Synthesis and Antibacterial\nProperties of Novel Quaternary\nAmmonium Lignins

The ongoing demand for effective antimicrobial materials\npersists,\nand lignin emerges as a promising natural antibacterial material with\nrenewable properties. The adaptability of lignin to various chemical\nmodifications offers avenues to enhance its antimicrobial activity.\nHere, we employed chloromethylation and subsequent functionalization\nwith variable tertiary <i>N-</i>alkyl dimethyl amines to\nproduce C6–C18 quaternary ammonium lignins (QALs) from hardwood\n(aspen), softwood (pine), and grass (barley straw). Successful synthesis\nof QALs was confirmed through NMR and FTIR analysis results along\nwith an increase in the surface ζ-potential. Antibacterial activity\nof QALs against clinical strains of Klebsiella pneumoniae and methicillin-resistant Staphylococcus aureus was assessed using minimal bactericidal concentration (MBC) assay\nand agar growth inhibition zone (ZOI) test. The antibacterial activity\nof QALs was found to be higher than that of the unmodified lignins.\nQALs with longer alkyl chains demonstrated an MBC of 0.012 mg/L against K. pneumoniae already after 1 h of exposure with\nsimilar effect size reached after 24 h for S. aureus. For all the lignins, an increase in alkyl chain length resulted\nin an increase in their bactericidal activity. MBC values of C14–C18\nQALs were consistently lower than the MBC values of QALs with shorter\nalkyl chains. Besides the alkyl chain length, MBC values of barley\nand pine QALs were negatively correlated with the surface ζ-potential.\nWhile alkyl chain length was one of the key properties affecting the\nMBC values in a liquid-based test, the agar-based ZOI test demonstrated\nan antibacterial optimum of QALs at C12–C14, likely due to\nlimited diffusion of QALs with longer alkyl chains in a semisolid\nmedium.