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

Abstract

Small-angle neutron scattering (SANS) experiments have been performed on large unilamellar liposomes prepared from 1,2-dilauroylphosphatidylcholine (DLPC), 1,2-dimyristoyl-phosphatidylcholine (DMPC) and 1,2-distearoylphosphatidylcholine (DSPC) in heavy water by extrusion through polycarbonate filters with 500 A pores. The neutron scattering intensity I(Q) in the region of scattering vectors Q corresponding to 0.0015 A(-2) < or = Q(2) < or = 0.0115 A(-2) was fitted using a step function model of bilayer 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 A and the water molecular volume intercalated 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 A, A(L) = 57.18+/-1.00 A(2) and N = 6.53+/-1.93 in DLPC at 20 degrees C, d(L) = 44.26+/-1.42 A, A(L) = 60.01+/-0.75 A(2) and N = 7.37+/-1.94 in DMPC at 36 degrees C, and d(L) = 49.77+/-1.52 A, A(L) = 64.78+/-0.46 A(2) and N = 8.67+/-1.97 in DSPC at 60 degrees C. After correcting for area thermal expansivity alpha approximately 0.00417 K(-1), the lipid surface area shows a decrease with the lipid acyl chain length at 60 degrees C: A(L) = 67.56+/-1.18 A(2) in DLPC, A(L) = 66.33+/-0.83 A(2) in DMPC and A(L) = 64.78+/-0.46 A(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 d(H) and the distance of the lipid phosphate group from the bilayer hydrocarbon region d(H1).

The effects of cholesterol and β-sitosterol on the structure of saturated diacylphosphatidylcholine bilayers

“…However, the drop of d L with increasing temperature is obvious again. The value d L = 3.74 ± 0.02 nm at 33°C found here is in good agreement with 3.86 ± 0.01 nm at 36°C (Balgavý et al 2001) and 3.83 ± 0.05 nm at 30°C (Kučerka et al 2004b). We observed a drop of d L of 0.52 ± 0.07 nm when the temperature increased from 15 to 45°C, similarly to Pencer et al (2005), who found a decrease of 0.54 nm for the same temperature change.…”

Interaction of long-chain n-alcohols with fluid DOPC bilayers: a neutron diffraction study

“…If more than 23-24 water molecules per lipid are present in the sample, they begin to form a separate aqueous phase. The bilayer structural parameters and ordering are influenced strongly by lateral stress and interbilayer forces at n W : n L ≤ 12 ± 1, and the bilayers become curved and thus the L α phase contains water-filled defects at n W : n L ≥ 23 ± 1 (Gawrisch et al 1985;Gallová et al 1995;Balgavý et al 1998;Karlovská et al 2004). Consequently, the bilayer structural parameters obtained from the repeat period d at 12 ± 1 ≥ n W : n L ≥ 23 ± 1 molar ratios can be thus flawed.…”

Influence of cholesterol on the structure of stratum corneum lipid model membrane

“…The analysis of the SANS curves has been performed using two methods, namely the 'model of separated form factors' (MSFF) [23], and the Kratky-Porod analysis of the Guinier approximation (GA) [24][25][26].…”

“…Another possibility to calculate the membrane thickness from a scattering curve is given by the GA [24][25][26]. In the q range valid for a homogeneous membrane approximation, the scattering intensity of ULVs dispersed in heavy water can be given by…”