INVESTIGATION OF ANION BINDING TO NEUTRAL LIPID-MEMBRANES USING H-2 NMR

The binding of aqueous anions (ClO4-, SCN-, I-, and NO3-) to lipid bilayer membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was investigated using deuterium (H-2) and phosphorus-31 (P-31) nuclear magnetic resonance (NMR) spectroscopy. The ability of these anions to influence the H-2 NMR quadrupole splittings of POPC, specifically labeled at the alpha or beta-position of the choline head group, increased in the order NO3- << I- < SCN- < ClO4-. In the presence of these chaotropic anions, the quadrupole splitting increased for alpha-deuterated POPC and decreased for beta-deuterated POPC, indicating a progressive accumulation of negative charge at the membrane surface. Calibration of the H-2 NMR quadrupole splittings with the amount of membrane-bound anion permitted binding isotherms to be generated for perchlorate, thiocyanate, and iodide, up to concentrations of 100 mM. The binding isotherms were analyzed by considering electrostatic contributions, according to the Gouy-Chapman theory, as well as chemical equilibrium contributions. For neutral POPC membranes, we obtained ion association constants of 32, 80, and 115 M-1 for iodide, thiocyanate, and perchlorate, respectively. These values increase in the order expected for a Hofmeister series of anions. We conclude that the factor determining whether a particular anion will bind to lipid bilayers is the ease with which that anion loses its hydration shell. A comparison of the calibrated sensitivity of the H-2 NMR quadrupole splitting to these and other ligands indicated that, in addition to charge, two factors dictate the level of the H-2 NMR response: first, whether the ligand is cationic or anionic; and second, whether the ligand is predominantly hydrophobic or hydrophilic in nature. Both of these factors can be seen to arise from the details of the "choline-tilt' model of the H-2 NMR response to surface charges.