Stratified Temperature-Responsive Multilayer Hydrogels of Poly(<i>N</i>‑vinylpyrrolidone) and Poly(<i>N</i>‑vinylcaprolactam): Effect of Hydrogel Architecture on Properties

We report on the effects of hydrophilicity and architecture on the temperature-responsive behavior and surface morphology of nonionic double-stack hydrogels prepared from cross-linked hydrogen-bonded layer-by-layer films. A hydrophilic poly­(<i>N</i>-vinyl­pyrrolidone) (PVPON)_<i>n</i> multilayer hydrogel is integrated with a relatively hydrophobic temperature-sensitive poly­(<i>N</i>-vinyl­caprolactam) (PVCL)_<i>m</i> network as either a top or bottom stratum, where <i>n</i> and <i>m</i> represent numbers of layers for each individual stratum. Neutron reflectometry revealed that all double-stack films in the dry state are well stratified with two distinct (PVPON) and (PVCL) strata of higher and lower scattering density, respectively, unlike highly mixed alternating (PVCL/PVPON) hydrogels. We have found that the order of stacking and stack thickness significantly influence hydration of the (PVPON)_<i>n</i>(PVCL)_<i>m</i> and (PVCL)_<i>m</i>(PVPON)_<i>n</i> networks at ambient temperature and above the LCST of PVCL. The hydration of the hydrogels consistently increases with PVPON amount within the network, resulting in suppressed temperature response. This effect is more pronounced for (PVPON)_<i>n</i>(PVCL)_<i>m</i> as compared to its mirror counterpart as explained by the two adjacent aqueous interfaces in which the (PVCL)_<i>m</i> stack is sandwiched between the hydrophilic (PVPON)_<i>n</i> stack below and the bulk of water above it. Our results yield new insights into controlling the temperature response and surface properties of nanostructured polymer networks, which is relevant to both fundamental and applied research where the dynamics of hydration, thickness, and control of surface hydrophobicity are important

Interaction of Silica Nanoparticles with a Flat Silica Surface through Neutron Reflectometry

Neutron reflectometry (NR) was employed to study the interaction of nanosized silica particles with a flat silica surface in aqueous solutions. Unlike other experimental tools that are used to study surface interactions, NR can provide information on the particle density profile in the solution near the interface. Two types of silica particles (25 and 100 nm) were suspended in aqueous solutions of varying ionic strength. Theoretical calculations of the surface interaction potential between a particle and a flat silica surface using the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory were compared to the experimental data. The theory predicts that the potential energy is highly dependent on the ionic strength. In high ionic strength solutions, NR reveals a high concentration of particles near the flat silica surface. Under the same conditions, theoretical calculations show an attractive force between a particle and a flat surface. For low ionic strength solutions, the particle concentration near the surface obtained from NR is the same as the bulk concentration, while depletion of particles near the surface is expected because of the repulsion predicted by the DLVO theory

Adsorption of α‑Synuclein to Supported Lipid Bilayers: Positioning and Role of Electrostatics

An amyloid form of the protein α-synuclein is the major component of the intraneuronal inclusions called Lewy bodies, which are the neuropathological hallmark of Parkinson’s disease (PD). α-Synuclein is known to associate with anionic lipid membranes, and interactions between aggregating α-synuclein and cellular membranes are thought to be important for PD pathology. We have studied the molecular determinants for adsorption of monomeric α-synuclein to planar model lipid membranes composed of zwitterionic phosphatidylcholine alone or in a mixture with anionic phosphatidylserine (relevant for plasma membranes) or anionic cardiolipin (relevant for mitochondrial membranes). We studied the adsorption of the protein to supported bilayers, the position of the protein within and outside the bilayer, and structural changes in the model membranes using two complementary techniquesquartz crystal microbalance with dissipation monitoring, and neutron reflectometry. We found that the interaction and adsorbed conformation depend on membrane charge, protein charge, and electrostatic screening. The results imply that α-synuclein adsorbs in the headgroup region of anionic lipid bilayers with extensions into the bulk but does not penetrate deeply into or across the hydrophobic acyl chain region. The adsorption to anionic bilayers leads to a small perturbation of the acyl chain packing that is independent of anionic headgroup identity. We also explored the effect of changing the area per headgroup in the lipid bilayer by comparing model systems with different degrees of acyl chain saturation. An increase in area per lipid headgroup leads to an increase in the level of α-synuclein adsorption with a reduced water content in the acyl chain layer. In conclusion, the association of α-synuclein to membranes and its adsorbed conformation are of electrostatic origin, combined with van der Waals interactions, but with a very weak correlation to the molecular structure of the anionic lipid headgroup. The perturbation of the acyl chain packing upon monomeric protein adsorption favors association with unsaturated phospholipids preferentially found in the neuronal membrane

Neutron Reflectometry and QCM-D Study of the Interaction of Cellulases with Films of Amorphous Cellulose

Improving the efficiency of enzymatic hydrolysis of cellulose is one of the key technological hurdles to reduce the cost of producing ethanol and other transportation fuels from lignocellulosic material. A better understanding of how soluble enzymes interact with insoluble cellulose will aid in the design of more efficient enzyme systems. We report a study involving neutron reflectometry (NR) and quartz crystal microbalance with dissipation monitoring (QCM-D) of the interaction of a fungal enzyme extract (T. viride) and an endoglucanse from A. niger with amorphous cellulose films. The use of amorphous cellulose is motivated by that the fact that several biomass pretreatments currently under investigation disrupt the native crystalline structure of cellulose and increase the amorphous content. NR reveals the profile of water through the film at nanometer resolution and is highly sensitive to interfacial roughness, whereas QCM-D provides changes in mass and film stiffness. NR can be performed using either H2O- or D2O-based aqueous reservoirs. NR measurement of swelling of a cellulose film in D2O and in H2O revealed that D/H exchange on the cellulose chains must be taken into account when a D2O-based reservoir is used. The results also show that cellulose films swell slightly more in D2O than in H2O. Regarding enzymatic digestion, at 20 °C in H2O buffer the T. viride cocktail rapidly digested the entire film, initially roughening the surface, followed by penetration and activity throughout the bulk of the film. In contrast, over the same time period, the endoglucanase was active mainly at the surface of the film and did not increase the surface roughness