Stability and Reactivity of Guaiacylglycerol-β-Guaiacyl Ether, a Compound Modeling β-O-4 Linkage in Lignin

Lignin, a complex and abundant biopolymer, is a major constituent of plant cell walls. Due to its chemical and structural complexity, lignin degradation is a challenging task for both natural and engineered systems. Therefore, investigation of lignin degradation using so called "model compounds" has been the focus of many research efforts in recent years. This study addresses the utility of guaiacylglycerol-beta-guaiacyl ether (G beta 2) as a model compound for evaluating the beta-O-4 bond cleavage under diverse thermal and aqueous medium conditions. Experimental conditions included varied pH (3-10), microbial biodegradation, subcritical water environment (150-250 degrees C), and mild pyrolysis (150-250 degrees C). A high-performance liquid chromatography with high-resolution mass spectrometry was employed for accurate detection and quantification of both G beta 2 and its degradation/modification products in an aqueous environment. Pyrolysis experiments were performed using gas chromatography-mass spectrometry analysis with a pyrolyzer. The results showed that G beta 2 remained stable under exposure to moderate pH and several bacterial strains, which were successfully used previously for biodegradation of other recalcitrant pollutants. We report, for the first time, differing G beta 2 breakdown pathways for subcritical water treatment vs. pyrolysis under an inert atmosphere. The scientific novelty lies in the presentation of differences in the degradation pathways of G beta 2 during subcritical water treatment compared to pyrolysis in an inert atmosphere, with water playing a key role. The observed differences are ascribed to the suppression of homolytic reactions by water as a solvent.