Figure 4.

Binding of Ca²⁺ ions to 5:1 POPC:POPS mixture membranes. Panels (a,b): behavior of POPS in the mixture. Panels (c,d): behavior of POPC in the mixture. All these panels show the order parameter response in a lipid bilayer as a function of the bulk concentration of Ca²⁺ in the system. Duplicated simulation lines correspond to two different order parameter signals of the α C–H bonds. Panel (e) compares the calcium charge density profiles centered around maximum phosphate density for the three force fields and panel (f) shows the calcium charge density profiles of prosECCo75 model at three different concentrations. Charge density profiles of Ca²⁺ shown by solid lines and Cl^– counterions with dashed lines. Na⁺ was used to neutralize POPS charges. Experimental data are from ref (91). In the case of CHARMM36, the multiple points at a bulk concentration of 0 mM result from all Ca²⁺ ions being bound to the lipids in several systems with different total numbers of Ca²⁺ per system.