Studies on mediators of manganese peroxidase for bleaching of wood pulps

doi:10.1016/S0960-8524(02)00132-3

Bioresource Technology

Volume 85, Issue 3, December 2002, Pages 249-252

https://doi.org/10.1016/S0960-8524(02)00132-3 Get rights and content

Abstract

In order to enhance the bleaching effect of manganese peroxidase (MnP), unsaturated fatty acids, thiol-containing compounds and various other organic compounds were applied in pulp bleaching experiments with MnP. Thiol-containing compounds did not improve the pulp bleaching effect by MnP. Some unsaturated fatty acids, linoleic acid and linolenic acid provided a better pulp bleaching effect than Tween 80. The correlation between the number of CC bonds in a fatty acid and its pulp bleaching effect was also investigated. The MnP pulp bleaching capability was shown to depend on the carboxylic acid used. A combination of Tween 80 and a carboxylic acid resulted in higher pulp brightness than that obtained with Tween 80 alone. A laccase mediator, 3-hydroxy-1,2,3-benzotriazin-4(3H)-one, could also enhance the MnP pulp bleaching effect.

Introduction

Pulp bleaching is conventionally achieved by treating pulps with molecular chlorine or chlorine-based chemicals. The effluents from this treatment contain chlorinated aliphatic and aromatic compounds that could be acutely toxic, mutagenic and even carcinogenic. To reduce the environmental pollution, researchers began to develop environmentally benign bleaching techniques such as elementary chlorine free (ECF) and even totally chlorine free (TCF) processes. Environmentally friendly enzymatic pulp bleaching techniques have been investigated intensively in recent years. The first enzyme to be used for pulp bleaching was xylanase (Viikari et al., 1986). Laccase has also been demonstrated to be effective in pulp bleaching when a redox mediator such as 2,2^′-azino-bis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS) or 1-hydroxybenzotriazole (1-HBT) is present (Bourbonnais and Paice, 1992; Call and Mücke, 1994). However, the lack of a cheap redox mediator is one of the hurdles for a commercial application of a laccase/redox mediator system for pulp bleaching today.

Manganese peroxidase (MnP), a ubiquitous peroxidase among white-rot fungi (Eggert et al., 1996), is another lignin-degrading enzyme that can be used for pulp bleaching. The activity of MnP is dependent on Mn²⁺. It has been proposed that MnP oxidizes Mn²⁺ to Mn³⁺ which in turn oxidizes lignin (Wariishi et al., 1988). It has been demonstrated that MnP is able to oxidize phenolic lignin subunits only (Wariishi et al., 1988). However, it was found that in the presence of glutathione (GSH), MnP could also oxidize non-phenolic lignin model compounds, veratryl, anisyl, and benzyl alcohols (Wariishi et al., 1989). They demonstrated that the Mn³⁺ formed oxidizes thiol to a thiyl radical that in turn abstracts a hydrogen from veratryl alcohol to form veratraldehyde. Bao et al. also demonstrated that MnP could oxidize a non-phenolic β-O-4 lignin model compound in the presence of Tween 80. Tween 80 contains an unsaturated fatty acid chain (Bao et al., 1994). They suggested that MnP oxidized the carbon–carbon double bond (CC) in Tween 80 to a peroxide which subsequently turned into a peroxyl radical. This radical was the actual oxidant for the oxidation of the non-phenolic model compound.

It was further demonstrated that MnP from Phanerochaete sordida YK-624 could bleach hardwood kraft pulp in the presence of Mn²⁺, Tween 80, malonate, and H₂O₂ (Kondo et al., 1994). However, the bleaching effect with MnP and Tween 80 is still not satisfactory. In an effort to enhance the pulp bleaching effect by MnP, we have evaluated a large number of organic compounds such as organic acids, thiol-containing compounds and their combinations for pulp bleaching. Some compounds are superior to Tween 80 for this purpose.

Section snippets

Pulp and chemicals

The unbleached hardwood kraft pulp used in these experiments (HWKP, brightness 35% ISO; kappa number 14.2) was obtained from a Georgian pulp mill. All chemicals were of analytical grade and obtained from commercial sources.

Enzymes

MnP was produced from Trametes versicolor strain 52J kindly provided by Michael G. Paice (Pulp and Paper Research Institute of Canada). Cultivation of the fungus and purification of the enzyme was performed as described by Paice et al. (1993). However, the culture medium was

Results and discussion

In this study, various compounds were screened for their ability to enhance the pulp bleaching effect by MnP. Brightness and kappa number values were measured to compare the effect of the compounds studied in these MnP-based pulp bleaching experiments.

The first group of compounds screened were thiol-containing compounds: cysteine, cystine, dithiothreitol and glutathione. As shown in Table 1, cysteine, dithiothreitol, and glutathione appeared to darken the pulp in comparison with the treatment

Conclusion

Although glutathione has previously been shown to enable MnP to oxidize non-phenolic lignin model compounds, in our studies thiol-containing compounds were not effective MnP mediators for pulp bleaching. Unsaturated fatty acids significantly increased the pulp bleaching effects by MnP. Unsaturated carbon–carbon double bonds in fatty acids appeared to play a very important role in the MnP-based pulp bleaching, which is consistent with the proposed mechanisms that lignin degradation is through

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Studies on mediators of manganese peroxidase for bleaching of wood pulps

Abstract

Introduction

Section snippets

Pulp and chemicals

Enzymes

Results and discussion

Conclusion

FEBS Lett.

J. Biol. Chem.

J. Biol. Chem.

Pulp. Pap. Mag. Can.

Appl. Microbiol. Biotechnol.

Holzforschung