Although the primary structure of proteins, nucleic acids, and carbohydrates can be readily determined, no sequencing method has been described yet for the second most abundant biopolymer on earth (i.e. lignin). Within secondary-thickened plant cell walls, lignin forms an aromatic mesh arising from the combinatorial radical-radical coupling of monolignols and many other less abundant monomers. This polymerization process leads to a plethora of units and linkage types that affect the physicochemical characteristics of the cell wall. Current methods to analyze the lignin structure focus only on the frequency of the major monomeric units and interunit linkage types but do not provide information on the presence of less abundant unknown units and linkage types, nor on how linkages affect the formation of neighboring linkages. Such information can only be obtained using a sequencing approach. Here, we describe, to our knowledge for the first time, a sequencing strategy for lignin oligomers using mass spectrometry. This strategy was then evaluated on the oligomers extracted from wild-type poplar (Populus tremula 3 Populus tremuloides) xylem. In total, 134 lignin trimers to hexamers were observed, of which 36 could be completely sequenced. Interestingly, based on molecular mass data of the unknown monomeric and dimeric substructures, at least 10 unknown monomeric units or interunit linkage types were observed, one of which was identified as an arylglycerol end unit.Lignin is an aromatic heteropolymer that is mainly present in secondary-thickened plant cell walls, allowing the transport of water and nutrients and providing the necessary strength for the plant to grow upwardly Vanholme et al., 2008Vanholme et al., , 2010. In angiosperms, lignin is predominantly composed of guaiacyl (G) and syringyl (S) units that are derived from combinatorial radical-radical coupling of the monolignols coniferyl and sinapyl alcohol, respectively Fig. 1A). Following oxidation of the monolignols by peroxidase and/or laccase, the resulting electron-delocalized radical has unpaired electron density at its 1-, 3-, O-4-, 5-, and 8-positions (Fig. 1B); note that much of the lignin literature uses Greek letters for the side chain, a, b, and g for the 7-, 8-, and 9-positions. As radical coupling at the 8-position is favored, coupling with another monolignol radical affords, after rearomatization, a mixture of dehydrodimers with 8-8-, 8-5-, and 8-O-4-linkages (Fig. 1C). Following dimerization, polymerization will continue by the coupling of the 8-position of an incoming monolignol radical to the O-4-position of the dimer's phenolic end. In the case of a G dimer, coupling can also occur, albeit at a lower frequency, to the 5-position. Thus, chain elongation creates 8-5-and 8-O-4-linkages (Adler, 1977). Besides the monolignols and other monomers that are present in minor amounts , the plasticity of lignin polymerization permits the incorporation of any phenolic that enters the lignification site, subject to its chemical cross-coupling propensit...

Stretchable and compressible hydrogels based on natural polymers have received immense considerations for electronics. The feasibility of using pure natural polymer-based hydrogels could be improved if their mechanical behaviors satisfy the requirements of practical applications. Herein, we report highly stretchable (tensile strain ∼126%) and compressible (compression strain ∼80%) cellulose ionic hydrogels (CIHs) among pure natural polymer-based hydrogels including cellulose, chitin, and chitosan via chemical cross-linking based on free radical polymerization of allyl cellulose in NaOH/urea aqueous solution. In addition, the hydrogels have good transparency (transmittance of ∼89% at 550 nm) and ionic conductivity (∼0.16 mS cm–1) and can be worked at −20 °C without freezing and visual loss of transparency. Moreover, the CIHs can serve as reliable and stable strain sensors and have been successfully used to monitor human activities. Significantly, the various properties of hydrogel can be controlled through rationally adjusting the chemically cross-linked density. Our methodology will prove useful in developing the satisfied mechanical and transparent CIHs for a myriad of applications in flexible electronics.

Caffeic acid O-methyltransferase (COMT) catalyzes preferentially the methylation of 5-hydroxyconiferaldehyde to sinapaldehyde in monolignol biosynthesis. Here, we have compared HPLC profiles of the methanol-soluble phenolics fraction of xylem tissue from COMT-deficient and control poplars (Populus spp.), using statistical analysis of the peak heights. COMT downregulation results in significant concentration differences for 25 of the 91 analyzed peaks. Eight peaks were exclusively detected in COMT-deficient poplar, of which four could be purified for further identification using mass spectrometry/mass spectrometry, nuclear magnetic resonance, and spiking of synthesized reference compounds. These new compounds were derived from 5-hydroxyconiferyl alcohol or 5-hydroxyconiferaldehyde and were characterized by benzodioxane moieties, a structural type that is also increased in the lignins of COMT-deficient plants. One of these four benzodioxanes amounted to the most abundant oligolignol in the HPLC profile. Furthermore, all of the differentially accumulating oligolignols involving sinapyl units were either reduced in abundance or undetectable. The concentration levels of all identified oligolignols were in agreement with the relative supply of monolignols and with their chemical coupling propensities, which supports the random coupling hypothesis. Chiral HPLC analysis of the most abundant benzodioxane dimer revealed the presence of both enantiomers in equal amounts, indicating that they were formed by radical coupling reactions under simple chemical control rather than guided by dirigent proteins.

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Stretchable and antifreezing conductive hydrogels, especially prepared from natural polymers, are beneficial for important and rapidly growing stretchable electronic devices. Unfortunately, the potential value of the pure natural polymer-based hydrogel in such applications is very limited because of its poor mechanical behavior and inevitable freeze of water-based hydrogels at subzero temperatures. Herein, we report the ultrastretchable and antifreezing double-cross-linked cellulose ionic hydrogels (DCIHs) prepared by ammonium persulfate initiating free radical polymerization of allyl cellulose and by NaCl inducing physical cross-linking. The acquired hydrogels display ultrastretchability (∼236% of tensile strain) and high compressibility (∼82% of compression strain) among pure polysaccharide-based hydrogels including cellulose, chitosan, and chitin. Simultaneously the soaking strategy in saturated NaCl solution also endows the DCIHs with excellent antifreezing properties that not only have good stretchability (strain up to ∼100%) at −24 °C but also surprisingly transform into high visual transparency under a wide range of low temperature (−30 to −16 °C). In addition, the DCIH with high reliability, response speed, as well as wide range strain sensor is also demonstrated by investigating the output electrical signals, showing the potential for applications in flexible electronics under a broad range of temperature.

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Using S-adenosyl-methionine as the methyl donor, caffeic acid O-methyltransferase from sorghum (Sorghum bicolor; SbCOMT) methylates the 5-hydroxyl group of its preferred substrate, 5-hydroxyconiferaldehyde. In order to determine the mechanism of SbCOMT and understand the observed reduction in the lignin syringyl-to-guaiacyl ratio of three brown midrib12 mutants that carry COMT gene missense mutations, we determined the apo-form and S-adenosyl-methionine binary complex SbCOMT crystal structures and established the ternary complex structure with 5-hydroxyconiferaldehyde by molecular modeling. These structures revealed many features shared with monocot ryegrass (Lolium perenne) and dicot alfalfa (Medicago sativa) COMTs. SbCOMT steady-state kinetic and calorimetric data suggest a random bi-bi mechanism. Based on our structural, kinetic, and thermodynamic results, we propose that the observed reactivity hierarchy among 4,5-dihydroxy-3-methoxycinnamyl (and 3,4-dihydroxycinnamyl) aldehyde, alcohol, and acid substrates arises from the ability of the aldehyde to stabilize the anionic intermediate that results from deprotonation of the 5-hydroxyl group by histidine-267. Additionally, despite the presence of other phenylpropanoid substrates in vivo, sinapaldehyde is the preferential product, as demonstrated by its low K m for 5-hydroxyconiferaldehyde. Unlike its acid and alcohol substrates, the aldehydes exhibit product inhibition, and we propose that this is due to nonproductive binding of the S-cis-form of the aldehydes inhibiting productive binding of the S-trans-form. The S-cisaldehydes most likely act only as inhibitors, because the high rotational energy barrier around the 2-propenyl bond prevents S-trans-conversion, unlike alcohol substrates, whose low 2-propenyl bond rotational energy barrier enables rapid S-cis/S-transinterconversion.

Catalytic oxidation via the application of molecular oxygen and copper complexes is a useful pathway toward valuable low molecular mass compounds from in situ or waste stream lignins. In this study, two dimeric β-ether model compounds, one β-ether oligomer, and a milled wood lignin sample from Loblolly pine were catalytically oxidized. Yields and stability of the aromatic aldehyde and acid products were measured. Nuclear magnetic resonance spectroscopy and gel permeation chromatography were used to monitor structure/composition and molecular mass changes of the lignin before and after catalytic oxidation to study the degree of depolymerization and structure of the residual lignin. Oxidized units appear to be derived from β-aryl ether, phenylcoumaran, and biphenyl ether components. To date, this method breaks down the lignin polymeric structure reasonably effectively, producing low molecular mass products; this work also highlights some of the issues that need to be overcome to optimize this approach.