Neutron reflectivity experiments on the interface of pure D2O against thin films of perdeuterated polystyrene (d-PS) spin-coated onto silicon blocks were performed to study the intrinsic structure of the interface of water against hydrophobic substrates. The experiments reveal nonvanishing scattering contrast at the polymer/water interface, although the two materials (d-PS and D2O) have closely similar scattering length densities. Organic (nondeuterated) contaminants or macroscopic air bubbles trapped at the polymer/water interface can be ruled out as the origin of this observation. From a systematic study of this system, it is concluded that the source of the nonvanishing contrast is a depletion of water in the boundary layer against the hydrophobic surface. It is conjectured that this depletion layer represents a precursor layer of submicroscopic gas bubbles recently observed by Tyrrell and Attard. The existence of such gas nanobubbles in the present system is confirmed by atomic force microscopy (AFM) of the surface of d-PS coatings in contact with bulk water. The thickness of the precursor gas layer as determined by neutron reflectometry is 2−5 nm, depending on the level of air saturation of the water sample and on the time elapsed after contacting it with the hydrophobic surface.
Bimetallic, oxalate-bridged compounds with bi- and trivalent transition metals comprise a class of layered materials which express a large variety in their molecular-based magnetic behavior. Because of this, the availability of the corresponding single-crystal structural data is essential to the successful interpretation of the experimental magnetic results. We report in this paper the crystal structure and magnetic properties of the ferromagnetic compound {[N(n-C(3)H(7))(4)][Mn(II)Cr(III)(C(2)O(4))(3)]}(n)() (1), the crystal structure of the antiferromagnetic compound {[N(n-C(4)H(9))(4)][Mn(II)Fe(III)(C(2)O(4))(3)]}(n)() (2), and the results of a neutron diffraction study of a polycrystalline sample of the ferromagnetic compound {[P(C(6)D(5))(4)][Mn(II)Cr(III)(C(2)O(4))(3)]}(n)() (3). Crystal data: 1, rhombohedral, R3c, a = 9.363(3) Å, c = 49.207(27) Å, Z = 6; 2, hexagonal, P6(3), a = 9.482(2) Å, c = 17.827(8) Å, Z = 2. The structures consist of anionic, two-dimensional, honeycomb networks formed by the oxalate-bridged metal ions, interleaved by the templating cations. Single-crystal field dependent magnetization measurements as well as elastic neutron scattering experiments on the manganese(II)-chromium(III) samples show the existence of long-range ferromagnetic ordering behavior below T(c) = 6 K. The magnetic structure corresponds to an alignment of the spins perpendicular to the network layers. In contrast, the manganese(II)-iron(III) compound expresses a two-dimensional antiferromagnetic ordering.
Neuronal plasma membranes are thought to be the primary target of the neurotoxic beta-amyloid peptides (Abeta) in the pathogenesis of the Alzheimer's disease. Histologically, Abeta peptides are observed as extracellular macroscopic senile plaques, and most biophysical techniques have indicated the presence of Abeta close to the lipid headgroup region but not in the core of the membrane bilayers. The focus of this study is an investigation of the interaction between Abeta and lipid bilayers from a structural point of view. Neutron diffraction with the use of selectively deuterated amino acids has allowed us to determine unambiguously the position of the neurotoxic fragment Abeta (25-35) in the membrane. Two populations of the peptide are detected, one in the aqueous vicinity of the membrane surface and the second inside the hydrophobic core of the lipid membrane. The location of the C terminus was studied in two different lipid compositions and was found to be dependent on the surface charge of the membrane. The localization of beta-amyloid peptides in cell membranes will offer new insights on their mechanism in the neurodegenerative process associated with Alzheimer's disease and might provide clues for therapeutic developments.
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