Stereoselective Formation of β-O-4 Structures Mimicking Softwood Lignin Biosynthesis: Effects of Solvent and the Structures of Quinone Methide Lignin Models

Abstract

p-Quinone methide (QM) is formed as an intermediate during lignin biosynthesis. The aromatization of the QM by the attack of a nucleophile at the α-position of its side chain generates a phenolic hydroxy group in a growing polymer and creates stereoisomeric forms in the side chain. A series of β-O-4-aryl ether QMs was reacted with water at 25 °C to replicate the formation of p-hydroxyphenyl (H) and guaiacyl (G) β-O-4 structures in plant cell walls. Water addition occurred in 3-methoxysubstituted QMs (G-type QMs) with half-lives (t 1/2 ) between 13 and 15 min, at pH 7, in 50% water solution (dioxane−water, 1:1). The rate increased as the water concentration increased to 99% (t 1/2 , 1.2−1.4 min). Similar solvent effects were observed for more reactive nonsubstituted QMs (H-type QMs with t 1/2 of <1 min). Consequently, t 1/2 of the H-type QMs was shorter than that of the G-type QMs under every solvent condition. Upon increasing the water concentration, the variation in the erythro/threo ratios of the four dimeric β-O-4 products increased. Interestingly, the effect of pH on the stereopreference, which was observed in 50% water solution, was small and became imperceptible as the water concentration increased to 99%, suggesting that the effect of the solvent, as well as the effect of the pH, plays an important role in understanding the reaction conditions in cell walls during lignin biosynthesis. The threo isomer was preferentially formed in the four dimeric β-O-4 structures, which is inconsistent with the structural features of compression wood lignin rich in H-units. However, the erythroselective formation was attained in an H-type QM at every pH studied (pH 3.5−7) by introducing a biphenyl structure into the β-etherified ring moiety.

A Quantitative Molecular Atlas for Interactions Between Lignin and Cellulose

“…Typical variations for a specific cellulose face and linkage are below 1 kT. This variability is outside the sampling error bounds determined from simulation but likely is not a productive approach to modulate lignin–cellulose affinity given the challenges associated with developing stereoselective lignin synthesis. , The differences that do exist derive from similar origins as the trends seen in the monomeric compounds, with different stereochemistries varying in their ability to intercalate similar to the examples from Figure A and B.…”

Size-Controlled and Super Long-Term Stable Lignin Nanospheres through a Facile Self-Assembly Strategy from Kraft Lignin

“…With the tight supply of petrochemical raw materials and the rise of oil prices, the development and utilization of lignin materials with extensive sources and excellent properties have been paid increasing attention in both industrial and academic worlds. − However, most of them focus on industrial fields such as building materials, coal water slurry, dyes, ceramics, and pesticide dispersants. − So far, there is still a lack of relevant research in the field of high-value utilization such as nanotechnology, , and thus, the application value of lignin nanomaterials is highly worth exploring. Nanomaterials derived from lignocellulose have many unique characteristics including optical properties, mechanical properties, and conductive characteristics, which ensure it with great application potential. , …”

Formation of lignin alkyl-O-alkyl ether structures via 1,6-addition of aliphatic alcohols to β-O-4-aryl ether quinone methides

“…As shown in Scheme 5, they act as acceptors and react with nucleophiles at that position to form benzylic adducts. In previous studies, 17,35,39 water-addition experiments were performed to systematically investigate the reactivity of QMs and the corresponding stereoselective formation of the β-O-4 structure. A limited number of studies previously began to explore the addition of alcohol-type nucleophiles to QMs.…”

“…32,33 A similar result was also found in a model experiment involving the addition of water to β-O-4-aryl ether QMs bearing different types of aromatic nuclei. 34,35 Considering that non-covalent interactions could govern the initial molecular geometry of reactants and promote the emerging bonds, 36 they may play a key role in lignin polymerization. Even though previous studies mentioned that such weak interactions regiospecifically control the lignol radical coupling step, 37,38 it was never emphasized that they were the crucial factor in the subsequent stereo-formation of β-O-4 linked structures.…”