Stereo-Recognition of Hydrogen Bond and Its Implications for Lignin Biomimetic Synthesis

Abstract

The hydrogen bond (H-bond) is essential to stabilizing the three-dimensional biological structure such as protein, cellulose, and lignin, which are integral parts of animal and plant cells; thus, stereo-recognition of the H-bond is extremely attractive. Herein, a methodology combining the variable-temperature 1H NMR technique with the density functional theory was established to recognize the underlying H-bonding patterns in lignin diastereomers. This method successfully classified the intramolecular and intermolecular H-bonds with slope values varying between 50.2–201.5 and 221.9–655.4, respectively, from the natural logarithm of the hydroxyl proton chemical shift versus the inverse of the temperature plot. Moreover, this slope was found to be correlated with the interaction distance between the H-bond donor and acceptor. Finally, it was proposed that the stereo-preferential formation of the β-O-4 structure (erythro vs threo form) during lignin biomimetic synthesis was probably influenced by their intramolecular H-bonding patterns, thus making it easier to reach thermodynamic equilibrium.

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

“…It's also supported by, products; *water-addition results from our previous study. 39 Paper Organic & Biomolecular Chemistry for example, the variation range of 31-58% (Δ27%) when increasing the number of β-etherified aromatic -OMe groups in QMs (GH-QM → GG-QM → GS-QM) that react with H 2 O. All these above results indicate that the steric bulkiness of both β-etherified aromatic rings and the nucleophiles contributes to the erythro-preferential formation of the adducts for QMs.…”

“…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.…”

“…Noticeable differences in t 1/2 values are found between metha-Scheme 5 Formation of erythro and threo forms of β-O-4 structures via a nucleophilic addition-reaction; *the water-addition experiment was performed in previous studies. 39 Fig. 4 The disappearance of quinone methides with addition of different alcohols at 25 °C.…”

“…It has been confirmed that these diastereomers of the α-OH/β-O-4 structure have different reactivities in many chemical reactions and different conformational behaviors in many solvent systems, consequently influencing the overall reactivity and molecular shape of lignin. 28,39,51 It is reasonable that organosolv lignins have different stereochemical structures and reactivities with varying biomass and alcohol types. Overall, these results can be further utilized to design smart extraction of such "native" lignins for subsequent selective depolymerization or material preparation, as the choice of biomass and solvent can significantly influence the structure and reactivity of the resultant organosolv lignins.…”

“…Fig.5Distribution of the diastereomers of the corresponding β-O-4 products; *water-addition results from our previous study 39. …”