Enzyme aggregation in ionic liquids studied by dynamic light scattering and small angle neutron scattering

Abstract

Candida antarctica Lipase B (CALB) formed a seemingly homogeneous solution in water, 1-ethyl-3-methylimidazolium dicyanamide ([C₂mim][N(CN)₂]) or dimethyl sulfoxide (DMSO). However, dynamic light scattering (DLS) and small angle neutron scattering (SANS) demonstrated that the enzyme formed aggregates in the non-aqueous solvents. In aqueous solution, SANS measurements revealed that CALB formed cylindrical nano-structures with a diameter of 5 nm and a length of 4 nm, equivalent to the dimensions of a single CALB molecule. The enzyme also formed cylindrical structures in DMSO but the diameter was 4 nm and the length was 12 nm, indicating that the enzyme had aggregated to form dimers or trimers. In [C₂mim][N(CN)₂], disc-shaped aggregates were formed, with an average diameter of 49 nm and a length of 3.8 nm, equivalent to the volume of 150 CALB molecules. In all cases, the hydrodynamic diameters measured by DLS matched the long axial lengths of the aggregates determined by SANS, indicating a good consistency between the two techniques. DLS measurements showed that CALB aggregates in [C₂mim][EtOSO₃] and [C₂mim][NO₃] had a smaller diameter than in [C₂mim][N(CN)₂]. In all cases, the aggregation observed in the solvents was associated with loss of enzymatic activity.