Enzymatic Synthesis of Inulin-Containing Hydrogels

Abstract

The Bacillus subtilis protease Proleather FG-F catalyzed the transesterification of inulin with vinyl acrylate (VA) in dimethylformamide (DMF). The reaction conversion for different VA concentrations was greater than 57% after 96 h at 50 °C. The degree of substitution (DS, defined as the amount of acrylate groups per 100 inulin fructofuranoside residues) with acrylate moieties can be controlled by varying the molar ratio of VA to inulin. Reasonable yields were obtained (44−51%, 2 days) using a two-step purification methodology. Inulin derivatized with VA (Inul-VA) was characterized by gel permeation chromatography, and its structure was established by 1H, 13C, and 1H−1H correlation spectroscopy and 1H−13C heteronuclear multiple quantum coherence NMR. The main positional isomer was at the 6 position of the fructofuranoside residue and two other minor isomers were observed at the 3 and 4 positions. Thus, the enzymatic reaction was largely regioselective. Furthermore, the inulin fructose residues were monosubstituted. Gels with swelling ratios at equilibrium of up to ca. 20 were prepared by free radical polymerization of aqueous solutions of Inul-VA with different DS and monomer concentrations. Gel pore sizes were calculated from swelling experiments and range from 19 to 57 Å. To our knowledge, this work reports the first successful enzymatic modification of a polysaccharide solubilized in 100% DMF solution.