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Non-enzymatic reduction of quinone methides during oxidative coupling of monolignols: implications for the origin of benzyl structures in lignins
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
Department of Chemistry, University of Helsinki.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
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2006 (English)In: Organic and biomolecular chemistry, ISSN 1477-0520, E-ISSN 1477-0539, Vol. 4, no 18, 3456-3461 p.Article in journal (Refereed) Published
Abstract [en]

Lignin is believed to be synthesized by oxidative coupling of 4-hydroxyphenylpropanoids. In native lignin there are some types of reduced structures that cannot be explained solely by oxidative coupling. In the present work we showed via biomimetic model experiments that nicotinamide adenine dinucleotide ( NADH), in an uncatalyzed process, reduced a beta-aryl ether quinone methide to its benzyl derivative. A number of other biologically significant reductants, including the enzyme cellobiose dehydrogenase, failed to produce the reduced structures. Synthetic dehydrogenation polymers of coniferyl alcohol synthesized ( under oxidative conditions) in the presence of the reductant NADH produced the same kind of reduced structures as in the model experiment, demonstrating that oxidative and reductive processes can occur in the same environment, and that reduction of the in situ-generated quinone methides was sufficiently competitive with water addition. In situ reduction of beta - beta-quinone methides was not achieved in this study. The origin of racemic benzyl structures in lignins therefore remains unknown, but the potential for simple chemical reduction is demonstrated here.

Place, publisher, year, edition, pages
2006. Vol. 4, no 18, 3456-3461 p.
Keyword [en]
Benzene; Biomimetic materials; Catalysis; Enzymes; Lignin; Polymers; Reaction kinetics; Cellobiose dehydrogenase; Non-enzymatic reduction; Oxidative coupling; Quinone methides
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-7968DOI: 10.1039/b606369aISI: 000240166800014Scopus ID: 2-s2.0-33748285226OAI: oai:DiVA.org:kth-7968DiVA: diva2:13161
Note
QC 20100811Available from: 2008-02-11 Created: 2008-02-11 Last updated: 2010-08-11Bibliographically approved
In thesis
1. Biochemical Control Aspects in Lignin Polymerization
Open this publication in new window or tab >>Biochemical Control Aspects in Lignin Polymerization
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Lignins are produced by all vascular plants and they represent one of the most abundant groups of biopolymers in nature. Lignin chemistry research, which has been of great importance for the progress of pulping technologies, has been plagued by the difficulties of its isolation and characterization. The pioneering work of Karl Freudenberg in the 1950’s with synthetic models of lignin paved the way for a detailed structural characterization of many lignin substructures. His work with the so-called “synthetic lignins” or dehydrogenative polymers (DHP) also laid a foundation for understanding how different lignin substructures are formed, reinforcing the already existing theory of lignin polymerization. However, subsequent structural characterizations of DHPs and lignins have repeatedly put this theory to the test. In the past decade, even a new radically different hypothesis for lignin polymerization has emerged and is sustained by a few researchers in the field.

In this work, DHPs were produced from phenolic monomers, mostly coniferyl alcohol, a common lignin monomer, in a variety of reaction conditions. This was done in order to establish how different chemical factors, potentially active in the plant cell wall during lignin polymerization, influence the polymer’s final properties. In the presence of nicotine amide adenine dinucleotide (NADH), a quinone methide model, which is an intermediate formed during lignin polymerization, was effectively reduced. An equivalent reduced structure was produced during DHP synthesis in the presence of NADH. These studies showed that reduction might take place during oxidative polymerization, possibly explaining how reduced lignin structures are formed in the plant cell wall. Another reductive agent, ascorbic acid, was also tested during synthesis of DHPs. It displayed a totally different effect than NADH, probably due to its anti-oxidant nature, by altering the final amounts of certain inter-unit substructures, in favour of β-O-4′ structures, which are so prominent in natural lignins. Furthermore, the new suggested model for lignin polymerization, stating that lignin itself possesses the ability for template replication, was tested by synthesizing DHPs in the presence of a simple β-β′ substructure model. The DHPs produced the same amounts of β-β′ substructures as a control synthesis without the model structure, indicating that no replication had occurred. Finally, the role of the monolignol γ-carbon oxidation state in lignin polymerization, was studied. Hypothetically, lignin- like polymers could be produced by the plant, using monolignol biosynthetic precursors which exhibit γ-carbonyl groups instead of an alcohol group, like the common lignin monomer. Synthetic lignins produced with ferulic acid, coniferaldehyde and the normal monolignol, coniferyl alcohol, displayed important differences in chemical and physical properties. Both the ferulic acid and coniferaldehyde polymers exhibited almost no saturated inter-unit substructures and very few cyclic structures, both of which are very common in coniferyl alcohol dehydrogenative polymers and natural lignins. This could have significant implications for the formation of an important type of lignin carbohydrate complexes (LCC). Also the hydrophobicity of the alcohol-type polymer was lower than the other two. The biological implications of all these findings are discussed and some suggestions are made to explain how all these factors might affect lignin polymerization and structure in nature.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. 60 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2008:10
Keyword
Lignin polymerization, DHP, Ascorbic Acid, NADH, template polymerization, melanin
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-4632 (URN)978-91-7178-864-1 (ISBN)
Public defence
2008-02-29, STFI Salen, STFI Packforsk, Dr Kristinas Väg 61, 10:00
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Note
QC 20100811Available from: 2008-02-11 Created: 2008-02-11 Last updated: 2010-08-11Bibliographically approved

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