Enzymatic esterification in organic media: role of water and organic solvent in kinetics and yield of butyl butyrate synthesis

Summary

The respective roles of organic solvent and of water in butyl butyrate synthesis from n-butanol and n-butyric acid in n-hexane by Mucor miehei lipase have been investigated by analysis of the kinetics and the reaction balances. Esterificaton was found to take place in both low water systems containing solid enzyme in hexane and in biphasic aqueous enzyme solution/hexane systems. In the solid enzyme system, the enzyme adsorbed the water produced, thus delaying the appearance of a discrete aqueous phase. As expected, the presence of some water was indispensable for this system, as its removal or exclusion by various means (adsorption, distillation) affected enzyme activity. However, water removal had little effect on the final yield of esterification. Reaction velocities were quite similar for the solid enzyme/hexane system and for the biphasic aqueous enzyme solution/hexane system. In the latter case, the butyl butyrate formed was almost exclusively found in the organic phase. Ethyl butyrate, a more polar compound, was synthesized with a lower yield. These results allow the conclusion that the reaction took place in a phase consisting of either solid hydrated enzyme with no discrete aqueous phase or of an aqueous enzyme solution by basically similar mechanisms according to the amount of water available to the system, the esterification being driven to completion by transfer of the ester into the organic phase because of a favourable partition coefficient.