamine by Candida antarctica lipase B in ionic liquids

The resolution of (R,S)-1-(1-naphthyl)ethylamine ((R,S)-NEA) by Candida antarctica lipase B (CALB) in ionic liquids (ILs) containing 1-alkyl-3-methylimidazolium cations ([Cnmim]+) and [Tf2N]−, [BF4]−, and [PF6]− anions was investigated. When the alkyl chain on the cation contained less than six carbons, the lipase activity corresponded with the hydrophobicity of the ILs, but further increase in the chain length suppressed the enzyme activity. The enzyme activity decreased depending on the anion, where [Tf2N]− > [PF6]− > [BF4]−. The effects of acyl donors, pH, temperature, water activity, and substrate concentration on the resolution were determined. Under the optimal conditions, the conversion of (R,S)-NEA and enantiomer excess of (R)-n-octyl acyl-NEA was 49.3% and 99.2%, respectively. The resolution kinetics of (R,S)-NEA by CALB in [C6mim][Tf2N] were studied and a ping-pong mechanism with a two substrate inhibition model was selected. The kinetic parameters of the fitting results were as follows: Michaelis constant of (R,S)-NEA Kma, 461.8 mmol/L; Michaelis constant of vinyl n-octanoateKmb, 262.1 mmol/L; inhibition constant of (R,S)-NEA Kia, 8737.2 mmol/L; inhibition constant of vinyl n-octanoateKib, 62336.8 mmol/L; maximum reaction rate rmax, 0.352 mmol/(mg min). Moreover, circular dichroism revealed that incubation of CALB in [C6mim][Tf2N] resulted in increased β-sheet content; its secondary structure was stable.