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Thermotropic phase transition in soluble nanoscale lipid bilayers - PubMed

  • ️Sat Jan 01 2005

Thermotropic phase transition in soluble nanoscale lipid bilayers

Ilia G Denisov et al. J Phys Chem B. 2005.

Abstract

The role of lipid domain size and protein-lipid interfaces in the thermotropic phase transition of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) bilayers in Nanodiscs was studied using small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and generalized polarization (GP) of the lipophilic probe Laurdan. Nanodiscs are water-soluble, monodisperse, self-assembled lipid bilayers encompassed by a helical membrane scaffold protein (MSP). MSPs of different lengths were used to define the diameter of the Nanodisc lipid bilayer from 76 to 108 A and the number of DPPC molecules from 164 to 335 per discoidal structure. In Nanodiscs of all sizes, the phase transitions were broader and shifted to higher temperatures relative to those observed in vesicle preparations. The size dependences of the transition enthalpies and structural parameters of Nanodiscs reveal the presence of a boundary lipid layer in contact with the scaffold protein encircling the perimeter of the disc. The thickness of this annular layer was estimated to be approximately 15 A, or two lipid molecules. SAXS was used to measure the lateral thermal expansion of Nanodiscs, and a steep decrease of bilayer thickness during the main lipid phase transition was observed. These results provide the basis for the quantitative understanding of cooperative phase transitions in membrane bilayers in confined geometries at the nanoscale.

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Figures

Figure 1
Figure 1

A crossection of MD simulated Nanodisc with DPPC . The numbers designate different cylindrical layers in the core – shell models used in the fitting of experimental SAXS data: (1) MSP double belt at the disc circumference, (2) phosphatidylcholine polar groups, (3) hydrocarbon acyl chains, (4) methyl end group layer in the middle of the lipid bilayer. D represents the overall diameter of the Nanodisc and H the bilayer height. The bar shown on top is a 100 Å scale.

Figure 2
Figure 2

Temperature dependence of SAXS measured from Nanodiscs of different sizes formed with DPPC. Experimental data – (•), data fits obtained by GNOM are indicated by full lines. Temperatures, from top to bottom are: left panel: (1) 5°C; (2) 22°C; (3) 37°C; (4) 42°C; (5) 46°C; (6) 48°C; (7) 52°C ; and the right panel: (1) 22°C; (2) 37°C; (3) 42°C; (4) 46°C; (5) 48°C; (6) 52°C. Scattering vector Q = 4πsin(θ)/λ. The data obtained with DMPC Nanodiscs reveal a similar pattern (not shown).

Figure 3
Figure 3

Temperature dependence of Nanodisc diameters determined from experimental SAXS curves using GNOM . (A) Smaller Nanodiscs formed with MSP1. (B) Larger Nanodiscs formed with MSP1E3. Filled symbols correspond to DMPC and open symbols to DPPC.

Figure 4
Figure 4

Density distribution functions p(R) at different temperatures determined from experimental SAXS curves using GNOM . (A) Smaller Nanodiscs formed with DMPC (top) and DPPC (bottom). (B) Larger Nanodiscs formed with DMPC (top) and DPPC (bottom). The arrows show the direction of p(R) changes with temperature increase. The temperatures for each data set are the same as in Figures 3 and 5.

Figure 5
Figure 5

The bilayer thickness of Nanodiscs as a function of temperature for Nanodiscs formed with DMPC (filled symbols) and DPPC (open symbols). (A) Smaller Nanodiscs formed with MSP1, (B) larger Nanodiscs formed with MSP1E3.

Figure 6
Figure 6

DSC scans of Nanodiscs of different sizes formed with DPPC (A) and DMPC (B). On both panels (1) indicates the smaller Nanodiscs formed with MSP1 and (2) the larger Nanodiscs formed with MSP1E3.

Figure 7
Figure 7

Laurdan GP values for MSP1 Nanodiscs (triangles) and MSP1E3 Nanodiscs (circles) formed with DPPC (empty symbols) and DMPC (filled symbols) as a function of temperature. The lines are added only to guide the eye.

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