Surface Potential Of Ppi Membranes And Calcium Ion Concentrations At The Ppi Membrane Surfaces

ABSTRACT The surface potentials of various PPI membranes facing a monovalent salt solution were calculated by using non-linearized Poisson-Boltzmann formula. The PPI used were PI, PIP and PIP2. As in our earlier study of surface potentials of these PPI membranes, we assumed that the surface charges of the PPI membranes are distributed on two different surface charge layers from the membrane surface. The first layer is at the membrane surface, where one of the phosphate group is linked to the glycerol group, while the second layer is located at a distance, d, from the membrane surface comprising phosphate groups on the inositol group in PIP(4) and PIP2(4/5). Such membrane surfaces are facing a uni-univalent electrolyte solution. We also measured experimentally the surface potentials of these PPI monolayers on various subphase solutions by using an air-ionizing electrode placed just above the monolayer surface. The subphase solutions used were the monovalent salt solution with different concentrations of calcium ions added. We recorded the potential ΔV (monovalent ion only subphase), and ΔV’ (with the subphse solution containing calcium ions in addition). We obtained the potential differences (ΔV - ΔV’) which are due to the calcium ion concentrations. With these data (experimental and theoretical), we deduced the calcium ion concentrations at the PPI membrane surfaces. It was concluded that the calcium ion concentrations at the PIP2 membrane surfaces are much higher compared with PI and PIP membranes, even with very small bulk calcium concentrations. This may imply important molecular roles of PIP2 membranes in many biological systems.