Understanding the Mechanical Properties of Ultradeformable Liposomes Using Molecular Dynamics Simulations

Abstract

Improving drug delivery efficiency to solid tumor sites is a central challenge in anticancer therapeutic research. Our previous experimental study (Guo et al., Nat. Commun. 2018, 9, 130) showed that soft, elastic liposomes had increased uptake and accumulation in cancer cells and tumors in vitro and in vivo respectively, relative to rigid particles. As a first step toward understanding how liposomes' molecular structure and composition modulates their elasticity, we performed all-atom and coarse-grained classical molecular dynamics (MD) simulations of lipid bilayers formed by mixing a long-tailed unsaturated phospholipid with a short-tailed saturated lipid with the same headgroup. The former types of phospholipids considered were 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dipalmitoleoyl-sn-glycero-3-phosphocholine (termed here DPMPC). The shorter saturated lipids examined were 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC), 1,2-didecanoyl-sn-glycero-3-phosphocholine (DDPC), 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Several lipid concentrations and surface tensions were considered. Our results show that DOPC or DPMPC systems having 25-35 mol % of the shortest lipids DHPC or DDPC are the least rigid, having area compressibility moduli K_A that are ∼10% smaller than the values observed in pure DOPC or DPMPC bilayers. These results agree with experimental measurements of the stretching modulus and lysis tension in liposomes with the same compositions. These mixed systems also have lower areas per lipid and form more uneven x-y interfaces with water, the tails of both primary and secondary lipids are more disordered, and the terminal methyl groups in the tails of the long lipid DOPC or DPMPC wriggle more in the vertical direction, compared to pure DOPC or DPMPC bilayers or their mixtures with the longer saturated lipid DLPC or DMPC. These observations confirm our hypothesis that adding increasing concentrations of the short unsaturated lipid DHPC or DDPC to DOPC or DPMPC bilayers alters lipid packing and thus makes the resulting liposomes more elastic and less rigid. No formation of lipid nanodomains was noted in our simulations, and no clear trends were observed in the lateral diffusivities of the lipids as the concentration, type of secondary lipid, and surface tension were varied.

Figure 1

Incorporation of short acyl chain lipids influences lipid packing in the bilayers of DOPC- (or DPMPC-) based liposomes.

Figure 2

Area per lipid of different bilayers as a function of the mole fraction of DOPC/DPMPC. In both figures, DOPC-based bilayers are shown by red solid lines and DPMPC-based bilayers are shown by blue dashed lines. (a) DOPC-DHPC = red circles; DOPC-DDPC = red squares; DPMPC-DHPC = blue diamonds; DPMPC-DDPC = blue triangles. (b) DOPC-DLPC = red circles; DOPC-DMPC = red squares; DPMPC-DLPC = blue diamonds; and DPMPC-DMPC = blue triangles.

Figure 3

Sum of the standard deviations (distances, in Å) from the average z coordinate of phosphorus atoms, as indicators of the surface unevenness of the bilayer structure. These values were calculated over 500 random simulation frames (see the text for details of how these summed distances were computed), as a function of the molar ratio of long tail lipids at a surface tension of γ = 0 mN/m. Secondary lipids considered are indicated by the top labels over each plot. Red solid lines with circles = DOPC; blue dashed lines with triangles = DPMPC.

Figure 4

Area compressibility modulus of bilayers with DOPC (red lines) or DPMPC (blue lines) as primary lipids as a function of their molar ratio. (a) DOPC-DHPC = red circles; DOPC-DDPC = red squares; DPMPC-DHPC = blue diamonds; and DPMPC-DDPC = blue triangles. (b) DOPC-DLPC = red circles; DOPC-DMPC = red squares; DPMPC-DLPC = blue diamonds; and DPMPC-DMPC = blue triangles. Data shown in these figures are presented in Table S2.

Figure 5

Relative order parameters of two acyl chains SN1 and SN2 for the primary lipid in DOPC-based bilayers. Relative order parameters are determined by dividing the value of the order parameter of each carbon atom in each tail SN1 and SN2 in a mixed bilayer by its counterpart in a pure bilayer system composed of the same primary lipid at the same value of surface tension (γ = 0 mN/m) and then multiplying by the carbon atom index. Therefore, the diagonal line corresponds to the pure DOPC system.

Figure 6

Relative order parameters of two acyl chains SN1 and SN2 for the primary lipid in DPMPC-based bilayers. Relative order parameters are determined by dividing the values of the order parameter of each carbon atom in each tail SN1 and SN2 in a mixed bilayer by their counterparts in a pure bilayer system composed of the same primary lipid at the same value of surface tension (γ = 0 mN/m) and then multiplying by the carbon atom index. Therefore, the diagonal line corresponds to the pure DPMPC system.

Figure 7

Vertical spatial distribution of terminal methyl groups (TMGs) in primary lipids for systems with 75% DOPC (top row) or DPMPC (bottom row) at a surface tension of γ = 0 mN/m. Results observed in systems with different secondary lipids are shown in vertical columns. Results for mixed bilayers (dashed, light-colored lines) are compared against those for pure bilayers (solid, dark-colored lines). Dark/light red, dark/light green, and dark/light blue represent the TMG distributions for leaflet 1, leaflet 2, and both leaflets, respectively.

Figure 8

Lateral diffusion coefficients of lipids in bilayers as a function of mole % of the primary lipid, at a surface tension of 0 mN/m. (a) DOPC in DOPC:DHPC (blue) or in DOPC:DDPC (red); (b) DHPC in DOPC:DHPC (blue) or DDPC in DOPC:DDPC (red); (c) DPMPC in DPMPC:DHPC (blue) or in DPMPC:DDPC (red); and (d) DHPC in DPMPC:DHPC (blue) or DDPC in DPMPC:DDPC (red). Although all of the binary mixtures depicted had the same compositions (65, 75, 85, 95, and 100 mol % of the long unsaturated lipid and for some systems 90%), all data points shown were slightly displaced horizontally around these compositions for ease of visualization.

Figure 9

Representative Voronoi diagrams for the 75 mol % DOPC (shown in green) and 25 mol % (a) DPPC (PC 16:0/18:0), (b) DLPC (PC 12:0/14:0), and (c) DTPC (PC 8:0/10:0) membrane bilayers in the Martini force field using GROMACS. Secondary lipids are shown in pink.

Figure 10

(a) Violin plots of the Voronoi cell area distribution of three bilayers with quartile dashed lines. The densities of DOPC and secondary lipids are colored in red and blue. (b) Histograms of Voronoi cell areas of primary lipid (DOPC), shown on the left, and secondary lipids (DTPC, DLPC, and DPPC), shown on the right, of three lipid bilayers.