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Understanding pulp delignification by LaccaseâMediator Systems through isolation and characterization of ligninâcarbohydrate complexes
The effects and mechanism of pulp delignification by laccases in the presence of redox mediators have been investigated on unbleached eucalyptus kraft pulp treated with laccases from Pycnoporus cinnabarinus (PcL) and Myceliophthora thermophila (MtL) and 1-hydroxybenzotriazole (HBT) and methyl syringate (MeS) as mediators, respectively. Determination of the corrected Îș number in eucalyptus pulps after the enzymatic treatments revealed that the PcLâHBT system exhibited a more remarkable delignification effect than the MtLâMeS system. To obtain further insight, ligninâcarbohydrate complexes were fractionated and subsequently characterized by nuclear magnetic resonance, thioacidolysis (followed by gas chromatography and size exclusion chromatography), and pyrolysisâgas chromatographyâmass spectrometry (pyrolysisâGCâMS) analyses before and after the enzymatic treatments and their controls. We can conclude that the laccaseâmediator
treatments altered the lignin structures in such a way that more lignin was recovered in the xylanâlignin fractions, as shown by
Klason lignin estimation, with smaller amounts of both syringyl (S) and guaiacyl (G) uncondensed units, as shown by thioacidolysis and gas chromatography, especially after the PcLâHBT treatment. The laccaseâmediator treatment produced oxidation at Cα and cleavage of Cα and CÎČ bonds in pulp lignin, as shown by pyrolysisâGCâMS. The general mechanism of residual lignin degradation in the pulp by laccaseâmediator treatments is discussed in light of the results obtained.This study was financially supported by the EU BIORENEW Project (NMP2-CT-2006-026456). Novozymes (Bagsvaerd, Denmark) and Beldem (Andenne, Belgium) are acknowledged for the M. thermophila and P. cinnabarinus laccases, respectively. J. Romero (ENCE) is acknowledged for providing the eucalyptus pulp samples and E. D. Babot for performing the enzymatic treatments. J.R. acknowledges the CSIC JAE-Doc contract of the program âJunta para la AmpliaciĂłn de Estudiosâ cofinanced by the European Social Fund (ESF).Peer reviewe