Synthesis and Antibacterial Properties of Oligomeric Dehydrogenation Polymer from Lignin Precursors

Abstract

The lignin precursors of coniferin and syringin were synthesised, and guaiacyl-type and guaiacyl-syringyl-type oligomeric lignin dehydrogenation polymers (DHP and DHP-GS) were prepared with the bulk method. The carbon-13 nuclear magnetic resonance spectroscopy showed that both DHP-G and DHP-GS contained β-O-4, β-5, β-β, β-1, and 5-5 substructures. Extraction with petroleum ether, ether, ethanol, and acetone resulted in four fractions for each of DHP-G (C11–C14) and DHP-GS (C21–C24). The antibacterial experiments showed that the fractions with lower molecular weight had relatively strong antibacterial activity. The ether-soluble fractions (C12 of DHP-G and C22 of DHP-GS) had strong antibacterial activities against E. coli and S. aureus. The C12 and C22 fractions were further separated by preparative chromatography, and 10 bioactive compounds (G1–G5 and GS1–GS5) were obtained. The overall antibacterial activities of these 10 compounds was stronger against E. coli than S. aureus. Compounds G1, G2, G3, and GS1, which had the most significant antibacterial activities, contained β-5 substructures. Of these, G1 had the best antibacterial activity. Its inhibition zone diameter was 19.81 ± 0.82 mm, and the minimum inhibition concentration was 56.3 ± 6.20 μg/mL. Atmospheric pressure chemical ionisation mass spectrometry (APCI-MS) showed that the antibacterial activity of G1 was attributable to a phenylcoumarin dimer, while the introduction of syringyl units reduced antibacterial activity.