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

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

Biodegradable, Colorless, and Odorless PLA/PBAT Bioplastics Incorporated with Corn Stover

The effective use of agricultural lignocellulose resources in biodegradable bioplastics has become the focus of people’s attention due to the inappropriate disposal of agricultural waste and plastic materials. However, the repulsive smell and uncontrollable color of agricultural lignocellulose-based materials limit their potential to replace common consumer plastics. In this study, when the corn stover (CS) meal enforced the PLA/PBAT matrix using the melt blending method, the lignin and hemicellulose components were shown to substantially impact the color darkening and unsavory odor release for the resultant bioplastic; therefore, an optimized scalable alkaline hydrogen peroxide oxidation that can be performed using traditional pulp and paper production lines was used to modify CS on a large scale (LCS) to enhance the properties of bioplastics fabricated with the PLA/PBAT matrix. As a result, the appearance color and odor character of the bioplastic enforced by LCS have been significantly improved compared to those of natural CS. In addition, the bioplastic (LCS/PLA/PBAT) exhibited good tensile strength (9.7 MPa), flexural strength (18.1 MPa), elongation at break (61.8%), and surface hydrophobicity with a contact angle value of 91.6°, which could meet the requirement of the Chinese National Standard for Packaging. The reinforcing effect of LCS on these performances lay in its rigid structure with a strong fiber network, high content of cellulose crystalline, and hydrophobic nature of lignin after treatment, as proven by FTIR, XRD, and SEM results. Therefore, our bioplastics filled with treated agricultural waste are attractive appearance-wise, economically competitive, and biodegradable, making them a sustainable alternative to common consumer plastics

Lignin-Biosynthetic Study: Reactivity of Quinone Methides in the Diastereopreferential Formation of <i>p</i>‑Hydroxyphenyl- and Guaiacyl-Type β‑<i>O</i>‑4 Structures

p-Quinone methides are involved in lignin biosynthesis as transient intermediates, and the aromatization step has a great impact on the chemical structure of the resulting lignin. A series of quinone methides (QMs) were synthesized and allowed to react with water in pH 3–7 buffers at 25 °C to mimic the formation of p-hydroxyphenyl- and guaiacyl-type (H- and G-type, respectively) β-O-4 structures in gymnosperm-plant cell walls. Water addition occurred in 3-methoxy-substituted QMs (G-type QMs) with half-lives of 1.4–15 min. In contrast, nonsubstituted QMs (H-type QMs) were very labile; they were aromatized to β-O-4 products with half-lives of only 10–40 s. The rapid aromatization in H-type QMs may provide an advantage over G-type species for efficiently driving the lignin-polymerization cycle, which possibly contributes to the development of highly lignified compression wood. In the water-addition reaction, the threo isomers of the β-O-4 products were stereopreferentially formed more than the erythro isomers from both G- and H-type QMs (erythro/threo ratios of 24:76 and 50:50, respectively). The proportion of erythro isomers was higher at lower-pH conditions. This pH-dependent trend agrees with findings from a previous study on 3,5-dimethoxy-substituted (syringyl-type, S-type) QMs; thus, this pH-dependent trend is common in H-, G-, and S-type lignin-related QMs. Higher threo-selectivity was obtained by changing the β-etherified aromatic rings from G- to H-type. A similar but weaker effect was also observed by changing the QM moiety from G- to H-type