Synthesis of highly water-soluble feruloyl diglycerols by esterification of an Aspergillus niger feruloyl esterase

Ferulic acid (FA) is a component of plant cell walls that has applications in food, cosmetic, and health products, but its applications are limited by its high insolubility. We synthesized water-soluble FA derivatives by esterification of FA with diglycerol (DG) using feruloyl esterase purified from a commercial enzyme preparation produced by Aspergillus niger. The major reaction product, FA-DG1, was determined to be γ-feruloyl-α,α′-DG by NMR and electrospray ionization mass spectrometry analyses. FA-DG1 is a sticky liquid whose water solubility (>980 mg/ml) is dramatically higher than that of FA (0.69 mg/ml). Suitable conditions for esterification of FA with DG were 100 mg of FA in the presence of 1 g of DG and 0.1 ml of 1 M phosphate buffer (pH 6.0) at 50 °C under reduced pressure. Under these conditions, 168 mg of feruloyl DGs (FA-DG1, 2, and 3) was obtained, corresponding to a 95 % conversion rate of FA. We also developed a batch method which resulted in synthesis of 729 mg of feruloyl DGs and 168 mg of diferuloyl DGs from 600 mg of FA and 1 g of DG (corresponding to conversion of 69 % of the FA to feruloyl DGs and 21 % of the FA to diferuloyl DGs). As an anti-oxidant, feruloyl DGs were essentially equal to FA and butyl hydroxytoluene in scavenging 1,1-diphenyl-2-picrylhydrazyl radicals. In contrast, the scavenging abilities of diferuloyl DGs were twice those of feruloyl DGs.