Eutectic mixtures as green solvents for laccase-catalyzed reactions

Abstract

Laccases are powerful biocatalysts in the biotechnological field; however, the substrates’ limited solubility in the reaction media may hinder the oxidation process. Herein, different natural deep eutectic solvents (NADES) were investigated as solvents for the laccase-assisted polymerization of catechol and phenolics from coffee. Citric acid, oxalic acid, DL-lactic acid, glucose, glycerol, ethylene glycol, and sodium DL-lactate were used as raw components to produce the NADES. Aiming to achieve high polymerization degrees, we ascertained the optimal reaction conditions considering the stability and activity of laccase in the presence of the NADES. The conformational structure of laccase remains practically unchanged in the presence of the NADES studied, as proven by molecular modeling and intrinsic fluorescence data. The polymer production, verified through 1 H NMR, revealed the formation of only one type of polycatechol using NADES containing DL-lactic acid/DL-sodium lactate:glycerol/ethylene glycol as solvents, at 20 °C. Four different types of polycatechol-derived structures are produced when the oxidation reactions are conducted in acetate buffer or NADES containing glucose:glycerol/ethylene glycol/DL-lactic acid. At 50 °C, catechol polymerization in the presence of NADES or acetate buffer tends to produce one single product. The 1 H NMR spectra at 20 and 50 °C revealed the consumption of phenolics from soluble coffee, suggesting the synthesis of polycaffeic acid. MALDI analysis evidenced polymer formation, with DP up to 40 units for polycatechol and 16 units for soluble coffee phenolics. According to these findings, NADES might be a green and cost-effective solvent for the bioproduction of novel polymers