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Biochemical Control Aspects in Lignin Polymerization
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.
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 [en]
Lignin polymerization, DHP, Ascorbic Acid, NADH, template polymerization, melanin
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-4632ISBN: 978-91-7178-864-1 (print)OAI: oai:DiVA.org:kth-4632DiVA: diva2:13165
Public defence
2008-02-29, STFI Salen, STFI Packforsk, Dr Kristinas Väg 61, 10:00
Opponent
Supervisors
Note
QC 20100811Available from: 2008-02-11 Created: 2008-02-11 Last updated: 2010-08-11Bibliographically approved
List of papers
1. Non-enzymatic reduction of quinone methides during oxidative coupling of monolignols: implications for the origin of benzyl structures in lignins
Open this publication in new window or tab >>Non-enzymatic reduction of quinone methides during oxidative coupling of monolignols: implications for the origin of benzyl structures in lignins
Show others...
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.

Keyword
Benzene; Biomimetic materials; Catalysis; Enzymes; Lignin; Polymers; Reaction kinetics; Cellobiose dehydrogenase; Non-enzymatic reduction; Oxidative coupling; Quinone methides
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-7968 (URN)10.1039/b606369a (DOI)000240166800014 ()2-s2.0-33748285226 (Scopus ID)
Note
QC 20100811Available from: 2008-02-11 Created: 2008-02-11 Last updated: 2010-08-11Bibliographically approved
2. Effects of a Biologically Relevant Antioxidant on the Dehydrogenative Polymerization of Coniferyl Alcohol
Open this publication in new window or tab >>Effects of a Biologically Relevant Antioxidant on the Dehydrogenative Polymerization of Coniferyl Alcohol
2008 (English)In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 9, no 12, 3378-3382 p.Article in journal (Refereed) Published
Abstract [en]

Dehydrogenation polymers (DHPs or synthetic lignins) were synthesized from coniferyl alcohol by enzymatic oxidation in the presence of ascorbic acid to study the potential effects of an antioxidant upon their structure. Specific interunit substructures (beta-O-4', beta-beta', and beta-5') were quantified by C-13 NMR, which showed how ascorbic acid altered their amounts compared with control syntheses without this antioxidant, especially by increasing the amount of beta-O-4' substructures. The effect of ascorbic acid increased with its concentration. Surprisingly, no influence on the sizes of the synthetic lignins, as determined by size exclusion chromatography, was observed. The chemistry of this antioxidant effect during dehydrogenative polymerization and the potential biological significance (cell wall lignification) of these observations are discussed.

Keyword
peroxidase-dependent oxidation, lignin biosynthesis, ascorbic-acid, cell-wall, hydrogen-peroxide, picea-abies, spruce, lignification, phenolics, cultures
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-18033 (URN)10.1021/bm800704k (DOI)000261510400006 ()2-s2.0-58149145606 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
3. In vitro monolignol dehydrogenative polymerization in the presence of a candidate dimer template model
Open this publication in new window or tab >>In vitro monolignol dehydrogenative polymerization in the presence of a candidate dimer template model
(English)Manuscript (Other academic)
Keyword
Dehydrogenation polymers; lignin; pinoresinol dimethyl ether; template polymerization
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-7970 (URN)
Note
QC 20100811Available from: 2008-02-11 Created: 2008-02-11 Last updated: 2010-08-11Bibliographically approved
4. On the role of the monolignol gamma-carbon functionality in lignin biopolymerization
Open this publication in new window or tab >>On the role of the monolignol gamma-carbon functionality in lignin biopolymerization
2009 (English)In: Phytochemistry, ISSN 0031-9422, E-ISSN 1873-3700, Vol. 70, no 1, 147-155 p.Article in journal (Refereed) Published
Abstract [en]

In order to investigate the importance of the monomeric gamma-carbon chemistry in lignin biopolymerization and structure, synthetic lignins (dehydrogenation polymers; DHP) were made from monomers with different degrees of oxidation at the gamma-carbon, i.e., carboxylic acid, aldehyde and alcohol. All monomers formed a polymeric material through enzymatic oxidation. The polymers displayed similar sizes by size exclusion chromatography analyses, but also exhibited some physical and chemical differences. The DHP made of coniferaldehyde had poorer solubility properties than the other DHPs, and through contact angle of water measurement on spin-coated surfaces of the polymeric materials, the DHPs made of coniferaldehyde and carboxylic ferulic acid exhibited higher hydrophobicity than the coniferyl alcohol DHP. A structural characterization with C-13 NMR revealed major differences between the coniferyl alcohol-based polymer and the coniferaldehyde/ferulic acid polymers, such as the predominance of aliphatic double bonds and the lack of certain benzylic structures in the latter cases. The biological role of the reduction at the gamma-carbon during monolignol biosynthesis with regard to lignin polymerization is discussed.

Keyword
Lignin, Monolignol biosynthesis, Lignin-polysaccharide networks, Plant, cell wall, Biopolymer, spruce wood, carbohydrate complexes, biosynthesis, dehydrogenation, polymerization, peroxidase, coniferyl, plants
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-18244 (URN)10.1016/j.phytochem.2008.10.014 (DOI)000264235800019 ()2-s2.0-59049107798 (Scopus ID)
Note
QC 20100525. Uppdaterad från manuskript till artikel (20100811). Tidigare titel: On the role of the monolignol gamma-carbon in lignin biopolymerizationAvailable from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved

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