Milled Lignin Research Articles

Exploring the relationship between lignin structure and antioxidant property using lignin model compounds

Lignin has a natural polyphenol structure that is expected to replace chemically synthesized antioxidants as a native antioxidant with biodegradable and convenient source characteristics. However, the improvement of the antioxidant property of lignin and its application as an antioxidant are still somewhat limited due to the lack of understanding of the relationship between specific lignin structures and antioxidant property. Therefore, the study of the relationship between lignin structure and antioxidant property is crucial to realize the high-quality application of lignin. In this experiment, the scavenging ability of free 1,1-diphenyl-2-picrylhydrazyl (DPPH·) radicals was determined for different grades of acetylated tannins, typical lignin model compounds and different structural units of milled wood lignin to investigate the relationship between lignin structure and antioxidant property. Based on the experimental results, some structure-activity relationships were proposed and the mechanism of the antioxidant property of lignin was discussed. The number of phenolic hydroxyl groups was linearly and positively correlated with antioxidant property, and the scavenging of DPPH radicals increased significantly with the increase in the number of methoxy groups in the model compounds. Moreover, aldehyde and carboxyl groups had a negative effect on the antioxidant property of lignin, while methoxy, alkyl and alcohol hydroxyl groups played a positive role. The guaiacyl (G) and syringyl (S) units favored the antioxidant property, so the difference in the content of structural units in lignin under certain conditions of phenolic hydroxyl content also affected the antioxidant property. Therefore, the antioxidant property of aspen milled lignin was higher than that of other milled lignin from different wood species. Finally, the mechanism of DPPH free radical scavenging by lignin was revealed to better understand the relationship between lignin structure and antioxidant property.

Lignin Composition and Structure Differs between Xylem, Phloem and Phellem in Quercus suber L.

The composition and structure of lignin in different tissues—phellem (cork), phloem and xylem (wood)—of Quercus suber was studied. Whole cell walls and their respective isolated milled lignins were analyzed by pyrolysis coupled with gas chromatography/mass spectrometry (Py-GC/MS), two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR) and derivatization followed by reductive cleavage (DFRC). Different tissues presented varied p-hydroxyphenyl:guaiacyl:syringyl (H:G:S) lignin compositions. Whereas lignin from cork has a G-rich lignin (H:G:S molar ratio 2:85:13), lignin from phloem presents more S-units (H:G:S molar ratio of 1:58:41) and lignin from xylem is slightly enriched in S-lignin (H:G:S molar ratio 1:45:55). These differences were reflected in the relative abundances of the different interunit linkages. Alkyl-aryl ethers (β–O–4′) were predominant, increasing from 68% in cork, to 71% in phloem and 77% in xylem, as consequence of the enrichment in S-lignin units. Cork lignin was enriched in condensed structures such as phenylcoumarans (β-5′, 20%), dibenzodioxocins (5–5′, 5%), as corresponds to a lignin enriched in G-units. In comparison, lignin from phloem and xylem presented lower levels of condensed linkages. The lignin from cork was highly acetylated at the γ-OH of the side-chain (48% lignin acetylation), predominantly over G-units; while the lignins from phloem and xylem were barely acetylated and this occurred mainly over S-units. These results are a first time overview of the lignin structure in xylem, phloem (generated by cambium), and in cork (generated by phellogen), in agreement with literature that reports that lignin biosynthesis is flexible and cell specific.

Fractionation and characterization of ball-milled and enzyme lignins from abaca fibre

An Erratum has been published for this article in Journal of the Science of Food and Agriculture 79(15) 1999, 2122. Ball-milled and enzyme lignins were produced from abaca fibre via ball milling for 6 days followed by cellulase treatment for 3 days. The crude lignin preparations were fractionated into milled lignin (ML), enzyme lignin (EL), hemicellulose-rich milled lignin (HRML), and lignin-rich enzyme lignin (LREL) fractions using a two-step precipitation method instead of a traditional ether precipitation procedure. The yield and chemical composition of the resulting lignin samples are reported. The ML and EL fractions contained low amounts of associated neutral sugars (2.0–3.3%) and uronic acids (1.4–1.5%), and showed relatively low average molecular weights (2500–2660), while the LREL and HRML fractions contained large amounts of bound polysaccharides (35.6–38.3%), and showed high molecular weights (8800–25000). The four lignin fractions are composed of a large proportion of syringyl units with fewer guaiacyl and p-hydroxyphenyl units. The ML is mainly composed of β–O–4 ether bonds between the lignin structural units. The less common β–β, β–5 and 5–5′ carbon–carbon linkages are also present in the lignin molecules. It was found that uronic acids and 41–63% of p-coumaric acids are esterified to lignin in the three lignin-rich fractions of ML, EL and LREL. This level increased to over 90% in the hemicellulose-rich fraction of HRML. For ferulic acids, 92–97% were found to be etherified to lignin in the three lignin-rich fractions of ML, EL and LREL, while in the hemicellulose-rich fraction of HRML this reduced to only 13%, suggesting that a majority of the ferulic acids are esterified to hemicelluloses or lignin in this fraction. © 1999 Society of Chemical Industry

Fractional and structural characterization of ball-milled and enzyme lignins from wheat straw

Ball-milled and enzyme lignins were extracted with 90 and 50% dioxane–water from 6 days ball-milled wheat straw, and subsequently cellulase-treated straw residues, respectively. The crude lignin preparations were purified using a two-step precipitation method instead of the traditional ether precipitation procedure, and fractionated into pure milled lignin (PML), pure enzyme lignin (PEL), hemicellulose-rich milled lignin (HRML), lignin-rich enzyme lignin (LREL), and solubilized lignin during enzyme treatment (SLET) fractions. The five lignin fraction were studied using spectroscopic and degradative tecyhniques. The PML and PEL fractions showed very low content of associated polysaccharides (2.36–2.86%). The PML is mainly composed of β-O-4 ether bonds in the lignin structural units. The less common β-5 and β-β carbon–carbon linkages are also present. The results obtained also indicated that the lignins in wheat straw cell walls appeared to be very closely associated to p-coumaric and ferulic acid, and glucuronic acid or 4-O-methylglucuronic acid. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1633–1641, 1998

Fractional and structural characterization of ball‐milled and enzyme lignins from wheat straw

Ball-milled and enzyme lignins were extracted with 90 and 50% dioxane–water from 6 days ball-milled wheat straw, and subsequently cellulase-treated straw residues, respectively. The crude lignin preparations were purified using a two-step precipitation method instead of the traditional ether precipitation procedure, and fractionated into pure milled lignin (PML), pure enzyme lignin (PEL), hemicellulose-rich milled lignin (HRML), lignin-rich enzyme lignin (LREL), and solubilized lignin during enzyme treatment (SLET) fractions. The five lignin fraction were studied using spectroscopic and degradative tecyhniques. The PML and PEL fractions showed very low content of associated polysaccharides (2.36–2.86%). The PML is mainly composed of β-O-4 ether bonds in the lignin structural units. The less common β-5 and β-β carbon–carbon linkages are also present. The results obtained also indicated that the lignins in wheat straw cell walls appeared to be very closely associated to p-coumaric and ferulic acid, and glucuronic acid or 4-O-methylglucuronic acid. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1633–1641, 1998

Isolation and Fractional Characterization of Ball-Milled and Enzyme Lignins from Oil Palm Trunk.

Ball-milled and enzyme lignins were isolated from the trunks of oil palms and subsequently studied. The lignins were fractionated into pure milled lignin (PML), pure enzyme lignin (PEL), lignin rich enzyme lignin (LREL), hemicellulose rich milled lignin (HRML), and solubilized lignin obtained during enzyme treatment (SLET) fractions by a two-step precipitation method. The chemical and structural composition of the five lignin preparations was determined by using UV, GPC, FT-IR, (13)C NMR spectroscopy and nitrobenzene oxidation. Pure milled lignin and pure enzyme lignin fractions were found to contain rather low amounts of neutral sugars, 2.2-3.1%. The lignin fractions contained a high proportion of noncondensed syringyl units with small amounts of noncondensed guaiacyl and p-hydroxyphenyl units. Trace amounts of p-coumaric and ferulic acids were also found to be associated with lignin in the oil palm trunk cell walls.

Thermally assisted hydrolysis and alkylation of lignins in the presence of tetra-alkylammonium hydroxides

Three different milled lignins isolated from bamboo, pine and beech, corresponding to the three different structural groups, have been subjected to pyrolysis in the presence of tetramethylammonium hydroxide (TMAH) and tetrabutylammonium hydroxide (TBAH). Pyrolysis of the lignins in the presence of TMAH releases the methyl esters and methyl ethers of the different lignin monomers. Several moieties bearing carboxylic acid groups, not previously detected using pyrolytic techniques, could be released from the three lignins, suggesting that the presence of these moieties in the lignin structure have been underestimated.