دانلود رایگان مقاله تبدیل کاتالیزی لاکاز-میانجی بوسیله مدل لیگنین مواد مرکب

Abstract

Laccases play an important role in the biological break down of lignin and have great potential in the deconstruction of lignocellulosic feedstocks. We examined a variety of laccases, both commercially prepared and crude extracts, for their ability to oxidize three model lignol compounds (p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol). We identified both mediated and non-mediated laccase-catalyzed reactions that converted p-coumaryl alcohol and sinapyl alcohol to p-hydroxybenzaldehyde and 2,6-dimethoxybenzoquinone, respectively. Interestingly, the products produced by the concerted action of the laccase mediator system on lignol substrates are the same as those produced by chemical catalytic approaches. The enzymatic approach affords the opportunity for a biological approach to the conversion of lignin to valuable specialty chemicals that have use in a variety of industrial, consumer and pharmaceutical applications.

3. Results/discussion

Lignin is a major structural component of woody plants, and one of the most abundant organic compounds on earth (Lora and Glasser, 2002). At present, however, it has limited commercial value. In contrast to the readily hydrolysable bonds between subunits of other wood polymers (e.g., cellulose and hemicellulose), lignin degradation requires oxidative attack on the carbon–carbon and ether inter-unit bonds (Martínez et al., 2005). Extracellular peroxidases and oxidases are thought to play an important role in the initial depolymerization of lignin (Martínez et al., 2005). The formation of monomeric phenolic compounds during various processes will be a direct result of lignin degradation (Martínez et al., 2005). Generally, three main aromatic constituents form the major part of the lignin with differing substitutions of the 3- and 5-positions of the aromatic nucleus. In order to improve the economics of ethanol production from lignocellulosic feedstocks, it will be necessary to identify valuable coproducts from lignin – the primary residual by-product. Currently, the commercial use for lignin is to burn for heating value. We have previously shown that it is important to screen a wide-range of enzyme-mediator pairs (Larson et al., 2013), as predicting activity of the laccase mediator system with a given substrate has been elusive. We screened three model lignin substrates, the common lignols p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Fig. 1), to evaluate the utility of laccasemediator combinations for synthetically useful oxidations. Enzymes were paired with mediators in a combinatorial fashion and reactions initiated by the addition of model lignin substrates. Samples were periodically withdrawn and products were determined by rapid LC/MS methods to determine the conversion and identify the type of transformation (Cotterill and Rich, 2005; Cotterill et al., 2008)