X-ray study on egg-yolk lecithin: Unit cell data and electron density profile

171

138

This is the first in a series of papers concerned with methods for the determination of the structures of fluid phospholipid bilayers in the liquid-crystalline (L alpha) phase. The basic approach is the joint refinement of quasimolecular models (King and White, 1986. Biophys. J. 49:1047-1054) using x-ray and neutron diffraction data. We present here (a) the rationale for quasimolecular models, (b) the nature of the resolution problem for thermally disordered bilayers, and (c) an analysis of the resolution of experiments in which Gaussian functions are used to describe the distribution of submolecular components. We show that multilamellar liquid-crystalline bilayers are best described by the convolution of a perfect lattice function with a thermally disordered bilayer unit cell. Lamellar diffraction measurements on such a system generally yield only 5-10 orders of diffraction data from which transbilayer profiles of the unit cell can be constructed. The canonical resolution of these transbilayer profiles, defined as the Bragg spacing divided by the index of the highest recorded diffraction order, is typically 5-10 A. Using simple model calculations, we show that the canonical resolution is a measure of the widths of the distributions of constituents of the unit cell rather than a measure of the spatial separation of the distributions. The widths provide a measure of the thermal motion of the bilayer constituents which can be described by Gaussian functions. The equilibrium positions of the centers of the distributions can be determined with a precision of 0.1-0.5 A based upon typical experimental errors.

Section: Structural Disorder and Resolutionmentioning

confidence: 80%

Fluid bilayer structure determination by the combined use of x-ray and neutron diffraction. I. Fluid bilayer models and the limits of resolution

Wiener¹,

White²

1991

171

138

“…The result is a range of 0.2 < K' < 0.5. At the other extreme of possible K' values, Sakurai et al (1977) obtained hm. = 25 orders from oriented egg phosphatidylcholine.…”

Section: (16)mentioning

confidence: 99%

“…In previous applications of atomic-level crystal structures or models, the atomic positions and Debye-Waller factors were adjusted to obtain the best fit to experimental data for crystalline (Hitchcock et al, 1975;Sakurai et al, 1977;Dorset, 1987) or liquid-crystalline (Franks, 1976) bilayers. We use phospholipid crystal data differently in this calculation.…”

Section: (16)mentioning

confidence: 99%

Fluid bilayer structure determination by the combined use of x-ray and neutron diffraction. II. "Composition-space" refinement method

Wiener

White

1991

126

This is the second of two papers describing a method for the joint refinement of the structure of fluid bilayers using x-ray and neutron diffraction data. We showed in the first paper (Wiener, M. C., and S. H. White. 1990. Biophys. J. 59:162-173) that fluid bilayers generally consist of a nearly perfect lattice of thermally disordered unit cells and that the canonical resolution d/hmax is a measure of the widths of quasimolecular components represented by simple Gaussian functions. The thermal disorder makes possible a "composition space" representation in which the quasimolecular Gaussian distributions describe the number or probability of occupancy per unit length across the width of the bilayer of each component. This representation permits the joint refinement of neutron and x-ray lamellar diffraction data by means of a single quasimolecular structure that is fit simultaneously to both diffraction data sets. Scaling of each component by the appropriate neutron or x-ray scattering length maps the composition space profile to the appropriate scattering length space for comparison to experimental data. Other extensive properties, such as mass, can also be obtained by an appropriate scaling of the refined composition space structure. Based upon simple bilayer models involving crystal and liquid crystal structural information, we estimate that a fluid bilayer with hmax observed diffraction orders will be accurately represented by a structure with approximately hmax quasimolecular components. Strategies for assignment of quasimolecular components are demonstrated through detailed parsing of a phospholipid molecule based upon the one-dimensional projection of the crystal structure of dimyristoylphosphatidylcholine. Finally, we discuss in detail the number of experimental variables required for the composition space joint refinement. We find fluid bilayer structures to be marginally determined by the experimental data. The analysis of errors, which takes on particular importance under these circumstances, is also discussed.

“…The characteristic spatial extent of thermally disordered atoms or small clusters of atoms is approximately d/h max , where d measures the unit cell size (Wiener and White, 1991a). For example, atoms smeared over a space of ϳ2 Å in a bilayer unit cell with d ϭ 50 Å will produce ϳ25 orders of diffraction (Sakurai et al, 1977;Suwalsky and Duk, 1987). Such a unit cell is intrinsically a high-resolution structure.…”

Section: Introductionmentioning

confidence: 99%

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

Hristova

White

1998

159

196

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.