Quaternary ammonium surfactants derived from leucine and methionine: Novel challenging surface active molecules with antimicrobial activity

Surfactants are versatile excipients, which are commonly employed in several pharmaceutical formulations as emulsifiers, stabilizing or solubilizing agents. In this field, current research is aimed at the design or discovery of amphiphilic molecules with improved characteristics in terms of safety (toxicological profile) and biological activities (e.g. antimicrobial activity, interaction with biomembranes). Quaternary ammonium surfactants derived from amino acids are one such class of compounds with significant potential for pharmaceuticals. In this work, quaternary ammonium surfactants derived from the amino acids leucine and methionine and esterified with fatty acids of different lengths (C10, C12 and C14) were synthesized. These amphiphiles were characterized in terms of critical micelle concentration (CMC) by tensiometry and conductivity measurements, cytotoxicity (MTS and LDH assay on Caco-2 and Calu-3 cell lines) and antimicrobial activity on selected Gram-positive (S. aureus and E. faecalis), Gram-negative (E. coli and P. aeruginosa) and the fungus C. albicans. The derivatives bearing a C12 or a C14 chain displayed CMC, EC50 (from cytotoxicity assays) and minimum inhibition concentration (MIC) values that are comparable to those of benzalkonium chloride (used as reference), especially towards Gram-positive and C. albicans. On the contrary, derivatives bearing a C10 chain have higher CMC, EC50 and MIC values, highlighting the crucial role of the hydrophobic tail in determining both physicochemical and biological properties. The C12-C14 synthesized derivatives may represent a promising alternative to benzalkonium chloride as surface active molecules and antimicrobial agents.