Determination of the Hydrocarbon Core Structure of Fluid Dioleoylphosphocholine (DOPC) Bilayers by X-Ray Diffraction Using Specific Bromination of the Double-Bonds: Effect of Hydration

Abstract

Changes in the structure of the hydrocarbon core (HC) of fluid lipid bilayers can reveal how bilayers respond to the partitioning of peptides and other solutes (Jacobs, R. E., and S. H. White. 1989. Biochemistry. 28:3421-3437). The structure of the HC of dioleoylphosphocholine (DOPC) bilayers can be determined from the transbilayer distribution of the double-bonds (Wiener, M. C., and S. H. White. 1992. Biophys. J. 61:434-447). This distribution, representing the time-averaged projection of the double-bond positions onto the bilayer normal (z), can be obtained by means of neutron diffraction and double-bond specific deuteration (Wiener, M. C., G. I. King, and S. H. White. 1991. Biophys. J. 60:568-576). For fully resolved bilayer profiles, a close approximation of the distribution could be obtained by x-ray diffraction and isomorphous bromine labeling at the double-bonds of the DOPC sn-2 acyl chain (Wiener, M. C., and S. H. White. 1991. Biochemistry. 30:6997-7008). We have modified the bromine-labeling approach in a manner that permits determination of the distribution in under-resolved bilayer profiles observed at high water contents. We used this new method to determine the transbilayer distribution of the double-bond bromine labels of DOPC over a hydration range of 5.4 to 16 waters per lipid, which reveals how the HC structure changes with hydration. We found that the transbilayer distributions of the bromines can be described by a pair of Gaussians of 1/e half-width A(Br) located at z = +Z(Br) relative to the bilayer center. For hydrations from 5.4 waters up to 9.4 waters per lipid, Z(Br) decreases from 7.97 +/- 0.27 A to 6.59 +/- 0.15 A, while A(Br) increased from 4.62 +/- 0.62 A to 5.92 +/- 0.37 A, consistent with the expected hydration-induced decrease in HC thickness and increase in area per lipid. After the phosphocholine hydration shell was filled at approximately 12 waters per lipid, we observed a shift in Z(Br) to approximately 7.3 A, indicative of a distinct structural change upon completion of the hydration shell. For hydrations of 12-16 waters per lipid, the bromine distribution remains constant at Z(Br) = 7.33 +/- 0.25 A and A(Br) = 5.35 +/- 0.5 A. The absolute-scale structure factors obtained in the experiments provided an opportunity to test the so-called fluid-minus method of structure-factor scaling. We found that the method is quite satisfactory for determining the phases of structure factors, but not their absolute values.

The effect of the protein corona on the interaction between nanoparticles and lipid bilayers

“…13,51 The NR characterisation of DOPC SLB agrees with data reported in literature. 49,[52][53][54] Curves are characterized by a minimum between 0.15 and 0.20 Å -1 related to the thickness of the bilayer, due to interference arising between the reflected waves at the top and the bottom of the SLB. The defects in the SLB can be evaluated from the amount of hydration water in the hydrophobic tails region of the SLB.…”

“…This asymmetry of the profiles can be due to the different distribution of hydration water and the thermal motions that affect the thickness of the head regions at the interfaces. [53][54][55] Moreover, the two lipid leaflets, the inner (L2-3) and the outer (L4-5) experienced different environments, as the inner leaflet was in direct contact with the SiO 2 layer and water confined between the solid surface and the headgroup layer is expected (L1). 41 The pre-formed SLB was then treated with the NPs dispersed in different environments.…”

Energetics of stalk intermediates in membrane fusion are controlled by lipid composition

“…For example, a locally varying lipid composition and, in case of cholesterol, redistribution between cis and trans monolayers by lipid flip-flop (62) can not be ruled out. A further issue is the interdependence of lipid headgroup hydration and curvature: At strong dehydration, water molecules are extracted from the first hydration shell directly associated to the lipid headgroups (63). This dehydration could change their effective molecular shape and thus κ,κ G , and c 0 .…”

Hydration of POPC bilayers studied by 1H-PFG-MAS-NOESY and neutron diffraction

“…Sample thickness was reduced and the proper corrections applied to eliminate an influence from extinction on data analysis (Bacon and Lowde 1948); (Worcester et al, unpublished). The magnitude of structure factors decreases somewhat with increasing hydration due to increases in bilayer disorder (Hristova 1998;Wiener and White 1991a). …”

“…5 are the Gaussian profiles reconstructed with the parameters found from the fit. The number of waters per lipid corresponding to the four humidity conditions, as reported in Table 1, were determined previously (Hristova 1998;White et al 1987). The uncertainty associated with water layer thickness was calculated based on the standard deviations of the experimental structure factors, using a Monte-Carlo sampling procedure as described by Wiener and White (Wiener and White 1992).…”