Abstract
The effect of head group modification on the solubility and antimicrobial potency of some quaternary nitrogen surfactants (QNSs) was investigated. Using high yielding Menshutkin reactions, a homologous series of monomeric, dimeric and polymeric QNSs based on aliphatic ammonium and aromatic pyridinium head groups were synthesised. The hydrophilicity of the head groups was increased via hydroxymethyl addition and decreased using ethyl substitution. The tails used were saturated, straight alkyl chains ranging from 6 to 22 carbons in length.
The hydrophobicity of the QNSs was established by calculating their predicted partition coefficient in an octanol/water mix (Log P) and by measuring their critical micelle concentration (cmc). As anticipated, the QNSs with longer alkyl tails were more hydrophobic with lower cmc and increased Log P values. Alterations to the head groups showed predictable results for all head groups except the hydroxymethyl addition on the pyridinium head group. The Log P value predicted an increased hydrophilicity, while the cmc suggested an increased hydrophobicity. This difference (lower cmc) was postulated to be due to decreased repulsion of the head groups allowing for micellisation at lower concentrations. This theory was supported by crystal structure analysis which showed that the hydroxymethyl addition caused the QNSs to pack more closely. Measurement of limiting molar conductivity (Λo) also showed that the hydroxymethylpyrdinium compounds were unexpectedly less solvated than the unsubstituted counterparts, and the micelle size was increased by the presence of the hydroxymethyl group. This data as a whole suggested the hydroxymethyl group was not solely acting to decrease the hydrophobicity.
The antimicrobial activity of each QNS was assessed against three Gram positive bacteria (Staphylococcus aureus, Listeria innocua, Bacillus subtilis); three Gram negative bacteria (Pseudomonas aeruginosa, Yersinia enterocolitica, Escherichia coli); a fungus (Aspergillus niger); and a yeast (Saccharomyces cerevisiae), using a minimum inhibitory concentration (MIC) assay. The longer chain compounds were generally found to be more effective against the bacteria with a cut-off seen in many cases at the hexadecyl derivative. The charge of the bacteria cell surface (ζ-potential) became increasingly positive at higher concentrations of QNS and transmission electron microscopy (TEM) showed that at concentrations above the MIC the cytoplasmic membrane had been compromised. These studies suggested the primary mode of action of the QNS against the bacteria was due to the attraction of the QNS to the cytoplasmic membrane of the bacterial cell which resulted in its destabilisation. Destabilisation of the bacterial cytoplasmic membrane occurred at lower concentration for QNSs with longer alkyl tails. It was therefore postulated that the hydrophobicity of a QNS could be used to predict its antimicrobial potency. However, when the MIC was related to Log P using a quantitative structure activity relationship (QSAR) it was demonstrated that the relationship was not linear, and that while parabolic and bilinear models provided a better approximation to the observed data no model sufficiently accounted for all the data.
The research presented in this thesis has shown that the physical properties and antimicrobial activity of QNSs varies greatly with altered head group hydrophobicity. It has been demonstrated that very small changes to the head group can lead to unexpected physical properties which changes their interactions with micro-organisms. The antimicrobial action of these compounds has been shown to rely on the attraction of the cationic head group to the cytoplasmic membrane which is then destabilised by the alkyl tail. Findings from this project will help in the development of more potent QNSs and a greater understanding of the interaction of QNSs with cytoplasmic membrane.