Structural insights on laccase biografting of ferulic acid onto lignocellulosic fibers

Treatment of high-kappa sisal pulp with Trametes villosa laccase and ferulic acid resulted in strong increases of kappa-number and acid-group content due to biografting of this phenolic acid, as shown by pyrolysis in the presence of tetramethylammonium hydroxide. The coupling linkages were investigated by 2D NMR of the lignin isolated from pulps. The aromatic region of the spectra showed incorporation of the cinnamic molecule, representing similar to 4% of the lignin content, that according to the displacement of its olefinic C-13(beta)-H-1(beta) signal to 117.0/6.40 ppm would be C-4-etherified. The aliphatic region of the spectra showed that ferulic acid also incorporates as the corresponding beta-beta' dilactone (another similar to 4% of the total lignin) with characteristic C-13(beta)-H-1(alpha) and C-13(beta)-H-1(beta) correlations at 81.8/5.69 and 47.9/4.19 ppm, respectively. The sisal lignin in the treated pulps was only slightly modified (including a small increase of C-alpha-oxidized units) revealing that the main effect of the treatment was ferulic acid biografting. (C) 2014 Elsevier B.V. All rights reserve