Effect of quaternary ammonium surfactants on biomembranes using molecular dynamics simulation

Research conducted both prior to and after the emergence of the COVID-19 pandemic reveals a notable rise in human exposure to cleaning products, hand sanitizers, and personal care items. Moreover, there has been a corresponding increase in the environmental release of these chemicals. Cleaning and disinfecting products often contain quaternary ammonium compounds (QACs) with alkyl chains as long as 8-12 carbon atoms. The attachment of quaternary ammonium surfactants to the membrane resulted in the deformation of the bilayer and membrane disruption. Before interactions with cell membranes, these surfactant molecules may form different aggregates depending on their architecture. Interaction of surfactant monomers or clusters with the cell membrane changes the physiochemical properties of the biomembranes. To investigate this interaction and its influence on membrane properties, we conducted molecular dynamics simulations of cationic quaternary ammonium surfactants interacting with dipalmitoylphosphatidylcholine (DPPC) membranes. Our simulations revealed significant interactions between the surfactants and the phospholipids, leading to substantial alterations in the structure of the bilayer. The results are compared with the simulated anionic (SDS) and nonionic surfactants/bilayer systems. Various aspects were considered, including the aggregation process, migration behavior, and eventual equilibrium of these molecules at the interface between the membrane and water. This analysis used various techniques such as density profiles, distribution functions, cluster analysis, order parameters, hydrogen bonding (H-bonding), and mean-square displacements. The results indicate that while surfactants with shorter alkyl tails (N-(2-hydroxyethyl)-N,N-dimethyloctan-1-aminium chloride (HEDMOAC)) make strong hydrogen bonds with the phosphate group and ester oxygen of the phosphatidylcholine bilayer and enter toward the bilayer in the monomer form, surfactants of longer alkyl tails aggregated on the membrane head-water interface and interact minimally with the head groups of the DPPC bilayer. For DDEDMEAC, a quaternary ammonium surfactant with a hydrophobic alkyl chain consisting of two decanoate groups, alteration of the structural and dynamical properties of the bilayer is expected to be governed by two different factors. First, the structural order of DPPC increases as surfactant aggregates interact with the membrane head group. Second, the decrease in the order of the bilayer occurs due to the insertion of surfactant monomers within the hydrophobic region of the bilayer. Strong interactions between constituents of tetraoctylammonium bromide (TOABr) and lipid head groups lead to a reduction in interlipid interactions and order, which further results in increased porosity of cellular membranes. Understanding the extent of these interactions plays a pivotal role in the toxicological assessment of these surfactants.