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PubMed1991

Detection of structural defects in phosphatidylcholine membranes by small-angle neutron scattering. The cluster model of a lipid bilayer.

The oriented DPPC multilayers hydrated by D2O have been studied by a small-angle neutron scattering method in the Guinier range, and the gyration radius of the structural inhomogeneities has been estimated at about 29 A. They are interpreted as the annular defects between adjacent clusters uniting the all-trans chain 'segments' adjacent to the polar head group regions. The angle of the 'segment' tilt is determined by the hydrated polar group area (59.2 A2 for DPPC bilayers) and has been estimated to be about 44 degrees under the given experimental conditions. The hydrocarbon interior of a bilayer can be suggested as a 'sandwich' that is formed by two clustered layers (approx. 7 A of the thickness) and the central disordered (liquid) layer. The average cluster size along the bilayer surface is estimated to be approx. 24 A which correlates with the estimations of the short order region dimensions from the halfwidth of the X-ray 'packing' reflex (4.6 A)-1. The average interchain separation of approx. 5 A and the average cross-section area of a chain in a cluster (21.4 A2) were estimated from the reflex position and the chain cross-section geometry. The total volume of defects and the fraction of a bilayer surface occupied by them were estimated too.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2003

Does sucrose influence the properties of DMPC vesicles?

Small-angle neutron and X-ray scattering, dynamic light scattering, X-ray diffraction coupled with differential scanning calorimetry, and Raman spectroscopy were applied to investigate unilamellar (ULVs) and multilamellar (MLVs) dimyristoylphosphatidylcholine (DMPC) vesicles in aqueous sucrose solutions with sucrose concentrations from 0 to 60% w/w. In case of ULVs, the addition of sucrose decreases the polydispersity of vesicle population. A minimum value of polydispersity was found at 20% sucrose. For sucrose concentration from 0 to 35% oligolamellar vesicles in the ULV population have a minimum presence. Vesicles with 5-10% sucrose exhibit the best stability in time. For the case of MLVs, sucrose influences the temperature of the phase transitions, but the internal membrane structure remains unchanged.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1984

Structure of bovine pancreatic trypsin inhibitor. Results of joint neutron and X-ray refinement of crystal form II.

The structure of form II crystals of bovine pancreatic trypsin inhibitor has been investigated by joint refinement of X-ray and neutron data. Crystallographic R factors for the final model were 0.200 for the X-ray data extending to 1 A resolution and 0.197 for the 1.8 A neutron data. This model was strongly restrained, with 0.020 A root-mean-square (r.m.s.) departure of bond lengths from their ideal values and 0.019 A r.m.s. departure of planar groups from planarity. The resulting structure was very similar to that of crystal form I (r.m.s. deviation for main chain atoms was 0.40 A); nevertheless larger deviations were observed in particular regions of the chain. Twenty out of 63 ordered water molecules occupy similar positions (deviation less than 1 A) in both models. Eleven amide hydrogens were found to be protected from exchange after three months of soaking the crystals in deuterated mother liquor at pH 8.2. Their locations were in excellent agreement with the results obtained by two-dimensional nuclear magnetic resonance, but the rates of exchange are much lower in the crystalline state.

Amino Acid Sequence↗
PubMed1984

Structure of the cholesteryl ester core of human plasma low density lipoproteins: selective deuteration and neutron small-angle scattering.

The structural arrangement of cholesteryl esters in human plasma low density lipoproteins (LDL) has been studied by selective deuteration and neutron small-angle scattering. LDL were labeled by in vitro exchange with two different kinds of deuterated cholesteryl esters, one labeled in the fatty acyl chain (cholesteryl myristate-d27) and the other in the branched side chain of cholesterol (cholesteryl-25,26,27-d7 oleate). Neutron scattering data from deuterated and protonated LDL were compared to identify the locations of the fatty acyl and cholesterol side chain moieties. Below the thermotropic transition, radii of gyration of 60 A and 70 A were obtained for these two domains, respectively, indicating that the cholesteryl nuclei are situated more distantly from the center than the fatty acyl chains. At 37 degrees C, above the thermotropic transition of the cholesteryl esters in LDL, both parts have similar radii of gyration of approximately 56 A. This information is used in a discussion of possible structural models for the apolar lipid core of LDL.

Cholesterol Esters↗
PubMed1999

Specular and diffuse scattering of highly aligned phospholipid membranes.

We present a quantitative study of specular and diffuse (non-specular) x-ray and neutron reflectivity from highly aligned phospholipid membranes deposited on solid substrates. The height-height correlation function could be obtained from the diffuse scattering without further model assumptions. The results differ significantly from the linear theory of smectic elasticity. We argue that the diffuse scattering is dominated by static liquid-crystalline defects, rather than thermal fluctuations.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1979

Hydrogen bonds in crystal structures of amino acids, peptides and related molecules.

The results of a survey of 439 hydrogen bonds in 95 recently determined crystal structures of amino acids, peptides and related molecules suggest that the following generalizations hold true for linear (angle X-H---Y greater than 150 degrees) hydrogen bonds. (1) The charge on the acceptor group does not influence the length of a hydrogen bond. (2) For a given acceptor group, the hydrogen bond lengths increase in the order imidazolium N--H less than ammonium N-H less than guanidinium N-H; this order holds true for oxygen anion acceptor groups. Cl-ions and the uncharged oxygen of water molecules. (3) The uncharged imidazole N-H group forms shorter hydrogen than the amide N-H GROUP. (4) The carboxyl O-H groups form shorter hydrogen bonds than other hydroxyl groups. (5) The hydrogen bonds involving a halogen ion are longer than hydrogen bonds with other acceptors when corrected for their longer van der Walls radii. The observed differences between the lengths of hydrogen bonds formed by different donor and acceptor groups in amino acids and peptides, imply differences in the energetics of their formation.

Amino Acids↗
PubMed1979

The shape of the fibrinogen molecule: sausage or banana?

The experimental results and the interpretation of small angle X-ray scattering measurements by the author and Hammel (1975) and of small angle neutron scattering measurements by Marguerie and Stuhrmann (1976) on dilute solutions of bovine fibrinogen are compared. The potential of both methods to discern straight and bent cylindrical structures is demonstrated. The study arrives at the conclusion that the question whether the fibrinogen molecule resembles more a banana or a straight sausage cannot be safely decided by the available data. This justifies the previous suggestion of the author to imagine the fibrinogen molecule as a swollen sausage 450 A long and 90 A thick with conceivable flexibility, a molecular model of sufficient universality to include the banana-shaped model of Marguerie and Stuhrmann (1976) as a special case.

Fibrinogen↗
PubMed2001

Ab initio structure determination of BiPb2VO6 from powder diffraction data.

The crystal structure of BiPb2VO6 has been determined from powder diffraction data using a combination of direct methods and the novel approach of applying simulated annealing methods simultaneously to X-ray and neutron data; BiPb2VO6 is a polar, noncentrosymmetric, second harmonic generation active material and its crystal structure is one of the more complex to be solved ab initio from powder diffraction data.

Journal Article↗
PubMed2003

N-alkyl perfluoroalkanamides as low molecular-mass organogelators.

A new class of low molecular-mass organogelators (LMOGs), N-alkyl perfluoroalkanamides, F(CF(2))(n)CONH(CH(2))(m)H, is described. The molecules are designed to exploit the incompatibilities of their three molecular parts, and the results demonstrate that this strategy can be used to tune molecular aggregation and gel stability. The gelating properties of these LMOGs have been examined in a wide variety of organic liquids (including alkanes, alcohols, toluene, n-perfluorooctane, CCl(4), and DMSO) as a function of the N-alkyl and perfluoroalkyl chain lengths by X-ray diffraction, polarizing optical microscopy, infrared spectroscopy, differential scanning calorimetry, and small-angle neutron scattering (SANS). The gels are thermally reversible and require generally very low concentrations (<2 wt %) of LMOG. Several of the gels are stable for very long periods at room temperature. The incompatibility of the fluorocarbon and hydrocarbon segments causes the LMOGs to aggregate, probably into lamellae within the fibrils that constitute the basic unit of the gel networks. The SANS studies show that the cross-sections of fibers in the gel networks of LMOGs with shorter perfluoroalkyl chains are much larger than those with longer ones. Comparisons with the gelating properties of some analogous esters (F(CF(2))(n)CO(2)(CH(2))(m)H) and diblock perfluoroalkylalkanes (F(CF(2))(n)(CH(2))(m)H) indicate that additional ordering within the aggregate units is enforced by the intermolecular H bonding among amide groups that is evidenced by IR spectroscopy. Analyses of these results and structure/solvent correlations are provided.

Journal Article↗
PubMed2006

Characterization and surface-reactivity of nanocrystalline anatase in aqueous solutions.

The chemical and electrostatic interactions at mineral-water interfaces are of fundamental importance in many geochemical, materials science, and technological processes; however, the effects of particle size at the nanoscale on these interactions are poorly known. Therefore, comprehensive experimental and characterization studies were completed, to begin to assess the effects of particle size on the surface reactivity and charging of metal-oxide nanoparticles in aqueous solutions. Commercially available crystalline anatase (TiO2) particles were characterized using neutron and X-ray small-angle scattering, electron microscopy, and laser diffraction techniques. The 4 nm primary nanoparticles were found to exist almost exclusively in a hierarchy of agglomerated structures. Potentiometric and electrophoretic mobility titrations were completed in NaCl media at ionic strengths from (0.005 to 0.3) mol/kg, and 25 degrees C, with these two experimental techniques matched as closely as the different procedures permitted. The pH of zero net proton charge (pHznpc, from potentiometric titration) and isoelectric point pH value (pHiep, from electrophoretic mobility titrations) were both in near perfect agreement (6.85 +/- 0.02). At high ionic strengths the apparent pHznpc value was offset slightly toward lower pH values, which suggests some specific adsorption of the Na+ electrolyte ions. Proton-induced surface charge curves of nanocrystalline anatase were very similar to those of larger rutile crystallites when expressed relative to their respective pHznpc values, indicating that the development of positive and negative surface charge away from the pHznpc for nanocrystalline anatase is similar to that of larger TiO2 crystallites.

Electrochemistry↗
PubMed1989

Different segmental flexibility of human serum transferrin and lactoferrin.

X-ray diffraction studies show that the diferric (holo) forms of human serum transferrin and lactoferrin have almost the same conformation in crystal. In solution, however, the two proteins exhibit different characteristics. The differences are even more pronounced in the apo forms. Small-angle X-ray and neutron scattering data show that lactoferrin is less compact, in apo and holo forms, than the corresponding forms of transferrin in solution. The comparison of primary structures of the two proteins suggests that one of the interdomain hinge regions is significantly longer in lactoferrin than its counterpart in transferrin. The difference in flexibility due to the long hinge region in lactoferrin may be responsible for many of the differences in the physicochemical characteristics of the two proteins.

Humans↗
PubMed2001

X-ray and neutron scattering analyses of hydration shells: a molecular interpretation based on sequence predictions and modelling fits.

Solution scattering is a low resolution diffraction method that provides important structural data on proteins. The ability to model scattering curves by recourse to known crystal structures for proteins under study significantly improves the resolution (and the utility) of the method because of the strict constraints that the crystal structures impose. For these structure determinations, a molecular description of the effect of hydration shells is needed. In calibration studies used for X-ray scattering curve modelling, it has been reproducibly found that a hydration shell is required. In molecular terms, this results from the higher electron density of the hydration shell compared to that of bulk water, which then becomes similar to that of the protein. This is well represented by a level of 0.3 g H(2)O/g glycoprotein and a water molecule volume of 0.0245 nm(3). Procedures for the addition of a hydration shell to a sphere model of a protein are described. For neutron scattering fits, it is not necessary to incorporate a hydration shell, as to a good approximation this is not detectable. In molecular terms, this apparent absence of the neutron hydration shell results from the effect of proton exchange on the scattering densities of bulk water and bound water which causes these to be similar but different from that of the protein.

Models, Chemical↗