Bilayer thickness and lipid interface area in unilamellar extruded 1,2-diacylphosphatidylcholine liposomes:: a small-angle neutron scattering study

Small-angle neutron scattering (SANS) experiments hale been performed on large unilamellar liposomes prepared from 1,2-dilauroylphosphatidylcholine (DLPC), 1,2-dimyristoyl-phosphatidylcholine (DMPC) and 1,2-distearoylphosphatidyl-choline (DSPC) in heavy water by extrusion through polycarbonate filters with 500 Angstrom pores. The neutron scattering intensity J(O) in the region of scattering vectors Q corresponding to 0.0015 Angstrom (-2) less than or equal to Q(2) less than or equal to 0.0115 Angstrom (-2) was fitted using a step function model of bilay er neutron scattering length density and supposing that the liposomes are spherical and have a Gaussian distribution of radii. Using the lipid volumetric data, and supposing that the thickness of bilayer polar region equals to d(H) = 9 +/- 1 Angstrom and the water molecular volume inrercalated in the bilayer polar region is the same as in the aqueous bulk aqueous phase, the steric bilayer thickness d(L), the lipid surface area A(L) and the number of water molecules per lipid molecule N intercalated in the bilayer polar region were obtained: d(L) = 41.58 +/- 1.93 Angstrom, A(L) = 57.18 +/- 1.00 Angstrom (-2) and N = 6.53 +/- 1.93 in DLPC at 20 degreesC. d(L) = 44.26 +/- 1.42 Angstrom. A(L) = 60.01 +/- 0.75 Angstrom (2) and N = 7.37 +/- 1.94 in DMPC at 36 degreesC, and d(L) = 49.77 +/- 1.52 Angstrom, 8.67 +/- 1.97 in DSPC at 60 degreesC. After correcting fur area thermal expansivity alpha similar to0.00417 K-1, the lipid surface area shows a decrease with the lipid acyl chain length at 60 degreesC: A(L) = 67.56 +/- 1.18 Angstrom (2) in DLPC, A(L) = 06.33 +/- 0.83 A(L) in DMPC and A(L) = 64.78 +/- 0.46 Angstrom (2) in DSPC. It is also shown that a joint evaluation of SANS and small-angle X-ray scattering on unilamellar liposomes can be used to obtain the value of dH and the distance of the lipid phosphate group from the bilayer hydrocarbon region d(HI). (C) 2001 Elsevier Science B.V. All rights reserved.