Structural insights on laccase biografting of ferulic acid onto lignocellulosic fibers

Abstract

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 ∼4% of the lignin content, that according to the displacement of its olefinic 13Cβ–1Hβ signal to 117.0/6.40 ppm would be C4-etherified. The aliphatic region of the spectra showed that ferulic acid also incorporates as the corresponding β–β′ dilactone (another ∼4% of the total lignin) with characteristic 13Cα–1Hα and 13Cβ–1Hβ 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 Cα-oxidized units) revealing that the main effect of the treatment was ferulic acid biografting.