Impact of deep eutectic solvents (DESs) and individual des components on alcohol dehydrogenase catalysis: connecting experimental data and molecular dynamics simulations
For a knowledge-based design of enzyme catalysis in deep eutectic solvents (DESs), the influence of the DES properties (e.g., water activity and viscosity) and the impact of DESs and their individual components must be assessed. This paper investigates three different DESs: choline chloride-glycerol (ChCl-Gly), choline chloride-ethylene glycol (ChCl-EG), and ethyl ammonium chloride-glycerol (EACl-Gly). The specific activity and half-life time of horse liver alcohol dehydrogenase (HLADH) were experimentally determined in these DESs with water contents ranging from 0%-100%. HLADH showed limited activity in neat DESs, which was enhanced by adding water. Experiments with individual DES components of ChCl-Gly were carried out to clarify their individual influence. Glycerol acts as a strong stabilizer for the enzyme, whereas choline chloride's results are deleterious. To understand the experimental findings, molecular dynamics (MD) simulations were carried out to quantify the solvation layer and calculate the spatial distribution of the solvent molecules around HLADH. The experimental and the in silico approach suggests that designing novel DESs with higher glycerol loadings would result in improved media for biocatalysis. This is demonstrated by performing the reduction of cinnamaldehyde to cinnamyl alcohol-a relevant compound for the food industry and cosmetics-in ChCl-Gly (1 : 9 molar ratio), with 20 vol% water to decrease the viscosity. HLADH is highly active and stable under these new conditions, giving promising productivity (15.3 g L-1 d-1). This paper demonstrates that DESs can be designed to be both substrate-solubilizers and enzyme-compatible, opening new research lines for green chemistry and biocatalysis.