Effect of Electrostatic Interactions on Phase Stability of Cubic Phases of Membranes of Monoolein/Dioleoylphosphatidic Acid Mixtures

Abstract

To elucidate effects of electrostatic interactions resulting from surface charges on structures and phase stability of cubic phases of lipid membranes, membranes of 1-monoolein (MO) and dioleoylphosphatidic acid (DOPA) (DOPA/MO membrane) mixtures have been investigated by small-angle x-ray scattering method. As increasing DOPA concentration in the DOPA/MO membrane at 30 wt% lipid concentration, a phase transition from Q(224) to Q(229) phase occurred at 0.6 mol% DOPA, and at and above 25 mol% DOPA, DOPA/MO membranes were in the L(alpha) phase. As NaCl concentration in the bulk phase increased, for 10% DOPA/90% MO membrane in excess water, a Q(229) to Q(224) phase transition occurred at 60 mM NaCl, and then a Q(224) to H(II) phase transition occurred at 1.2 M NaCl. Similarly, for 30% DOPA/70% MO membrane in excess water, at low NaCl concentrations it was in the L(alpha) phase, but at and above 0.50 M NaCl it was in the Q(224) phase, and then at 0.65 M NaCl a Q(224) to H(II) phase transition occurred. These results indicate that the electrostatic interactions in the membrane interface make the Q(229) phase more stable than the Q(224) phase, and that, at larger electrostatic interactions, the L(alpha) phase is more stable than the cubic phases (Q(224) and Q(229)). We have found that the addition of tetradecane to the MO membrane induced a Q(224)-to-H(II) phase transition and also that to the 30% DOPA/70% MO membrane induced an L(alpha)-to-H(II) phase transition. By using these membranes, the effect of the electrostatic interactions resulting from the membrane surface charge (DOPA) on the spontaneous curvature of the monolayer membrane has been investigated. The increase in DOPA concentration in the DOPA/MO membrane reduced the absolute value of spontaneous curvature of the membrane. In the 30% DOPA/70% MO membrane, the absolute value of spontaneous curvature of the membrane increased with an increase in NaCl concentration. On the basis of these new results, the phase stability of DOPA/MO membranes can be reasonably explained by the spontaneous curvature of the monolayer membrane and a curvature elastic energy of the membrane.

Stability of Membrane Proteins: Relevance for the Selection of Appropriate Methods for High-Resolution Structure Determinations

“…Its potentials have by no means been exhausted as, due to an intense interest in lipid polymorphism (Lindblom, 1996;Luzzati, 1997), much information concerning geometry, fluidity, and stability of cubic phases is available (Li et al, 2001;Templer et al, 1998). Applying the results of these studies to LCPC might broaden its scope significantly.…”

On structural transitions in a discontinuous micellar cubic phase loaded with sodium diclofenac

“…Therefore, solubilized bioactives in the LLC systems would be released at rates depended on the mesophase structure and type of dissolved solutes [10,13,14]. The electrolytes can also affect the lipids assemblies and structure since they control factors such as curvature, surface area per surfactant, solubilization capacity, susceptibility of the headgroups to solubilization of bioactives, melting, and phase transitions [8,[15][16][17]. Electrolytes may also strongly affect surfactant-water interactions and can impart structural transitions.…”

“…From previous studies it is clear that hydration of the headgroups plays an important role in the stability of mesophases [15,16] and that salinity can stabilize the mesophases, depending on the hydration of the headgroups [16,29].…”

Low pH Stabilizes the Inverted Hexagonal II Phase in Dipalmitoleoylphosphatidylethanolamine Membrane

“…For this purpose, we added 16 wt% tetradecane into the DPOPE membrane, because tetradecane can fill the interstitial region of the H II phase and relax the acyl chain packing stress, and therefore allow the DPOPE membranes in the H II phase [19,22,23]. As shown in Figure 4, the DPOPE membranes containing 16 wt% tetradecane were in the H II phase in various pH values at fully hydrated condition, and the basis vector length, d, of the DPOPE/tetradecane membranes did not change (d = 7.8 nm) with a decrease in pH values ( Figure 4).…”

“…To take the information of spontaneous curvature of DPOPE monolayer membrane, we have investigated the structures of lipid membranes containing 16 wt% tetradecane. The method is the same as that described in our previous paper [19] else than the incubation for 10 hours for equilibration from the addition of the various pH values buffer to DPOPE/tetradecane. X-ray diffraction experiments were performed by using Nickel-filtered Cu K α X-ray (λ = 0.154 nm) from rotating anode type X-ray generator (Rigaku, Rotaflex, RU-300) at the operating condition (40 kV × 200 mA).…”