# Synthesis and Antibacterial Properties of Novel Quaternary Ammonium Lignins

## TL;DR

Researchers created new lignin-based antimicrobial materials that show strong antibacterial effects against dangerous bacteria like Klebsiella and MRSA.

## Contribution

The study introduces a novel synthesis method to produce quaternary ammonium lignins with enhanced antibacterial activity.

## Key findings

- QALs with longer alkyl chains showed significantly lower MBC values against K. pneumoniae and S. aureus.
- Barley and pine QALs with higher surface ζ-potential had better antibacterial performance.
- Agar tests showed optimal antibacterial activity at C12–C14 QALs due to limited diffusion of longer chains.

## Abstract

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