Ionic strength and composition govern the elasticity of biological membranes. A study of model DMPC bilayers by force- and transmission IR spectroscopy

•The main phase transition of SLB in complex ion mixtures is 8–10°C broad.•In complex ion mixtures ion binding to lipids is a competitive process.•Competitive ion binding in seawater strengthens lipid bilayers. Infrared (IR) spectroscopy was used to quantify the ion mixture effect of seawater (SW), particularly the contribution of Mg2+ and Ca2+ as dominant divalent cations, on the thermotropic phase behaviour of 1,2-dimyristoyl-sn-glycero-3-posphocholine (DMPC) bilayers. The changed character of the main transition at 24°C from sharp to gradual in films and the 1°C shift of the main transition temperature in dispersions reflect the interactions of lipid headgroups with the ions in SW. Force spectroscopy was used to quantify the nanomechanical hardness of a DMPC supported lipid bilayer (SLB). Considering the electrostatic and ion binding equilibrium contributions while systematically probing the SLB in various salt solutions, we showed that ionic strength had a decisive influence on its nanomechanics. The mechanical hardness of DMPC SLBs in the liquid crystalline phase linearly increases with the increasing fraction of all ion-bound lipids in a series of monovalent salt solutions. It also linearly increases in the gel phase but almost three times faster (the corresponding slopes are 4.9nN/100mM and 13.32nN/100mM, respectively). We also showed that in the presence of divalent ions (Ca2+ and Mg2+) the bilayer mechanical hardness was unproportionally increased, and that was accompanied with the decrease of Na+ ion and increase of Cl− ion bound lipids. The underlying process is a cooperative and competitive ion binding in both the gel and the liquid crystalline phase. Bilayer hardness thus turned out to be very sensitive to ionic strength as well as to ionic composition of the surrounding medium. In particular, the indicated correlation helped us to emphasize the colligative properties of SW as a naturally occurring complex ion mixture.