Synthesis and enzymatic hydrolysis of a diaryl benzyl ester model of a lignin-carbohydrate complex (LCC)

Specific degradation of the bonds between lignin and carbohydrates is an important step towards separating individual lignocellulosic biopolymers for sustainable production of materials and chemicals. One of the most established covalent lignin-carbohydrate (LC) interactions is the ester bond between the alpha- or gamma-hydroxyl group of a lignin phenylpropane unit and a glucuronic acid side chain of xylan. In this work, a model of the LC benzyl ester bond was synthesized in a one-pot reaction from a beta-O-4 lignin unit and D-glucuronic acid, both from commercial sources. The resulting lignin-carbohydrate complex (LCC) model was unstable in aqueous solution. However, at pH 4, the rate of spontaneous hydrolysis was sufficiently low to allow for enzymatic splitting experiments. The enzymatic hydrolysis of the LC benzyl ester bond of the LCC model was demonstrated by means of the glucuronoyl esterase StGE2 from Sporotrichum thermophile, which showed a preference for erythro forms of the LCC model.