Tuning the hydrophobicity of glycine ester through N-acylation and introducing quaternary pyridinium groups enable the selectivity against Gram-positive bacteria

Abstract

The membrane, which plays numerous fundamental biological roles, is a key target for drug discovery. To selectively gain access to the bacterial membrane, we synthesized a new series of 2-((N-(acylglycine ester)methyl)-1-methylpyridin-1-ium iodide) (QPyNAGe) compounds from glycine esters with tunable hydrophobicity by N-acylating them with fatty acids and introducing a cationic charge using quaternary pyridinium groups. The screening for antibacterial activity using the zone of inhibition method revealed that QPyNAGe is highly selective against Gram-positive bacteria. Among the compounds studied, 2-((N-(2-methoxy-2-oxoethyl)palmitamido)methyl)-1-methylpyridin-1-ium (QPyN16Ge) showed promising antibacterial activity (MIC, 3.91–7.81 µM), and 2-((N-(2-methoxy-2-oxoethyl)decanamido)methyl)-1-methylpyridin-1-ium (QPyN10Ge) had low antibacterial activity (MIC, 500–1000 µM) against the tested Gram-positive strains, including the methicillin-resistant Staphylococcus aureus. The hemocompatibility assay results showed that all QPyNAGe derivatives were more selective for bacteria than for human red blood cells. QPyN16Ge displayed the highest selectivity for bacteria, indicating that the hydrophobicity of the N-palmitoyl chain played a crucial role in enhancing its activity. QPyN16Ge exhibited rapid killing kinetics against planktonic MRSA compared with the standard antibiotic ciprofloxacin. The dipropylthiacarbocyanine iodide release experiment and the propidium iodide uptake assay demonstrated that QPyN16Ge depolarizes the cytoplasmic membrane more than QPyN10Ge and has greater membrane permeability. Scanning electron microscopy studies support the notion that QPyN16Ge damages the bacterial membranes, allowing their cellular contents to leak and inducing cell death. Resistance studies suggest that QPyN16Ge is less likely than ciprofloxacin to develop resistance against S. aureus. The therapeutic potential was demonstrated by rescuing MRSA-infected zebrafish with QPyN16Ge, reducing the bacterial burden in the animals.