Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Neutron Diffraction”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3Linked to original sources
PubMed2002

Anomalous spin-density distribution on oxygen and Ru in Ca(1.5)Sr(0.5)RuO(4): polarized neutron diffraction study.

By means of polarized neutron diffraction in a magnetic field of 7.0 T at 1.6 K an anomalously large magnetization density is observed on the in-plane oxygen in Ca(1.5)Sr(0.5)RuO(4). Field-induced moments of different ions are determined by refinement on the flipping ratios, yielding micro(Ru)=0.346(11)micro(B), micro(O1)=0.076(6)micro(B), and micro(O2)=0.009(6)micro(B). The moment on the oxygen arises from the strong hybridization between the Ru-4d and O-2p orbitals. The maximum entropy magnetization density reconstruction reveals a strongly anisotropic density at the Ru site, consistent with the distribution of the xy (t(2g) band) d orbitals.

Journal Article↗
PubMed1983

Neutron diffraction of chromatin in interphase nuclei and metaphase chromosomes.

We have used neutron diffraction to study chromatin structure in interphase nuclei and metaphase chromosomes as a function of decreasing ion concentration. Aliquots of a suspension of rat liver nuclei prepared in a polyamine-free buffer were washed in buffers of 1/3, 1/6 and 1/12 if the original concentration of monovalent and divalent cations (40 mM KCl; 20 mM NaCl; 1.2 mM MgCl2). After the first dilution step (1/1 to 1/3), only small changes occurred in the diffraction pattern. They can be interpreted by a loosening of the original structure, i.e. by the formation of isolated buffer-filled spaces with an overall size of the order of 35-45 nm. Drastic changes in the diffraction pattern were observed, however, when the nuclei were washed in the more diluted buffers (1/6 and 1/12). The profiles of the distances distribution functions indicate the formation of supranucleosomal particles with an overall diameter of 40-50 nm. The compact chromatin structure disassembled directly into these fundamental structural units. Structural transformations in the Chinese hamster ovary metaphase chromosomes were induced by diminishing the Ca2+ ion concentration of the buffer from originally 3.0 mM to 0.3 mM and/or by increasing the pH value of the buffer from originally 7.0 up to 8.0. The neutron diffraction patterns remained essentially unchanged during these treatments, i.e. the decondensation of the chromosomes as observed in the light microscope is not accompanied by disassembly at the ultrastructural level between 2 nm and 150 nm.

Animals↗
PubMed1983

Absolute configuration of a chiral CHD group via neutron diffraction: confirmation of the absolute stereochemistry of the enzymatic formation of malic acid.

Neutron diffraction has been used to monitor the absolute stereochemistry of an enzymatic reaction. (-)(2S)malic-3-d acid was prepared by the action of fumarase on fumaric acid in D2O. After a large number of cations were screened, it was found that (+)(R) alpha-phenylethylamine forms the large crystals necessary for a neutron diffraction analysis. The subsequent structure determination showed that (+)(R) alpha-phenylethylammonium (-)(2S)malate-3-d has an absolute configuration of R at the CHD site (i.e., the C3 carbon of malate). This result confirms the absolute stereochemistry of fumarate-to-malate transformation as catalyzed by the enzyme fumarase.

Malates↗
PubMed2003

Recent results from the in situ study of hydrothermal crystallisations using time-resolved X-ray and neutron diffraction methods.

We present new time-resolved powder diffraction data measured in situ during the hydrothermal crystallisation of two families of crystalline inorganic materials. In the first study, we have used time-resolved energy-dispersive X-ray diffraction (EDXRD) to follow the formation of zeolitic zinc phosphates from amine phosphates and zinc oxide in acidic solutions at 60-150 degrees C. The advantage of this method is the ability to penetrate a laboratory-sized reaction vessel and to measure data in short (< 1 min) time intervals. Integration of the Bragg peak intensities during the crystallisation of the product allows accurate crystallisation curves to be produced. In addition, in a number of cases, we observe the formation of transient crystalline intermediate phases which can be identified by use of a new three-element detector that allows a large amount of diffraction data to be measured during the experiment. We are thus able to show that three-dimensional zinc phosphate architectures often form via low-dimensional chain and layered phases, which is consistent with a recent aufbau model proposed for their formation. In the second study, we focus on the hydrothermal formation of ferroelectric barium titanate from TiO2 and barium salts in alkaline solution using time-resolved neutron diffraction. Although the time resolution of the neutron diffraction experiment is lower than the EDXRD experiment (data are measured in intervals of 5 min), we are able to penetrate reaction mixtures that are highly absorbing towards X-rays, and thus can measure data in a large volume reaction cell. Neutron diffraction data were collected on one of the highest-flux/highest detector-coverage diffractometers currently available; the GEM diffractometer at ISIS, UK. These experiments reveal that BaTiO3 crystallises after a large amount of TiO2 has been consumed; this implies that a dissolution crystallisation mechanism predominates. Additional mechanistic information is inferred by the observation of transient crystalline phases under certain reaction conditions.

Journal Article↗
PubMed1979

Stress distribution in the scapula studied by neutron diffraction.

High intensity neutron beams provide a method of measuring the preferred orientation of apatite crystals in bulk samples of bone. Measurements at seven different sites on the scapula show that the c axes of the crystals lie preferentially along the directions of pull of the attached muscles. The highest orientation is found at positions under the influence of only a single group of muscles, such as M. teres major or M. infraspinatus. In intermediate regions a multiple distribution of crystals is found, able to withstand stresses in more than one direction. The technique provides a method of assessing the distribution of stress in bones.

Animals↗
PubMed2000

Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction. II. 3-1 photoisomerization of a cobal-oxime complex

Single crystal neutron diffraction analysis of photo-exposed (3-cyanopropyl-d2(alpha,alpha))-[(R)-1-phenylethylamine-d11]bis(dimethylglyoximato-d14)cobalt(III) was carried out in order to clarify the mechanism of the crystalline-state photoisomerization of the 3-cyanopropyl group bonded to the Co atom in some cobaloxime complexes. Before irradiation the two H atoms bonded to the C1 atom of the 3-cyanopropyl group were exchanged with the D atoms such as --CH2CH2CD2CN. On exposure to a xenon lamp, the cell dimensions of the crystal were gradually changed. After 7 d exposure the change became insignificantly small. The structure was analyzed by neutron diffraction. The 3-cyanopropyl group was transformed to the 1-cyanopropyl group such as --CD(CN)C(H1/2,D1/2)2CH3 with retention of the single-crystal form. This indicates that one of the D atoms bonded to C1 migrates to either position bonded to C2. The other atoms of the complex remained unchanged. These results indicate that photoisomerization proceeded in two steps: the 3-cyanopropyl group was isomerized to the 2-cyanopropyl group in the first place and then the 2-cyanopropyl group was transformed to the 1-cyanopropyl group. Moreover, it was made clear that the second-step isomerization was irreversible, since one of the D atoms was retained. The disordered structure at C2 is estimated to be caused by the interconversion between the 1-cyanopropyl group produced and its dehydrogenated olefin after the photoisomerization.

Journal Article↗
PubMed2001

Long-range magnetic order in Mn[N(CN)2]2(pyz) (pyz = pyrazine). Susceptibility, magnetization, specific heat, and neutron diffraction measurements and electronic structure calculations.

Using dc magnetization, ac susceptibility, specific heat, and neutron diffraction, we have studied the magnetic properties of Mn[N(CN)2]2(pyz) (pyz = pyrazine) in detail. The material crystallizes in the monoclinic space group P2(1)/n with a = 7.3248(2), b = 16.7369(4), and c = 8.7905 (2) A, beta = 89.596 (2) degrees, V = 1077.65(7) A(3), and Z = 4, as determined by Rietveld refinement of neutron powder diffraction data at 1.35 K. The 5 K neutron powder diffraction data reflect very little variation in the crystal structure. Interpenetrating ReO3-like networks are formed from axially elongated Mn(2+) octahedra and edges made up of mu-bonded [N(CN)2](-) anions and neutral pyz ligands. A three-dimensional antiferromagnetic ordering occurs below T(N) = 2.53(2) K. The magnetic unit cell is double the nuclear one along the a- and c-axes, giving the (1/2, 0, 1/2) superstructure. The crystallographic and antiferromagnetic structures are commensurate and consist of collinear Mn(2+) moments, each with a magnitude of 4.15(6) mu(B) aligned parallel to the a-direction (Mn-pyz-Mn chains). Electronic structure calculations indicate that the exchange interaction is much stronger along the Mn-pyz-Mn chain axis than along the Mn-NCNCN-Mn axes by a factor of approximately 40, giving rise to a predominantly one-dimensional magnetic system. Thus, the variable-temperature magnetic susceptibility data are well described by a Heisenberg antiferromagnetic chain model, giving g = 2.01(1) and J/k(B) = -0.27(1) K. Owing to single-ion anisotropy of the Mn(2+) ion, field-induced phenomena ascribed to spin-flop and paramagnetic transitions are observed at 0.43 and 2.83 T, respectively.

Journal Article↗
PubMed1986

Determining bilayer hydrocarbon thickness from neutron diffraction measurements using strip-function models.

Neutron diffraction methods provide information about the distribution of matter in biological and model membrane systems. The information is derived from plots (profiles) of scattering length density along an axis normal to the membrane plane. Without the use of specific deuteration, the generally low resolution of the profiles limits their interpretation in terms of specific chemical constituents (e.g., lipid headgroup, lipid hydrocarbon, protein, and water). A fundamental and useful structural assignment to make is the boundary between the headgroup and hydrocarbon regions of bilayers. We demonstrate here that strip-function model representations of neutron scattering length density profiles of bilayers are sufficient to determine accurately the position of the headgroup-hydrocarbon boundary. The resulting hydrocarbon thickness of the bilayer is useful for determining the area per lipid molecule and consequently the molecular packing arrangements of the membrane constituents. We analyze data obtained from dioleoylphosphatidylcholine (DOPC) bilayers at 66% RH using standard Fourier profile analyses and from DOPC deuterated specifically at the C-2 carbon of the acyl chains using difference Fourier analysis. We demonstrate that strip-function models accurately define the positions of the C-2 carbons and thus the hydrocarbon thickness (dhc) of the bilayer. We then show, using quasi-molecular models, that the strip-model analysis probably provides an accurate measure of dhc because of the exceptionally high scattering length density difference between the carbonyl and methylene groups.

Deuterium↗
PubMed1976

Neutron diffraction studies of biological membranes and membrane components.

In this paper the methods and techniques of neutron diffraction studies of biological membranes and membrane components have been described and demonstrated. Foremost among these are the methods of deuterium exchange and deuterium labeling, since they can directly provide locations of membrane components. In addition, the methods of contrast variation with H2O-D2O mixtures, swelling, direct scaling of data from deuterium-labeled structures, and magnetic orientation have provided useful information in neutron diffraction analyses of membrane structures. Although this field is only in its infancy, it clearly has very encouraging prospects.

Cell Membrane↗
PubMed1989

Tertiary structure of bacteriorhodopsin. Positions and orientations of helices A and B in the structural map determined by neutron diffraction.

Positions and rotations of two helices in the tertiary structure of bacteriorhodopsin have been studied by neutron diffraction using reconstituted, hybrid purple membrane samples. Purple membrane was biosynthetically 2H-labeled at non-exchangeable hydrogen positions of leucine and tryptophan residues. Two chymotryptic fragments were purified, encompassing either the first two or the last five of the seven putative transmembrane segments identified in the amino acid sequence of bacteriorhodopsin. The 2H-labeled fragments, diluted to variable extents with the identical, unlabeled fragment, were mixed with their unlabeled counterpart; bacteriorhodopsin was then renatured and reconstituted. The crystalline purple membrane samples thus obtained contained hybrid bacteriorhodopsin molecules in which certain transmembrane segments had been selectively 2H-labeled to various degrees. Neutron diffraction powder patterns were recorded and analyzed both by calculating difference Fourier maps and by model building. The two analyses yielded consistent results. The first and second transmembrane segments in the sequence correspond to helices 1 and 7 of the three-dimensional structure, respectively. Rotational orientations of these two helices were identified using best fits to the observed diffraction intensities. The data also put restrictions on the position of the third transmembrane segment. These observations are discussed in the context of folding models for bacteriorhodopsin, the environment of the retinal Schiff base, and site-directed mutagenesis experiments.

Amino Acid Sequence↗
PubMed1985

Neutron diffraction studies of collagen in fully mineralized bone.

Neutron diffraction measurements have been made of the equatorial and meridional spacings of collagen in fully mineralized mature bovine bone and demineralized bone collagen, in both wet and dry conditions. The collagen equatorial spacing in wet mineralized bovine bone is 1.24 nm, substantially lower than the 1.53 nm value observed in wet demineralized bovine bone collagen. Corresponding spacings for dry bone and demineralized bone collagen are 1.16 nm and 1.12 nm, respectively. The collagen meridional long spacing in mineralized bovine bone is 63.6 nm wet and 63.4 nm dry. These data indicate that collagen in fully mineralized bovine bone is considerably more closely packed than had been assumed previously, with a packing density similar to that of the relatively crystalline collagens such as wet rat tail tendon. The data also suggest that less space is available for mineral within the collagen fibrils in bovine bone than had previously been assumed, and that the major portion of the mineral in this bone must be located outside the fibrils.

Achilles Tendon↗
PubMed1976

Neutron diffraction studies of retinal rod outer segment membranes.

Neutron diffraction measurements on isolated retinal rod outer segments show that most of the visual pigment protein, rhodopsin, is embedded in the hydrophobic core of the disk membrane. A very slight outward shift of protein at the cytoplasmic side of the membrane is associated with pigment bleaching.

Animals↗
PubMed2002

X-ray and neutron diffraction studies of the non-linear optical compounds MBANP and MBADNP at 20 K: charge-density and hydrogen-bonding analyses.

Neutron-diffraction studies of the compounds 5-nitro-2-[[1-phenylethyl]amino]pyridine (methylbenzylaminonitropyridine, hereafter MBANP) and 3,5-dinitro-2-[[1-phenylethyl]amino]pyridine (methylbenzylaminodinitropyridine, hereafter MBADNP) are presented, and a charge-density study of the latter is reported. The studies were conducted in order to relate the structural attributes of these materials to their physical properties. MBANP exhibits a second-harmonic generation (SHG) output, chi(2), over eight times higher than that of MBADNP, despite their very similar molecular characteristics and the seemingly more SHG-favourable molecular features present in MBADNP. The neutron-diffraction studies show that intramolecular hydrogen bonding is responsible for this apparent discrepancy. The charge-density study on MBADNP confirms this and reveals that the pyridine group is the principal moiety responsible for the SHG effect on the molecular scale. Moreover, the strong intramolecular hydrogen bond present in MBADNP is proven to result from an electrostatic interaction. The dipole moment of MBADNP is also deduced from the charge-density study.

Journal Article↗
PubMed1991

Structure and thermal vibrations of adenosine from neutron diffraction data at 123 K.

The crystal structure of adenosine, C10H13N5O4, Mr = 267.24, has been refined by full-matrix least-squares methods using single-crystal neutron diffraction data (sin theta/lambda less than 0.79 A-1) measured at 123 K. Crystal data at 123 K: monoclinic, P2l, Z = 2, a = 4.7885 (8), b = 10.240 (2), c = 11.772 (2) A, beta = 99.59 (2) degrees, V = 569.2 A3, lambda = 1.0402 (1) A, Dn = 1.559 Mg m-3, mu = 191.1 m-1, F(000) = 175.72 fm, R(F2) = 0.044, wR(F2) = 0.055, S = 1.13 for 2450 reflections with F2 greater than 0.0. Bond lengths and angles have e.s.d.'s less than 0.002 A and 0.1 degrees for non-H atoms and less than 0.004 A and 0.3 degrees for H atoms. Values are in general agreement with those previously obtained from room-temperature X-ray data and with those obtained for 9-methyladenine by neutron diffraction at 126 K. Small displacements of amino H atoms from the least-squares plane through the non-H atoms of the adenine part appear to be the result of hydrogen bonding. There is an intermolecular interaction C2--H2...O2' with an H...O distance of 2.20 A. In its thermal vibrations, the adenosine molecule behaves as two rigid segments with a torsional libration about the glycosidic bond of 14.4 deg2 at 123 K. The force constant for this libration, as determined from the diffraction data, is 73 (10) J mol-1 deg-2.

Adenosine↗
PubMed1986

Equatorial diffraction spacing as a function of water content in fully mineralized cow bone determined by neutron diffraction.

Variation of the equatorial diffraction spacing of soft type I collagen tissues with water content using X-rays has been known for many years. Recently, a generalized model for collagen molecule packing within fibrils was deduced from this information for different collagenous tissues. It is now known that the eq. diff. sp. of mineralized tissues can be less than for soft tissues and is inversely dependent on the wet density. A determination of the eq. dif. sp. dependence on water content using neutron diffraction of fully mineralized cow bone was undertaken for comparison. Specimens with various partial water content between 0 and 100% were tested. Data show collagen molecules pack more closely together as water content decreases, just as for soft tissues.

Animals↗
PubMed1988

Analysis of solvent structure and hydrogen exchange in proteins on the basis of neutron diffraction data from deuterated and hydrogenous crystals.

A method has been developed to determine the structure of bound solvent and the positions of exchanged hydrogens in proteins, on the basis of neutron diffraction from hydrogenous and deuterated crystals. In this method phases for the hydrogenous and for the deuterated model are refined simultaneously, and an average model is imposed in the volume occupied by non-hydrogen atoms. The densities in the areas of bulk solvent are replaced by their average values, while no modifications are performed in the vicinity of ordered solvents and potentially exchangeable hydrogens. The method was tested on 1.8 A neutron diffraction data collected from two crystals of bovine pancreatic trypsin inhibitor, one of them deuterated and the other hydrogenous. Significant improvement was observed for the densities corresponding to many partially occupied solvent sites, as well as to partially exchanged hydrogens. The algorithm presented here has been compared with a different approach published recently by Shpungin & Kossiakoff [Methods Enzymol.(1986), 127, 329-342].

Amides↗
PubMed2001

Influence of hydrogen bonding on the second harmonic generation effect: neutron diffraction study of 4-nitro-4'-methylbenzylidene aniline.

A neutron diffraction study of the non-linear optical (NLO) material 4-nitro-4'-methylbenzylidene aniline (NMBA) is presented. NMBA exhibits a large macroscopic second-order NLO susceptibility, chi((2)), and this study shows that hydrogen bonding is, in part, responsible for this. No hydrogen bonding was reported in the X-ray study [Ponomarev et al. (1977). Sov. Phys. Crystallogr. 22, 223-225], whereas the present work shows that C-H.X hydrogen bonds (where X = N, O or pi) direct the nature of the three-dimensional lattice. C-H.X (X = N or O) hydrogen bonds are common; however, C-H.pi hydrogen-bond motifs are relatively rare. Such intermolecular interactions help extend the molecular charge transfer into the supramolecular realm, the charge transfer originating as a consequence of the high level of molecular planarity and strong donor-to-acceptor interactions. Molecular planarity, coupled with the favourable nature of the hydrogen bonds, results in parallel stacking of molecules in both the a and c crystallographic directions with extremely close interplanar spacings. Such a combination of influential hydrogen-bonding characteristics accounts, in part, for the large second-order NLO output of the material since the phenomenon is so critically dependent upon the nature of the charge transfer.

Journal Article↗
PubMed1994

The in vitro binding of acetaldehyde to collagen studied by neutron diffraction.

The location of acetaldehyde binding sites in the axial unit cell of tendon collagen was investigated by neutron diffraction. Acetaldehyde forms spontaneous cross-links with specific residues in collagen. The use of deuterated acetaldehyde increased the neutron scattering length of these groups. The introduction of deuterated acetaldehyde at specific locations allowed the acetaldehyde-reacted collagen to be treated as multiple isomorphous derivatives for neutron fibre diffraction. The low resolution axially projected structure was determined using amplitudes of the first eight meridional reflections (d = 67 nm). Results indicate that the process of acetaldehyde labelling takes place at different rates at different sites within the collagen fibril. The position of acetaldehyde attachment correlates well with the position of lysine and hydroxylysine residues especially in the regions of the molecular termini. This information is relevant to the process of cirrhosis and fibrosis of the liver since adduction of collagen by acetaldehyde may interfere with normal Schiff base cross-link formation at the C- and N-termini. This may result in subsequent alterations in the intra- and inter-molecular cross-linking pattern of collagen molecules.

Acetaldehyde↗