Galloylation (esterification with gallic acid) of catechins was achieved using a tannase from Aspergillus niger in room temperature ionic liquids. Immobilization of the tannase on Eupergit C substantially increased the esterification activity. Six out of seven tested ionic liquids proved adequate media for the esterification of (-)-epicatechin, with the highest yield (3.5%) in 1-butyl-3-methylimidazolium 2-(2-methoxyethoxy)-ethylsulfate. The reaction is equilibrium-controlled. Synthesis of esters was favoured with increasing concentrations of gallic acid (6.0% yield, 2 M gallic acid) and decreasing water content. However, water concentrations lower than 20% (v/v) resulted in a decrease of conversion due to inactivation of the tannase. Significant differences in the reaction yields were observed for the galloylation of epicatechin (5.4%), epigallocatechin (3.1%) and catechin (1.3%), but not for the individual (-)- and (+)-enantiomers. Tannase showed a broad specificity for the alcohol moiety and an absolute specificity for the acid portion of the ester.
- Enzymatic synthesis