Quantum-chemical and empirical calculations on phospholipids. VI: The stereospecific interaction of Na+ with phosphatidylethanolamine and with phosphatidylcholine layer systems

Using 1-6-12 atom-atom potential functions with a solvent-averaged electrostatic potential U = q_Iq_j/(?D(r)r_1j between the charges q_I and q_j the stereospecific interaction of Na⁺ with clusters of l-glycero-phosphorylethanolamine (GPE) and -choline (GPC) in a two-dimensional planar lattice was calculated. With neutral PE headgroups only a weak attractive interaction of ?140 kJ/mol was found. This should be compensated in PE/water systems by reorientational processes within the Na⁺ hydration shell to get an optimum binding of Na⁺ with the PE headgroup. With a phosphorylethanolamine anion within a neutral PE headgroup environment a stereospecific binding energy of ?500 kJ/mol was calculated. The interaction of Na⁺ or Ca²⁺ with clusters of phosphatidylcholine is always repulsive. These results are in a very good agreement with experimental ones where a binding of Na⁺ with PE bilayers at pH > 7.5, but not with PC bilayers was detected.