Santos, J. Ignacio et al.Lignosulfonates, isolated from the black liquors of sulfite cooking to obtain cellulose pulp (Sjöström 1993), are the only available commercial lignins nowadays, with different industrial and agricultural applications. This is because the lignin released during the kraft cooking of wood, largely the process most widely used for paper pulp manufacture, is burnt for recovery of chemicals and energy supply to the mill. The commercial utilization of lignosulfonates largely depends on tailoring their physico-chemical properties for the different applications intended, including molecular weight distribution and presence of functional groups (Lebo et al. 2008). Different chemical treatments, including oxidative modifications, can be used to change these lignosulfonate properties, but the economic costs of some of them limit their industrial applicability, especially when low-value products are to be produced.
Oxidative enzymes, including laccases (benzenediol: oxygen oxidoreductase, EC 1.10.3.2) in the presence of redox mediators, have been largely investigated for the removal of lignin-derived compounds in paper pulp manufacture (Call et al. 1997) and more recently also for pitch control (Gutiérrez et al. 2009) and multipurpose functionalization of pulp fibers and other wood products (Widsten et al. 2008). Very recently the potential of the above enzymatic system to modify lignosulfonates is being investigated as a potential alternative to the chemical treatments, which often require harsh application conditions (Prasetyo et al. 2010).
Despite sulfite cooking historically preceded kraft cooking, the chemical structure of lignosulfonates is not known into detail. This is because some of the most powerful tools for the chemical characterization of complex macromolecules, such as multidimensional nuclear magnetic resonance (NMR) that enabled the discovery of new substructures in wood lignin (Zhang et al. 2006), are only recently being applied to lignosulfonates (Lutnaes et al. 2008). In the present study we used two different NMR techniques - namely solution 1H-13C correlation 2D-NMR in HSQC (heteronuclear single quantum correlation) experiments and solid-state CPMAS (cross-polarization and magic angle spinning) 13C NMR - to investigate the nature and extent of the chemical changes produced by the laccase-mediator system when applied on a softwood lignosulfonate.The work was supported by the BIORENEW project (NMP2-CT-2006-026456, www.biorenew.org). Guro E. Fredheim (Borregaard, Norway) is acknowledged for the lignosulfonate sample.Peer reviewe
