David L. Worcester

Interdigitated hydrocarbon chains in C20 and C22 phosphatidylcholines induced by hydrostatic pressure

Abstract Saturated diacyl-phosphatidylcholines (PCs) can exist in an unusual gel phase in which the acyl chains from opposing monolayers interdigitate. This pahse is induced by a variety of small amphiophillic molecules such as ethanol and by hydrostatic pressure. Pressure-temperature (P-T) phase diagrams previously established conditions for interdigitation of dipalmitoyl-PC (C16) and distearoyl-PC (C18). Measurements have now been made for P-T phase diagrams up to 1 kbar pressure for diarachidoyl-PC (C20) and dibehenoyl-PC (C22) using the small-angle neutron scattering facilities at NIST. The minimum pressure for interdigitation was found to be about 0.6 kbar for diarachidoyl-PC and slightly less for dibehenoyl-PC. Together with previous results for distearoyl-PC and dipalmitoyl-PC, these results establish that the minimum pressures for interdigitation, which occur at about 60°C except for dibehenoyl-PC, decrease monotonically with acyl chain length. The data are fit reasonably well by an exponential. Also, the curved phase boundaries for interdigitation show that entropy changes associated with interdigitation change sign with increasing temperature at about 60°C.

Solutions to the one-dimensional inverse scattering problem with missing phase information

Abstract The conversion of intensity data to structural models is made difficult by the uniqueness problem when phase information is lacking. Several different structures can give the same scattering intensities. For the one-dimensional problem in which the scattered amplitude is given by a finite Fourier transform, specification of all possible structures can be made using the analytic properties of the intensity function, since it is uniquely determined by the values of its complex zeros. The concepts of zero encoding are reviewed to show how reflectivity data in the first Born approximation can be converted into all possible structural models that give the measured intensities.

Structure, red-shifted absorption and electron transport properties of specific aggregates of chlorophylls

Abstract Neutron small-angle scattering from chlorophyll aggregates formed by hydration in octane/toluene mixtures show that specific structures can form which are long hollow cylinders. The cylinder diameters, absorption spectra and electron transport properties depend on the chlorophyll type. Results for 2-acetyl chlorophyll-a (a synthetic derivative of chlorophyll-a) and chlorophyll a/b mixtures are presented.

Neutron inverse optics in layered materials

Neutron specular reflectivity data obtained with a new grazing angle neutron spectrometer (GANS) from a NiC/Ti-multilayer sample were analyzed and modeled for reconstructing the scattering length density profile as a periodic step potential for the layered material. There is some ambiguity in the results due to the uniqueness problem with missing phase information. For more complex layered materials, there is often insufficient knowledge about the layers to use modeling reconstruction without phase information. In the second part, we present a method in which this problem is solved for diffraction data from lipid multilayers: due to changes in chemistry (isomorphous heavy atom method) the phases are determined directly and therefore the density profile of the lipid bilayer can be uniquely determined.

Phase determination using transform and contrast-variation methods in neutron diffraction studies of biological lipids

Biological lipid bilayer structures have been studied which give neutron diffraction data to better than 0.3 nm resolution. The general problem of phase retrieval in these one- dimensional, centrosymmetric cases was investigated using transform method together with deuterium incorporation (contrast variation) in order to establish experimental procedures of general use for such neutron scattering studies. The findings demonstrate procedures which, in many cases, quickly provide the experimental data necessary for reliable phase retrieval.

Intercalation of Organic Compounds in Lipid Bilayers

Using neutron diffraction we studied the incorporation of small hydrophobic compounds into bilayers consisting of phosphatidylcholine (PC) and cholesterol. They were found to be localized in a narrow band at the center of the hydrocarbon region, between the two halves of the bilayer. The structures formed by introduction of the compounds are therefore intercalated structures with the long axis of the intercalated molecules lying in the plane of the bilayer. We worked with several bilayers which differed by the length of the hydrocarbon chain of the PC. The quality of the localization depended on the presence of cholesterol, the water content and the PC chain length.

Neutron Guinier camera

The feasibility of Guinier cameras for small angle neutron scattering (SANS) is analyzed theoretically and experimentally. Small angle X-ray scattering (SAXS) is commonly measured with Guinier cameras1 that use bent perfect crystals to focus to detector beams from point sources of characteristic X-rays. Neutron Guinier cameras do not exist yet, although focusing to detector has occasionally been tried. The philosophy of current SANS pinhole instruments is to gain intensity from broad wavelength bands at tight collimation. With characteristic X-rays, intensity gains can only come from broad angular divergences. Neutron focusing instruments represent a return, at a higher level, to the philosophy of characteristic X-rays. Such a return is advocated in this paper for SANS. The resolution of Guinier cameras is defined not by the collimation (which is relaxed), but by the beam size at focus and the spatial resolution of the position sensitive detector (which should match each other). Within the recent concept of neutron imaging2 multi-wafer monochromators can provide image sizes comparable to the thickness of one wafer in the bent packet. The imaging may be non-dispersive, at broad wavelength bands, like with mirrors in conventional optics. These are the right ingredients for convergent neutron beams in Guinier cameras. The paper addresses the question whether the increased angular divergence can compensate for the reduced size of the source that is imaged into a sharp spot at detector. A neutron Guinier camera at thermal neutron energies is evaluated. It turns to be quite feasible, providing moderate resolution at high intensity with detection systems in current use for high-resolution neutron diffraction. High-resolution SANS is also possible with detection by image plates or microchannel plate systems. Tests were performed using a single wafer and a packet of bent silicon wafers in both Bragg and Laue (transmission) geometry in non-dispersive imaging arrangements. Experiments have confirmed expectations. SANS data obtained in neutron Guinier camera conditions on samples of collagen and lipids are presented.

Wavelet Inverse Neutron Scattering Study of Layered Metallic NiC-Ti Composites

Composites are prevalent in high technology devices such as aircraft, computers, automobiles and communications systems. They improve brittleness and provide a lower density which enhances mechanical strength. Electron and light manipulating composites will be used more and more in the future. It is necessary to have a capability of inspecting composites, both to assure production quality and as a baseline for later NDE. In this paper, we present a study using wavelet, inverse neutron optics and the grazing angle neutron spectrometer, GANS, at the Missouri University Research Reactor, MURR.

Intercalation of Small Hydrophobic Molecules in Lipid Bilayers Containing Cholesterol

Partitioning of small hydrophobic molecules into lipid bilayers containing cholesterol has been studied using the 2XC diffractometer at the University of Missouri Research Reactor. Locations of the compounds were determined by Fourier difference methods with data from both deuterated and undeuterated compounds introduced into the bilayers from the vapor phase. Data fitting procedures were developed for determining how well the compounds were localized. The compounds were found to be localized in a narrow region at the center of the hydrophobic layer, between the two halves of the bilayer. The structures are therefore intercalated structures with the long axis of the molecules in the plane of the bilayer.