Solvent Polarity Trend and Recommended Choices

A very commonly observed trend is that activity is highest in the least polar solvents. In many of these cases this is an effect of water or substrate availability, as just noted. Hexane is regularly identified as the best medium, because the low solubility of water and most substrates makes them most available to the enzyme, when comparisons are made at equal concentrations. Nevertheless, even when water and substrate availability have been allowed for, non-polar solvents seem to offer the highest activity. The probable explanation involves the tendency for solvent molecules to migrate from the bulk phase into the immediate environment of the enzyme. The picture is simplest when there is a discrete aqueous phase (albeit of very small volume) around the enzyme molecules. The more hydrophobic the bulk solvent, the lower will be the (saturating) concentration in the aqueous phase, which is what is experienced by the enzyme. Even in the absence of an identifiable aqueous 

it is often best to select a non-polar or hydrophobic solvent, at least for initial trials. Some reasons why this might not be the best choice are  

The following list presents some choices for a more general solvent screening exercise  

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