No abstract

Fructans, a family of oligo- and polyfructoses, are implicated to play a drought-protecting role in plants. Inulin-type fructan is able to preserve the membrane barrier during dehydration. However, whether other fructans would be able to perform this function is unknown. In addition, almost nothing is known about the organization of these systems, which could give insight into the protective mechanism. To get insight into these questions the effect of different fructans on phosphatidylcholine-based model systems under conditions of dehydration was analyzed. Using a vesicle leakage assay, it was found that both levan- and inulin-type fructans protected the membrane barrier. This suggests that fructans in general would be able to protect the membrane barrier function. Furthermore, both fructan-types inhibited vesicle fusion to a large extent as measured using a lipid-mixing assay. Using x-ray diffraction, it was found that in the presence of both inulin- and levan-type fructans the lamellar repeat distance increased considerably. From this it was concluded that fructans are present between the lipid bilayers during drying. Furthermore, they stabilize the L(alpha) phase. In contrast to fructans, dextran did not increase the lamellar repeat distance and it even promoted L(beta) phase formation. These data support the hypothesis that fructans can have a membrane-protecting role during dehydration, and give insight into the mechanism of protection.

Self-aggregation of rigid-rod poly(sodium p-phenylenesulfonate) in aqueous solution and inside water-swollen polyacrylamide gel was studied by small-angle neutron scattering. It was shown that both inside the hydrogel and in solution polyelectrolyte rods self-assemble into cylindrical aggregates having eight to nine single polymer chains in the cross-section, the chains being aligned parallel to the axis of the aggregate. The length of these aggregates is much higher than the contour length of a single chain. Gels with embedded rods were studied by contrast variation method in order to examine separately the scattering by the gel and by the rods. Two important observations were made. First, it was shown that the ordering of the rods in the gel resembles that in solution. Second, it was shown that the gel itself is more homogeneous in the presence of rods. Most probably, this effect is due to mobile counterions of rods, which counteract the formation of spatial inhomogeneities in the network during synthesis, because in an inhomogeneous network mobile counterions should be also distributed nonuniformly that is associated with significant translational entropy losses.

The structure of oriented multilayers of the zwitterionic phospholipid palmitoyloleoylphosphatidylcholine containing nonionic surfactants (oligo(oxyethylene glycol) monododecyl ethers, C12En with n ) 2, 4, and 6) was studied by X-ray and neutron diffraction. Investigations were done at T ) 20 °C in the lamellar liquid-crystalline phase with surfactant/lipid molar ratios RA/L ) 0.5 and 1 and hydration levels corresponding to 85 and 97% relative humidity. The transbilayer distribution of the surfactant was determined by neutron diffraction using selectively deuterated C12En. The incorporation of surfactant causes a decrease of the repeat distance and of the thickness of the hydrocarbon core of the membrane. The effect increases with the number of oxyethylene units and with the concentration of the surfactant as well as with the relative humidity. The R-methylene group of the surfactants is anchored near the boundary of the hydrophobic core. The oxyethylene moieties are mainly located in the polar membrane/water interface region. The impact of molecular disorder on the diffraction data is discussed.

The structure of amphiphilic hydrogels of copolymers of acrylic acid and n-alkylacrylate swollen by D2O was studied by small-angle neutron scattering (SANS). For most of the uncharged gels, a scattering peak is observed. It is attributed to the correlation between hydrophobic domains formed by self-assembled n-alkyl side chains. From the SANS data, the aggregation number of the hydrophobic domains was derived. It increases with increasing length of hydrophobic groups and also as a result of absorption of hydrophobic additives that are solubilized inside hydrophobic domains. The most important observation of this work consists of the fact that the introduction of charged groups into the gel leads to microphase separation with the formation of hydrophobic regions including several densely packed hydrophobic domains that alternate with hydrophilic regions swollen by water where most of the charged repeat units and counterions are located. The size of the hydrophobic regions decreases with increasing charge content. The microphase separation seems to be due to the effective interplay of two counteracting tendencies:  hydrophobic association and electrostatic repulsion. When salt screening is added, the microphase separation disappears.

No abstract

Small-angle neutron scattering data were collected from aqueous dispersions of unilamellar vesicles (ULVs) consisting of mixtures of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine and a homologous series of N,N-dimethyl-N-alkylamine-N-oxides (CnNO, n = 12, 14, 16, and 18, where n is the number of carbon atoms in the alkyl chain). A modeling approach was applied to the neutron scattering curves to obtain the bilayer structural parameters. Particularly, the external (2)H2O/H2O contrast variation technique was carried out on pure dioleoylphosphatidylcholine (DOPC) ULVs to determine the hydrophilic region thickness [Formula: see text] = 9.8 ± 0.6 Å. Consequently, the hydrocarbon region thickness [Formula: see text], the lateral bilayer area per one lipid molecule [Formula: see text], and the number of water molecules located in the hydrophilic region per one lipid molecule [Formula: see text] were obtained from single-contrast neutron scattering curves using the previously determined [Formula: see text]. The structural parameters were extracted as functions of [Formula: see text] (the CnNO:DOPC molar ratio) and n. The dependences [Formula: see text] provided the partial lateral areas of CnNOs ([Formula: see text]) and DOPC ([Formula: see text]) in bilayers. It was observed that the [Formula: see text]'s were constant in the investigated interval of [Formula: see text] and for n = 12, 14, and 16 equal to 36.6 ± 0.4 Å(2), while [Formula: see text] increased to 39.4 ± 0.4 Å(2). The bilayer hydrocarbon region thickness [Formula: see text] decreased with intercalation of each CnNO. This effect increased with [Formula: see text] and decreased with increasing CnNO alkyl chain length. The intercalation of C18NO changed the [Formula: see text] only slightly. To quantify the effect of CnNO intercalation into DOPC bilayers we fit the [Formula: see text] dependences with weighted linear approximations and acquired their slopes [Formula: see text].

The structural characteristics of polycarbosilane dendrimers with different molecular architecture were determined in solutions by small angle neutron and X-ray scattering. The same linear dimensions were sized up for the dendrimers both in benzene and chloroform. A solvent molecules penetration inside dendrimer structure in amount up to 30 vol.-% was found from the comparison of the partial and effective scattering volume for the dendrimers in solution.