Synthesis and Antibacterial Properties of Lignin-Based Quaternary Ammonium and Phosphonium Salts ^†

Lignin, a naturally occurring aromatic polymer, possesses a range of biological functions. Notably, plants have harnessed lignin’s properties as a defense mechanism against invading pathogenic microbes. Consequently, the utilization of isolated lignin as an ecofriendly antimicrobial agent holds great promise for enhancing the value of lignin. Furthermore, given lignin’s green and sustainable origin through plant photosynthesis, its integration into the antimicrobial industry has the potential to reduce carbon emissions. Numerous studies have explored the utilization of lignin for the development of antimicrobial agents tailored to various applications. However, lignin is a highly heterogeneous polymer, characterized by variations in monomer composition, linkages, molecular weight, and functional groups. Consequently, the relationship between lignin’s structure and properties, as well as its mechanism of action as an antimicrobial agent, remains unclear [1]. To address these gaps in knowledge, we conducted a study in which we prepared forty-two quaternary ammonium/phosphonium organosolv lignin samples from aspen, pine, and barley straw (representing hardwood, softwood, and grass sources, correspondingly) using a versatile intermediate, known as chloromethylated lignin, developed recently [2]. These samples were then assessed for their potential antibacterial properties against clinical isolates of Gram-positive (Staphylococcus aureus, MRSA) and Gram-negative (Klebsiella pneumonia) pathogenic bacterial strains. Our findings suggest that the antibacterial activity of these lignin samples exhibits an increase with the length of the hydrophobic chain, up to C14, after which it begins to decline. Additionally, ongoing research is being conducted to investigate the antimicrobial activity of ammonium and phosphonium surfactant materials, both individually and in combination. In this research, chemical modification has significantly boosted the antimicrobial efficacy of lignin, and incorporating extra chemical structures, e.g., cationic functional groups through chemical modification, presents a viable strategy to enhance the future potential of lignin’s antimicrobial activity.