What can we learn about the lipid vesicle structure from the small-angle neutron scattering experiment?
Small-angle neutron scattering (SANS) on the unilamellar vesicle (ULV) populations (diameter 500 and 1,000 Å) in D2O was used to characterize lipid vesicles from dimyristoylphosphatidylcholine (DMPC) at three phases: gel Lβ′, ripple Pβ′ and liquid Lα. Parameters of vesicle populations and internal structure of the DMPC bilayer were characterized on the basis of the separated form factor (SFF) model. Vesicle shape changes from nearly spherical in the Lα phase to elliptical in the Pβ′ and Lβ′ phases. This is true for vesicles prepared via extrusion through pores with the diameter 500 Å. Parameters of the internal bilayer structure (thickness of the membrane and the hydrophobic core, hydration and the surface area of the lipid molecule) were determined on the basis of the hydrophobic–hydrophilic (HH) approximation of neutron scattering length density across the bilayer ρ(x) and of the step function (SF) approximation of ρ(x). DMPC membrane thickness in the Lα phase (T=30°C) demonstrates a dependence on the membrane curvature for extruded vesicles. Prepared via extrusion through 500 Å diameter pores, vesicle population in the Lα phase has the following characteristics: average value of minor semi-axis 266±2 Å, ellipse eccentricity 1.11±0.02, polydispersity 26%, thickness of the membrane 48.9±0.2 Å and of the hydrophobic core 19.9±0.4 Å, surface area 60.7±0.5 Å2 and number of water molecules 12.8±0.3 per DMPC molecule. Vesicles prepared via extrusion through pores with the diameter 1,000 Å have polydispersity of 48% and membrane thickness of 45.5±0.6 Å in the Lα phase. SF approximation was used to describe the DMPC membrane structure in Lβ′ (T=10°C) and Pβ′ (T=20°C) phases. Extruded DMPC vesicles in D2O have membrane thickness of 49.6±0.5 Å in the Lβ′ phase and 48.3±0.6 Å in the Pβ′ phase. The dependence of the DMPC membrane thickness on temperature was restored from the SANS experiment.
KeywordsPhospholipids Lipid membrane Vesicles Small-angle neutron scattering
Small-angle neutron scattering
Small-angle X-ray scattering
Separated form factor
Differential scanning calorimetry
Liquid crystalline phase
This work is partly based on the experiments performed at Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. The authors are grateful to V.L. Aksenov for his support, H. Schmiedel and A.M. Balagurov for fruitful discussions and K. Schwarz for performing DSC measurements. The investigation was supported by Grant of Leading Scientific Scholl, Grant of the Federal State of Saxony-Anhalt (project no. 3482A/1102L) and RFBR (grant no. 03-01-00657). The authors would like to thank Lipoid (Moscow) for the gift of DMPC.
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