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 la rge unilamellar liposomes prepared from 1,2-dilauroylphosphatidylcholine (D LPC), 1,2-dimyristoyl-phosphatidylcholine (DMPC) and 1,2-distearoylphosphat idyl-choline (DSPC) in heavy water by extrusion through polycarbonate filte rs with 500 Angstrom pores. The neutron scattering intensity J(O) in the re gion of scattering vectors Q corresponding to 0.0015 Angstrom (-2) less tha n or equal to Q(2) less than or equal to 0.0115 Angstrom (-2) was fitted us ing a step function model of bilay er neutron scattering length density and supposing that the liposomes are spherical and have a Gaussian distributio n of radii. Using the lipid volumetric data, and supposing that the thickne ss 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 lip id 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 jo int evaluation of SANS and small-angle X-ray scattering on unilamellar lipo somes can be used to obtain the value of dH and the distance of the lipid p hosphate group from the bilayer hydrocarbon region d(HI). (C) 2001 Elsevier Science B.V. All rights reserved.