Anion binding to neutral and positively-charged lipid membranes

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Abstract
Aqueous anion binding to bilayer membranes consisting of 1-palmitoyl-2-oleoxyl-sn-glycero-3-phosphocholine (POPC) was investigated by using deuterium and phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy. Only those anions that exhibit chaotropic properties showed significant binding to POPC membranes. A detailed investigation of thiocyanate binding to neutral POPC and to positively charged mixed POPC/dihexadecyldimethylammonium bromide (DHDMAB) (8:2 mol/mol) membranes revealed changes in the 2H NMR quadrupole splittings from POPC specifically deuteriated at either the .alpha.-segment or the .beta.-segment of the choline head group which were consistent with a progressive accumulation of excess negative charge at the membrane surface with increasing SCN- concentration. Both the 2H and 31P NMR spectra indicated the presence of fluid lipids in a bilayer configuration up to at least 1.0 M NaSCN with no indication of any phase separation of lipid domains. Calibration of the relationship between the change in the 2H NMR quadrupole splitting and the amount of SCN- binding provided thiocyanate binding isotherms. At a given SCN- concentration the positively charged membranes bound levels of SCN- 3 times that of the neutral membranes. The binding isotherms were analyzed by considering both the electrostatic and the chemical equilibrium contributions to SCN- binding. Electrostatic considerations were accounted for by using the Gouy-Chapman theory. For 100% POPC membranes as well as for mixed POPC/DHDMAB (8:2 mol/mol) membranes the thiocyanate binding up to concentrations of 100 mM was characterized by a partition equilibrium with an association constant of K .apprxeq. 1.4 .+-. 0.3 M-1. Hence the greater levels of thiocyanate binding occurring in the presence of positively charged membranes were primarily the result of electrostatic effects. The 2H NMR results further indicated that the anions were binding in the plane of the POPC choline head group quaternary nitrogen. The factor critical to the ability of an aqueous anion to bind to lipid membrane surfaces appeared to be the ease with which its hydration shell water molecules could be removed.

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