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

Lignin, a naturally occurring aromatic polymer, possesses diverse biological functions, particularly in the defense mechanisms of plants against pathogenic microbes. The prospect of utilizing isolated lignin as an eco-friendly antimicrobial agent presents a promising avenue for enhancing the value of lignin. Furthermore, as lignin derives from plant photosynthesis, its integration into the antimicrobial industry carries the potential to reduce carbon emissions. While numerous studies have explored lignin's utilization for the development of antimicrobial agents across various applications, the highly heterogeneous nature of lignin, encompassing variations in monomer composition, linkages, molecular weight, and functional groups, has obscured the relationship between lignin's structure and its antimicrobial properties.<br/>To bridge these knowledge gaps, we conducted a comprehensive study in which we synthesized forty-two quaternary ammonium/phosphonium organosolv lignin samples from Aspen, Pine, and Barley straw, representing hardwood, softwood, and grass sources, respectively, employing a recently developed versatile intermediate known as chloromethylated lignin. These lignin samples were systematically evaluated for their antibacterial potential against <i>E.coli</i> and clinical isolates of Gram-positive (MRSA) and Gram-negative (<i>K. pneumonia</i>) bacterial strains.<br/>Our findings indicate that the antibacterial activity of these lignin samples exhibits a notable increase with the length of the hydrophobic chain, up to C14, beyond which it begins to decline. Ongoing research is also exploring the antimicrobial activity of ammonium and phosphonium surfactant materials, both individually and in combination. This research underscores the significant enhancement in lignin's antimicrobial efficacy through chemical modification and highlights the potential for further improvements by incorporating additional chemical structures, such as cationic functional groups through chemical modification. This study opens new avenues for maximizing the antimicrobial potential of lignin, contributing to sustainable and eco-friendly solutions for pathogen control.