EFFECT OF HEADGROUP STRUCTURE AND COUNTERION CONDENSATION ON PHASE-EQUILIBRIA IN ANIONIC PHOSPHOLIPID WATER-SYSTEMS STUDIED BY H-2, NA-23, AND P-31 NMR AND X-RAY-DIFFRACTION

The phase equilibria, hydration, and sodium counterion association for the systems DOPA-(H2O)-H-2 DOPS-(H2O)-H-2, DOPG-(H2O)-H-2, and DPG-(H2O)-H-2 were investigated with H-2, Na-23, and P-31 NMR and X-ray diffraction. The following one-phase regions were found in the DOPA-water system: a reversed hexagonal liquid-crystalline (H(II)) phase up to about 35 wt % water and a lamellar liquid-crystalline (L(a)) phase between about 55 and 98 wt % water. The area per DOPA molecule was 36-65 angstrom 2 in the H(II) phase (10-40 wt % water) and 69 angstrom 2 in the L(a) phase (60 wt % water). DOPS and DOPG with 10-98 wt % water, and DPG with 20-95 wt % water formed an L(a) phase at temperatures between 25 and 55-degrees-C. At temperatures above 55-degrees-C, DPG with 20 and 30 wt % water formed a mixture of L(a), H(II), and cubic liquid-crystalline phases, the mole percent of lipid forming nonlamellar phases being smaller at 30 wt % water than at 20 wt % water. DPG with 10 wt % water probably formed a mixture of an L(a) phase and at least one nonlamellar liquid-crystalline phase at 25 and 35-degrees-C, and a pure H(II), phase at 45-degrees-C and higher temperatures. At water concentrations above about 50 wt % the Na-23 quadrupole splitting was constant for all four lipid-water systems studied, implying that the counterion association to the charged lipid aggregates did not change upon dilution. These experimental observations can be described with an ion condensation model but not with a simple equilibrium model. The fraction of counterions located close to the lipid-water interface was calculated to be >95%. The H-2 and Na-23 NMR quadrupole splittings of (H2O)-H-2 and sodium counterions, respectively, indicate that the molecular order in the polar head-group region decreases for the L(a) phase in the order DOPA almost-equal-to DPG > DOPS > DOPG.