Structural modification of eucalypt pulp lignin in a totally chlorine-free bleaching sequence including a laccase-mediator stage

Abstract

Structural modification of eucalypt pulp lignin was investigated in a totally chlorine-free (TCF) bleaching sequence including a laccase-mediator stage. This stage was applied after two oxygen delignification stages, and was followed by an alkaline peroxide stage. After two oxygen delignification stages, two more stages with a laccase mediator and alkaline peroxide were applied. The residual lignins were enzymatically isolated from the different pulps and analyzed by spectroscopic techniques and analytical pyrolysis. The latter revealed high amounts of syringyl units ()70%) in the lignins. Moreover, oxidative modification of the major syringyl units was shown by C 2,6 -H 2,6 HSQC correlations and by the presence of oxidized pyrolysis markers in pyrograms. The existence of both C a keto and carboxyl groups in the residual lignin, together with normal (C ahydroxylated) units, was revealed by heteronuclear multiple bond correlation (HMBC) between aromatic H 2,6 and side-chain carbons. These C a -oxidized structures represent nearly 60% of total units in the lignin isolated from the enzymatically treated pulp. Analysis of residual lignin after the final peroxide stage compared with a simple alkaline treatment revealed that most of the oxidatively altered lignin was removed by the alkali used in the peroxide stage. Thus, the kappa number decreased and the final residual lignin was more structurally related to that found before the oxidative stages, although it contained less resinols and more carboxyl group-bearing units. However, the action of peroxide is necessary to attain the high brightness required ()90% ISO).

HSQC-NMR analysis of lignin in woody (Eucalyptus globulus and Picea abies) and non-woody (Agave sisalana) ball-milled plant materials at the gel state 10th EWLP, Stockholm, Sweden, August 25–28, 2008

“…The S units showed a prominent signal for the C (Ralph et al 2004;Ibarra et al 2007b) were present in the HSQC spectra of the eucalypt wood and sisal fiber, as also observed in their MWL. The relative abundance of these oxidized moieties estimated from the HSQC is similar (approximately 10% of total S-lignin units) in the whole eucalypt wood and in its MWL.…”

Lignin Composition and Structure in Young versus AdultEucalyptus globulusPlants

“…Lignin and carbohydrate contours in the HSQC spectra were assigned by comparison with the literature (Ä mmälahti et al, 1998;Ralph et al, 1999Ralph et al, , 2004bCapanema et al, 2001Capanema et al, , 2004Capanema et al, , 2005Balakshin et al, 2003Balakshin et al, , 2005Liitiä et al, 2003;Ha et al, 2005;Golovchenko et al, 2007;Ibarra et al, 2007aIbarra et al, , 2007bdel Río et al, 2008del Río et al, , 2009Kim et al, 2008;Rencoret et al, 2008Rencoret et al, , 2009Ç etinkol et al, 2010;Kim and Ralph, 2010;Ralph and Landucci, 2010). The main lignin correlation assignments are listed in Table III, and the main lignin substructures found in the different eucalypt woods are depicted in Figure 5.…”

Engineering Platforms for Directed Evolution of Laccase from Pycnoporus cinnabarinus

“…Laccases produced by the basidiomycetes white rot fungiinvolved in lignin biodegradation-generally exhibit a higher redox potential at the T1 site (close to ϩ800 mV), which broadens the range of possible substrates to be oxidized by the enzyme (1). Moreover, in the presence of natural or synthetic redox mediators, laccase can transform compounds with higher redox potentials than the laccase itself, as well as complex polymers (lignin, starch, cellulose), circumventing steric hindrance difficulties (8,30,40). Among high-redox-potential laccases (HRPLs), the laccase from Pycnoporus cinnabarinus (PcL) shows particularly interesting biochemical characteristics in terms of stability and turnover rates for natural and synthetic substrates (36,67).…”