Effect of Salts on Laccase-Catalyzed Polymerization of Lignosulfonate

Enzymatic polymerization of lignosulfonate (LS) has a high potential for various applications ranging from coatings to adhesives. Here, the effect of different ions in low concentrations on enzymatic polymerization of LS was investigated, including salt solutions consisting of mono- and dicarboxylic acids, sulfate, phosphate and chloride with sodium as counter ion. LS polymerization was followed by viscometry and size exclusion (SEC) chromatography. Interestingly, there was only a small effect of ions on the activity of the laccase on standard substrate ABTS, while the effect on polymerization of LS was substantially different. The presence of acetate led to a 39 % higher degree of polymerization (DP) for LS. Small angle X-ray scattering (SAXS) revealed that the structure of the enzyme was largely unaffected by the ions, while the determination of the zeta potential showed that those ions conveying higher negative surface charges onto LS particles showed lower DPs, than those not affecting the surface charge. Further, electron paramagnetic resonance (EPR) spectroscopy showed 5-times higher intensity in phenoxyl radicals for the monovalent ions compared to the divalent ones. It was concluded that the DPs of LS could be tuned in the presence of certain ions, by facilitating the interaction between the laccase substrate-binding site and the LS molecules.