Biomedical subjects

Giovanna Fragneto

Publications and source records attributed to Giovanna Fragneto.

8 recordsLinked to original sources

PubMed2007

Human saliva forms a complex film structure on alumina surfaces.

Films formed from saliva on surfaces are important for the maintenance of oral health and integrity by protection against chemical and/or biological agents. The aim of the present study was to investigate adsorbed amounts, thickness, and structure of films formed from human whole saliva on alumina surfaces by means of in situ ellipsometry, neutron reflectivity, and atomic force microscopy. Alumina (Al2O3, synthetic sapphire) is a relevant and interesting substrate for saliva adsorption studies as it has an isoelectric point close to that of tooth enamel. The results showed that saliva adsorbs rapidly on alumina. The film could be modeled in two layers: an inner and dense thin region that forms a uniform layer and an outer, more diffuse and thicker region that protrudes toward the bulk of the solution. The film morphology described a uniformly covering dense layer and a second outer layer containing polydisperse adsorbed macromolecules or aggregates.

Adsorption↗

PubMed2005

Effect of low amounts of cholesterol on the swelling behavior of floating bilayers.

The effect of the addition of 1, 2, 4, and 6 mol % cholesterol to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) floating bilayers has been investigated by neutron reflectivity. All samples exhibited fully stable and reversible gel and fluid phases. Around the main lipid phase transition temperature, DPPC double bilayers exhibit large increases in the water layer separating the bilayers and the upper bilayer roughness. The inclusion of low amounts of cholesterol reduced the swelling of the water layer between the bilayers and the upper bilayer roughness and progressively widened the temperature range over which swelling occurs. Results from asymmetric bilayers are also reported. A higher amount of cholesterol in the lower bilayer induces a smaller swelling of the water layer between the bilayers than in the symmetric case. Finally, the effect of the inclusion of a leaflet of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) was investigated. The presence of a leaflet with a higher gel-transition temperature (T(m)) modifies the phase behavior of the lower T(m) leaflet.

Journal Article↗

PubMed2005

Cholesterol induced suppression of large swelling of water layer in phosphocholine floating bilayers.

The effect of the addition of 10 mol % cholesterol to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) floating bilayers has been investigated by means of neutron reflectivity measurements. The large swelling of the water layer between the two bilayers found in pure phosphocholine systems around the lipid main phase transition is greatly reduced in the 9:1 DPPC:cholesterol mixture. The analysis of the structure of the bilayer reveals that in the gel phase cholesterol induces the presence of a high rms roughness that disappears in the fluid phase.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2005

Locating counterions in guanosine quadruplexes: a contrast-variation neutron diffraction experiment in condensed hexagonal phase.

Guanosine, one of the primary components of nucleic acids, self-associates in water to form G-quadruplexes, four-stranded helicoidal aggregates, made by stacked planar tetramers, consisting of four planar guanine molecules. Essential for the stability of these supramolecular aggregates is the presence of monovalent cations. As G-quadruplexes show a lyotropic polymorphism, neutron diffraction, in combination with the H2O/D2O contrast variation technique, has been applied to study the cation structural features of quadruplexes in hexagonal phase at different hydrations and counterion concentrations. The guanosine 5'-monophosphate, dipotassium salt, was considered and G-quadruplexes in hexagonal phase were prepared in the different experimental conditions (contrast, hydration and KCl solution concentration) by using the osmotic stress technique. The calculated scattering length density distribution maps show that counterions fill the helix inner cavity and that atmospheric cations are bound to a second site, close to the external phosphate groups. The site occupancy turned out to be very high: we found on the inner site 0.87 K ions per tetramer in G-quadruplexes prepared in pure water and 0.97 K ions per tetramer in G-quadruplexes prepared in KCl 0.5 M, while in all cases 6 ions per unit cell were detected to occupy the outer sites, partially neutralizing the two formal negative charges per phosphate group. The very large K ions concentration difference between the binding sites and the bulk solution demonstrates that counterions are structurally involved in the formation and in the stabilization of the helices.

Cations↗

PubMed2005

Composition of supported model membranes determined by neutron reflection.

We have investigated the formation of supported bilayers by coadsorption of dipalmitoyl phosphatidylcholine (DPPC) with the nonionic surfactant beta-D-dodecyl maltoside. The adsorption of mixed phospholipid-surfactant micelles on hydrophilic silica surfaces at 25 degrees C was followed as a function of bulk concentration by neutron reflection. Using chain-deuterated d(25)-beta-D-dodecyl maltoside and d(62)-DPPC, we demonstrate that it is possible to determine the composition of the bilayers at each stage of a sequential dilution process, which enriches the adsorbed layer in phospholipid and leads to complete elimination of the surfactant. The final supported bilayers have thicknesses of 51 +/- 3 A and are stable to heating to 37 degrees C once all surfactant has been removed, and the structures agree well with other published data on DPPC supported bilayers. The coadsorption of cholesterol in a DPPC-surfactant mixture was also achieved, and the location and volume fraction of cholesterol in the DPPC bilayer was determined. Cholesterol is located in a 18 +/- 1 A thick layer below the lipid headgroup region and leads to an increased bilayer thickness of 58 +/- 2 A at 26 mol % of cholesterol.

Absorption↗

PubMed2005

Phospholipase A2 hydrolysis of supported phospholipid bilayers: a neutron reflectivity and ellipsometry study.

We have investigated the phospholipase A(2) catalyzed hydrolysis of supported phospholipid bilayers using neutron reflection and ellipsometry. At the hydrophilic silica-water interface, hydrolysis of phosphatidylcholine bilayers by phospholipase A(2) from Naja mossambica mossambica venom is accompanied by destruction of the bilayer at an initial rate, which is comparable for DOPC and DPPC but is doubled for POPC. The extent of bilayer destruction at 25 degrees C decreases from DOPC to POPC and is dramatically reduced for DPPC. Neutron reflectivity measurements indicate that the enzyme penetrates into the bilayers in increasing order for DOPC, POPC, and DPPC, while the amount of enzyme adsorbed at the interface is smallest for DPPC and exhibits a maximum for POPC. Penetration into the hydrophobic chain region in the bilayer is further supported by the fact that the enzyme adsorbs strongly and irreversibly to a hydrophobic monolayer of octadecyltrichlorosilane. These results are rationalized in terms of the properties of the reaction products and the effect of their accumulation in the membrane on the kinetics of enzyme catalysis.

Animals↗

PubMed2005

Structural studies of the neural-cell-adhesion molecule by X-ray and neutron reflectivity.

The structures of adhesion proteins play an important role in the formation of intercellular junctions and the control of intermembrane spacing. This paper describes the combination of neutron and X-ray specular reflectivity measurements to investigate the structure of the ectodomain of the neural-cell-adhesion molecule (NCAM). The measurements with unmodified NCAM suggest the presence of a bend in the extracellular region. Measurements with the polysialic-acid-modified form of NCAM reveal that, at physiological ionic strength, the carbohydrate chains extend beyond the range of the unmodified protein. The excluded volume of the polymer is also ionic-strength-dependent, as expected for a polyelectrolyte. The structural characteristics obtained from these independent analyses of X-ray and neutron reflectivity data agree with each other, with prior reflectivity studies, and with molecular dimensions obtained from direct-force measurements. These results provide structural insights into the configuration of the NCAM ectodomain and the regulation of NCAM adhesion by post-translational modification.

Animals↗

PubMed2005

Distorted hexagonal phase studied by neutron diffraction: lipid components demixed in a bent monolayer.

The recent discovery of a distorted hexagonal phase in 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine/1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPE/DOPC) mixtures raised the intriguing question as to whether lipid mixtures demix in a bent monolayer. We performed neutron diffraction on a mixture of headgroup deuterated DOPC-d(13) and nondeuterated DOPE to study the lipid distribution in the distorted hexagonal phase. The 1:1 lipid mixture in full hydration and 25 degrees C was in a homogeneous lamellar phase. Upon dehydration the mixture transformed to a rhombohedral phase, then to a distorted inverted hexagonal phase, and finally to a regular inverted hexagonal phase. In the distorted hexagonal phase, the diffraction pattern showed a two-dimensional monoclinic lattice with two reciprocal vectors of equal length (1.5 nm(-)(1)) forming an angle 53 degrees between them. Diffraction intensities measured while varying the D(2)O/H(2)O ratio in the humidity was used to solve the phase problem. The neutron scattering length density distribution of the distorted hexagonal phase was constructed. The constant density contours are approximately elliptical. The difference in the eccentricities of the contours between the water and lipid distributions indicates that the DOPE/DOPC ratio is not uniform around the elliptical lipid tube in the unit cell. DOPE is preferentially distributed at the vertex regions where the curvature is the highest. Thus for the first time it is shown that when a monolayer of a homogeneous lipid mixture is bent, the lipid components are partially demixed in reaching the free energy minimum.

Lipids↗