Understanding the antimicrobial activity of water soluble γ-cyclodextrin/alamethicin complex

In our previous investigation (Zhang et. al., J. Functional Foods 40 (2018) 700-706), we have proposed a method of complexation of alamethicin (ALM) with gamma-cyclodextrin (gamma-CD) to increase the solubility and demonstrated an enhancement in its antimicrobial activity against Listeria monocytogenes. In this study, transmission electron microscopy of L. monocytogenes treated with gamma-CD/ALM complex indicated cell lysis due to pore formation. This was further corroborated by fluorescent dye leakage from DMPC/cholesterol liposomes when exposed to gamma-CD/ALM complex for molar ratios of 1, 5 and 10. The extent of dye leakage increased with ALM-lipid ratio in the range of 0.00015 to 0.16. Dye leakage of gamma-CD/ALM complex was found to be the highest for molar ratio of 5, consistent with our earlier results of antimicrobial activity of the complex against L. monocytogenes. All atom molecular dynamics (MD) simulation showed that gamma-CD/ALM complex can effectively bind to the 3:1 POPE/POPG bilayer, a mimic of bacterial cell membrane. In addition, circular dichroism spectrum indicated that ALM in the complex has a helical conformation in solution as well as in the presence of liposome. Transmembrane MD simulation of six gamma-CD/ALM complex aggregates in a helical conformation showed water channel with barrel stave like pore structure.