Cationic DMPC/DMTAP lipid bilayers: molecular dynamics study

Abstract

Cationic lipid membranes are known to form compact complexes with DNA and to be effective as gene delivery agents both in vitro and in vivo. Here we employ molecular dynamics simulations for a detailed atomistic study of lipid bilayers consisting of a mixture of cationic dimyristoyltrimethylammonium propane (DMTAP) and zwitterionic dimyristoylphosphatidylcholine (DMPC). Our main objective is to examine how the composition of the DMPC/DMTAP bilayers affects their structural and electrostatic properties in the liquid-crystalline phase. By varying the mole fraction of DMTAP, we have found that the area per lipid has a pronounced nonmonotonic dependence on the DMTAP concentration, with a minimum around the point of equimolar DMPC/DMTAP mixture. We show that this behavior has an electrostatic origin and is driven by the interplay between positively charged TAP headgroups and the zwitterionic phosphatidylcholine (PC) heads. This interplay leads to considerable reorientation of PC headgroups for an increasing DMTAP concentration, and gives rise to major changes in the electrostatic properties of the lipid bilayer, including a significant increase of total dipole potential across the bilayer and prominent changes in the ordering of water in the vicinity of the membrane. Moreover, chloride counterions are bound mostly to PC nitrogens implying stronger screening of PC heads by Cl ions compared to TAP headgroups. The implications of these findings are briefly discussed.

FIGURE 1

Chemical structures of the two lipids considered in this work: a zwitterionic dimyristoylphosphatidylcholine (DMPC) and a cationic dimyristoyltrimethylammonium propane (DMTAP).

FIGURE 2

Time evolution of the area per lipid, A(t), for different mixtures of DMPC and DMTAP. The low concentration results (χ_TAP ≤ 0.31) are shown on the top, and the high concentration regime (χ_TAP ≥ 0.63) is illustrated at the bottom. For clarity's sake, the results at intermediate concentrations are not shown here.

FIGURE 3

Average area per lipid 〈A〉 as a function of the DMTAP mole fraction, χ_TAP.

FIGURE 4

Average area per lipid 〈A〉_V based on the Voronoi analysis in two dimensions. The results for DMPC and DMTAP are shown separately as a function of χ_TAP.

FIGURE 5

Deuterium order parameter |S_CD| averaged over sn-1 and sn-2 chains for DMPC (top) and for DMTAP (bottom) as a function of DMTAP mole fraction. Small carbon atom numbers correspond to those close to the headgroup.

FIGURE 6

Plateau order parameter S_ave calculated by averaging S_CD over C2–C8 hydrocarbons. Shown are S_ave for DMPC (solid lines with solid circles) and DMTAP (dashed lines with open circles).

FIGURE 7

Results for the probability distribution function P(α) versus the angle α between the P-N vector (of DMPC headgroups) and the bilayer normal.

FIGURE 8

The average angle 〈α〉 between the P-N vector of DMPC and the bilayer normal, shown as a function of DMTAP mole fraction. (•) Results averaged over all DMPC molecules in a given system. (○) Results averaged over only those DMPC molecules that are beside DMTAP. See text for details.

FIGURE 9

Scaled number densities ρ_N(z) for three DMPC/DMTAP mixtures with χ_TAP = 0.0 (top), χ_TAP = 0.5 (middle), and χ_TAP = 1.0 (bottom). The case z = 0 corresponds to the center of the bilayer.

FIGURE 10

Maxima of the density profiles, z_max, for phosphorus and nitrogen atoms from the DMPC headgroups, and of the density profile of chloride ions. The maxima are shown for the z coordinate in the direction of the membrane normal, shown as a function of DMTAP molarity. The dashed line marks the position of the membrane-water interface determined from the condition that the densities of water and lipids (in Fig. 9) are equal.

FIGURE 11

Charge densities ρ(z) across a single leaflet for χ_TAP equal to 0.0 (top), 0.5 (middle), and 1.0 (bottom). The case z = 0 corresponds to the center of the bilayer. Charge densities are shown as solid lines. In addition, the componentwise contributions due to DMPC (•), DMTAP (○), water (dashed line), and chloride ions (_*) are displayed. To reduce the noise in the data, the charge densities shown here were first fitted to splines (Thijsse et al., 1998). The error bars are of the same size as the symbols.

FIGURE 12

Electrostatic potential V(z) across cationic bilayers at different DMTAP mole fractions.

FIGURE 13

Projection of the water dipole unit vector onto the interfacial normal →, yielding . Here, z = 0 corresponds to the center of the bilayer, and the bilayer normal → is chosen to point away from the bilayer center.

FIGURE 14

(Top) Coordination numbers N_C of DMPC phosphorus with DMPC nitrogen (solid line with solid circles) and with DMTAP nitrogen (dashed line with open circles) plotted versus χ_TAP. (Bottom) Coordination numbers N_C of DMPC nitrogen (solid line with solid circles) and DMTAP nitrogen (dashed line with open circles) with Cl ions.

FIGURE 15

A proposed schematic picture of the observed change in the area per lipid versus χ_TAP. Only headgroups of the lipids are shown here, and water (not shown) is above the headgroups.