SEARCH · PubMed Health
Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Neutron diffraction studies show that the inositol ring in the headgroup of phosphatidylinositol extends perpendicular to the membrane surface but that phosphorylation of the 4-position causes the ring to tilt over.
The structure of the digalactosyldiacylglycerol bilayer is calculated using neutron diffraction data. The polar head group of this lipid is oriented parallel to the plane of the bilayer such that the galactose moieties are tightly packed at the bilayer surface into a 0.8 nm thick polar layer. The thickness of this layer is independent of water activity over a wide range (15-100% relative humidity). The constant thickness of both the galactose layer and the hydrocarbon layer constrain the structure factor amplitudes to lie on a single continuous transform for repeat periods between 4 and 5 nm.
By using neutron diffraction, the headgroup conformation of purified phosphatidylglycerol from Escherichia coli membranes has been investigated. Measurements at 25 degrees C and 15% relative humidity on oriented multilayers of lipid selectively deuterated at the sn-3-position of the glycerol backbone and of the gamma-position of the glycerol headgroup show that the labels are at a mean distance of 23.0 A and 27.6 A from the centre of the hydrocarbon chain region. This suggests that the negatively charged headgroup is oriented at about 30 degrees to the membrane surface. The orientation of the phosphatidylglycerol headgroup makes the negatively charged phosphate group easily accessible to cations present in the adjacent water layer.
Purple membranes were prepared by growing Halobacterium halobium in a medium containing nicotine (which inhibits biosynthesis of retinal) and the oxidation products of fully deuterated beta-carotene. This allowed the in vivo incorporation of deuterated retinal into the membranes. The labelled membranes were crystalline and isomorphous with native membrane as determined by X-ray diffraction, and their optical absorption spectra were very similar. Neutron diffraction data for the two dimensional in-plane lattice from labelled and native membranes were analysed by difference Fourier and direct methods to 8.6 A resolution. The difference Fourier shows the retinal to be located in the centre of the bacteriorhodopsin molecule. The best fit to the data was obtained with the projection of retinal as a 10 A long rod forming an angle of -40 degrees +/- 10 degrees with the x axis centred at x = -0.19 +/- 0.02, y = -0.35 +/- 0.02 in fractional unit cell coordinates. The main peak in the difference Fourier map is at x = -0.17, y = -0.33.
The localization of the RNA in a spherical plant virus is established by neutron diffraction. For BMV, this localization is different at low pH than it is at high pH (swollen virus).
Small-angle neutron scattering was used to determine the intralamellar location of (-)-delta 8-tetrahydrocannabinol (delta 8-THC) in hydrated dipalmitoylphosphatidylcholine (DPPC) bilayers. Nuclear scattering density profiles were calculated from measurements on deuterium and non-deuterium-labelled inclusions (8.3% (w/w)) of delta 8-THC in DPPC multilayer samples. By comparing pairs of such nuclear density profiles, the locations of the deuterium labels were determined. Present results on the topography of delta 8-THC in membranes are compared with earlier X-ray measurements using iodine labelling. Whereas the position of the phenolic hydroxy group is similar in both types of measurement, a difference is found in the conformation of the terminal methyl groups of the cannabinoid side-chains. The X-ray measurements on dimyristoylphosphatidylcholine (DMPC) indicated that the iodine-labelled cannabinoid side-chains assume an all-trans orientation with the terminal iodine atom pointing inward into the membrane away from the tricyclic region while the neutron measurements indicate that the terminal CH3 group of delta 8-THC aligns itself at the level of the tricyclic ring system implying that the side chain exists in a more compact conformation perpendicular to the DPPC hydrocarbons. A Gaussian function analysis of the data indicates that the delta 8-THC molecule is significantly delocalized in the DPPC membrane in the liquid crystal phase. The mean location of delta 8-THC suggests that the active site on a membrane-embedded receptor protein will lie near the polar interface at the base of the phospholipid headgroups.
A neutron diffraction study of aqua(N-salicylideneglycinato)copper(II) hemihydrate, [Cu(C9H7NO3)-(H2O)].0.5H2O, Mr = 267.7, a model for vitamin B6-amino-acid-related metal complexes, has been carried out at 130 K. Least-squares refinement based on 3800 reflections [(sin theta/lambda)max = 0.786 A-1] converged at R(F2) = 4.5%. The crystal is monoclinic, space group C2/c, Z = 8 with cell parameters at 130 K: a = 17.030 (3), b = 6.665 (1), c = 17.469 (5) A, beta = 111.13 (1) degree, V = 1849.50 A3. The bond distances between non-H atoms have been determined with estimated standard deviations in the range 0.0008-0.0012 A; the corresponding values for distances involving H atoms are 0.0017-0.0020 A. The presence of a Cu11 ion does not significantly change the bond distances around the alpha-C atom from those observed in glycine or glycylglycine. There are, however, indications of differences in distance and direction relative to the pi system for the two alpha-C-H bonds. These bond vectors are inclined at angles of 30 and 71 degrees to the plane of the conjugated pi system. A planar conjugated pi-system is stabilized by chelation to the Cu ion.
The microscopic structures of calcium-ammonia solutions have been established by using neutron diffraction. Total structure factors measured at 230 K reveal immediately the evolution of an uncommonly intense diffraction prepeak in the metallic solutions. As concentration is increased from 4 mole percent metal to 10 mole percent metal (i.e., saturation), this feature intensifies and shifts from 0.6 to 0.9 A(-1). It is therefore evidence of well developed intermediate-range ordering among the solvated cations, and is a microstructural signature of the observed strong phase separation of metallic (concentrated) and nonmetallic (dilute) solutions. The technique of isotopic labelling of *N by 15N was then used in conjunction with difference analysis to focus on the solvent structure in metallic solutions at 4 and 10 mole percent metal. These nitrogen-centered functions are analyzed in conjunction with classical Monte Carlo computer simulation techniques, to provide us with detailed insight into the calcium solvation and the extent of hydrogen bonding. We find that calcium is solvated by approximately 6-7 ammonia molecules, with a Ca-N distance of around 2.45 A. There is evidence of hydrogen bonding among the solvent molecules, even in the saturated 10 mole percent metal solution.
For confirmation of some general aspects of phospholipid conformation in membranes and extension of previous neutron diffraction studies on dipalmitoyllecithin, measurements have now been made on 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) in the gel phase by the same method. Three selectively deuterated samples were investigated; in one of the specimens the first methylene segment close to the glycerol backbone in both chains was deuterated, and in the other two samples one of the methylene segments in the phosphoethanolamine group was replaced by CD2. Together with the undeuterated DPPE, these probes were investigated at very low water content (about 1.5--2 molecules of water per lipid) as oriented samples at 25 degrees C. The intensities of the first 12 reflections were collected and phased, and the mean positions of the segments were determined. The results confirm the idea that the conformation of a DPPE molecule in the gel state is very similar to the crystal structure of rac-1,2-dilauroyl-sn-glycero-3-phosphoethanolamine. The two main features are (1) the chains remain in all all-trans conformation having an axial displacement of about 3--4 A, (2) the zwitterionic dipoles in the head groups of both compounds are found to be aligned almost parallel to the bilayer surface. The main advantage of the method results in the fact that the combination of neutron scattering with selectively deuterated probes allows the determination of the mean label position to an accuracy of up to +/- 1 A.
The gas phase of hydrogen fluoride has been investigated by neutron diffraction experiments at three different particle densities. All investigated states are within the liquid-gas coexistence region of hydrogen fluoride. From the obtained diffraction data we deduced information about the local structure of the gas phase, which consists of small agglomerates. This has been expected as liquid hydrogen fluoride forms the strongest hydrogen bonds known. Molecular dynamics simulations with a modified potential have been carried out for all experimentally investigated states. The results confirmed that the size of the formed agglomerates in the gas phase is growing with increasing density of the gas phase.
The distribution of calcium in lamellar phases of dipalmitoyllecthin (DPPC) multilayers was directly determined by neutron diffraction and stable isotope substitution of 44Ca for 40Ca. A significant resonance effect on the intensities of the lamellar diffraction pattern was observed for millimolar concentrations of these calcium isotopes. The calcium difference profile indicated that calcium was localized in the phospholipid headgroup region, being excluded from the hydrocarbon core as was water separately determined from the water profile structure obtained by H2O/D2O exchange. A reciprocal space analysis of the difference structure factors indicated that calcium binds preferentially to within 1-2 A of the phosphate moiety of the phospholipid head groups of the DPPC bilayer.
A rubidium titanyl arsenate single-crystal has been studied by neutron diffraction (lambda = 1.207 A). The polished sample used was 5 x 3 x 2 mm and was cut from a crystal made by top-seeded solution growth. The crystal showed severe extinction. It was, however, possible to obtain a structural model with well defined oxygen sites and reasonable anisotropic displacement parameters.
Pentacoordinate hydrogen atoms were identified by single-crystal neutron diffraction analysis of [N(CH3)4]3[H2Rh13(CO)24]. The hydrogen atoms are located in square pyramidal cavities of the Rh13 cluster, in positions almost coplanar with the Rh4 faces on the surface of the cluster. They are slightly displaced inward, toward the central rhodium atom of the cluster, with average H-Rh(central) and H-Rh(surface) distances of 1.84(2) and 1.97(2) angstroms, respectively. This result shows that hydrogen, which normally forms only one bond, can be attached to five other atoms simultaneously in a large metal cluster.
The D2 clathrate hydrate crystal structure was determined as a function of temperature and pressure by neutron diffraction for the first time. The hydrogen occupancy in the (32+X)H2.136H(2)O, x=0-16 clathrate can be reversibly varied by changing the large (hexakaidecahedral) cage occupancy between two and four molecules, while remaining single occupancy of the small (dodecahedral) cage. Above 130-160 K, the guest D2 molecules were found in the delocalized state, rotating around the centers of the cages. Decrease of temperature results in rotation freezing followed by a complete localization below 50 K.
A boron-rich sodium boride, formerly known as NaB15, has been subjected to a comprehensive structural reinvestigation using X-ray single-crystal and powder diffraction, low-temperature neutron and electron diffraction, high-resolution transmission electron microscopy, and 23Na solid-state NMR spectroscopy. The results indicate that the previously published orthorhombic space group is incorrect. Consistent with all of the experimental results a modified structural description is developed in the monoclinic space group Ilml (a = 585.92(3), b= 1039.92(6), c = 833.17(5) pin, beta = 90.373(5) from powder data). Because one of the interstitial boron atom positions remains unoccupied, the accurate compositional formula is NaB145 or Na2B29.
Accurate structure factors up to sin theta/lambda = 1.6 A(-1) have been measured with 316.5 keV gamma-rays from CoF(2), both at room temperature and in the antiferromagnetic state at 10 K. The same crystal was used to collect extended time-of-flight neutron diffraction data in the two magnetic states, which allowed an accurate determination of the fluorine positional parameter. For room temperature, the standard structural parameters are reported. At 10 K, a complete charge-density study has been carried out. The total number of 3d electrons on Co is found to be 6.95 (3). The experimental populations of the d orbitals agree with expectation from crystal field theory. The fluorine valence region exhibits a strong dipolar deformation. Electronic properties at the bond critical points and integrated atomic properties are derived from the static model electron density, revealing the Co-F interactions as purely ionic. On magnetic ordering, a shift of the fluorine ions of 1.5 (4) x 10(-3) A is found which confirms a prediction from theory of optical birefringence. The effect of magnetostriction on the distortion of the ligand coordination octahedra is compared for the late members of the 3d transition-metal difluorides. From neutron powder diffraction, an ordered magnetic moment of 2.60 (4) mu(B) per cobalt ion is found. Despite the strong deviation from the ideal spin value of 3 mu(B), there is still an appreciable orbital contribution to the local magnetic moment.
The structure of crystalline purple membrane reconstituted from purified bacteriorhodopsin (BR) chymotryptic fragments has been studied by neutron diffraction. In one of the samples studied, the fragment C-2, encompassing the first two predicted transmembrane segments, was prepared from deuterated purple membrane. The diffraction changes when the natural C-2 fragment is substituted by a deuterated one are analysed in terms of a seven-helix model for BR. The assignment of the labelled fragment to one end of the molecule placed new constraints on folding models for the protein.
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.