Elsevier

Process Biochemistry

Volume 59, Part A, August 2017, Pages 111-115
Process Biochemistry

Two distinct enzymatic approaches for coupling fatty acids onto lignocellulosic materials

https://doi.org/10.1016/j.procbio.2017.01.008Get rights and content

Highlights

  • Fatty acids were covalently attached to lignocellulosic material.

  • Covalent bonding was demonstrated using lignin model substrates.

  • Environmentally friendly alternative to the known methods of functionalizing wood.

Abstract

Two enzyme based strategies for hydrophobic functionalization of lignocellulose materials were developed and mechanistically compared using sinapic acid and the dimer syringylglycerol β-guaiacyl ether as models respresenting (hardwood) lignin substructures for coupling fatty acid esters. Coupling of lipase/hydrogen peroxide treated methyl linoleate to sinapic acid indeed resulted in a 1:1 coupling product with an m/z peak at 575.5 measured with HPLC–MS. Using laccase for coupling, oligo/polymerization of sinapic acid seems to prevent a coupling-reaction to methyl linoleate. However, methyl linoleate was successfully coupled by laccase 1:1 onto syringylglycerol β-guaiacyl ether through the ether bond at position four. The efficient enzyme mediated incorporation of fatty acid esters as hydrophobic molecules were further confirmed when triglycerides were used to treat veneers resulting in a water contact angle increase from 58.3° to 93.5°. Thus, this study demonstrates for the first time that both (laccase mediator and lipase-hydrogen peroxide) systems can act as promising strategies for introducing fatty acid esters in wood leading to increased hydrophobization.

Introduction

The modification and functionalization of wood surfaces for increasing the hydrophobicity have been subject of research for a long period of time. Increased hydrophobicity of wood surfaces improves strength, aesthetic properties and reduces biodeterioration of the material. Increasing the wood hydrophobicity is conventionally achieved by chemical and physical methods. Chemical methods include the use of silicon compounds, hydrophobic oil and different water repellants like waxes, oils and natural or synthetic resins [1]. Recently, fatty acids and fatty acid chlorides, e.g. palmitoyl chloride, have also been used to graft kraft pulps and modify wood using physicochemical approaches [2].

Although these physicochemical methods are well established, it is known, that they reduce mechanical properties such as the tensile strength [3] and are highly energy demanding. Another disadvantage of many strategies is that water repellant substances are only adsorbed to the cell walls by rather weak Van der Waal forces and are therefore released to the environment after long time exposure to water [4]. In addition, physicochemical methods become more and more unpopular due to rising eco- and energy-sensitivity of the society. Environmentally-friendly processes for wood surface modification are increasingly being investigated, among them enzymatic grafting [5]. In this context, enzymes of interests are oxidoreductases especially laccases, lipases and peroxidases [6]. Laccases ((EC 1.10.3.2), p-diphenol: dioxygen oxidoreductase) catalyze the oxidation of phenolic molecules via hydrogen abstraction with a concomitant reduction of the co-substrate dioxygen to water [7]. These polyphenol oxidases are found in fungi, various bacteria and plants. They have been used in different studies e.g. to mediate the grafting of fluorophenols [8], long chain alkylamines [9], reactive phenolic amines and wood preservatives [10], [11], [12] onto wood surfaces. Furthermore, lignin present in different materials was functionalized by enzymatic methods. Liu et al. [13] as well as Dong et al. [14] successfully demonstrated the possibility to graft lignocellulosic jute fabrics in two different ways using a horseradish peroxidase/hydrogen peroxide system. On the one hand polyacrylamide was grafted onto the surface in order to increase the hydrophilicity of the fabric [13], while on the other hand, octadecylamine was used for increasing the hydrophobicity of the surface [14]. The same material was modified using a laccase from Myceliophthora thermophila by grafting acrylamide in combination with tert-butyl hydroperoxide in order to increase hydrophilicity, thermostability, dye uptake and dyeing depth of the jute fabric [15]. An increase in the hydrophobicity of the surface of jute fibers was achieved when using laccase in combination with dodecyl gallate [16]. Recently, Dong et al. showed the possibility to increase the mechanical properties as well as the surface hydrophobicity of jute fiber membranes due to treatment of the material using various laccase/mediator systems as well xylanase/laccase and cellulase/laccase combinations [17]. Amines and phenols were grafted onto flax and coconut fibers by Herrero Acero et al. [18] using a laccase/mediator system.

This work mechanistically investigates for the first time chemo-enzymatic and enzymatic coupling of fatty acids onto lignin model substrates. Further, the study demonstrates the ability of these enzymes to mediate hydrophobization of wood veneers.

Section snippets

Chemicals and enzymes

Candida antarctica lipase B (≥5000 U/g) immobilised on macroporous acrylic resin (L4777 from Sigma), methyl linoleate with a purity of ≥99% (ML; L1876 from Sigma), sinapic acid with a purity of ≥98% (SA; D7927), soybean oil (SBO; S7381 from Sigma) and 35% hydrogen peroxide (H2O2) were purchased from Sigma Aldrich. Laccase from Trametes hirsuta (ThL) was produced and purified as previously described by Almansa et al. [19]. To determine the activity of the laccase, the oxidation of

Coupling of methyl linoleate to sinapic acid using lipase and H2O2

Sinapic acid was used as the model-substrate to represent the hardwood lignin substructure in order to mechanistically investigate covalent enzymatic functionalization of lignocellulose materials with fatty acids. Methyl linoleate was successfully coupled onto sinapic acid according to LC–MS analysis. The molecular mass acquired in positive mode showed a coupling product with an m/z peak at 575.5 (Fig. 1). Based on the findings of Kudanga et al. [21] and Warwel et al. [23], methyl linoleate was

Conclusions

Concluding the results, this study shows the great potential of enzymes in wood surface functionalization or modification. In addition, it was clearly shown here by using model substrates that both enzyme based coupling strategies lead to covalent attachment of hydrophobic molecules. Consequently, this strategy ensures that functional molecules are not released from lignocellulose materials like it is currently the case with conventional strategies based on simple adsorption (“impregnation”).

Acknowledgements

This study was performed within the Austrian Centre of Industrial Biotechnology ACIB, the MacroFun project and COST Action 868. This work has been supported by the Federal Ministry of Science, Research and Economy (BMWFW), the Federal Ministry of Traffic, Innovation and Technology (bmvit), the Styrian Business Promotion Agency SFG, the Standortagentur Tirol, the Government of Lower Austria and Business Agency Vienna through the COMET-Funding Program managed by the Austrian Research Promotion

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