Interfacial Electrostatic Interactions Research Articles

Charged Residues on the Intracellular Intersubunit Assembly Interface Contribute to Calcium-Sensitivity of BK Channels

The large-conductance calcium-activated K+ (BK) channel contains a large C-terminal intracellular Ca2+-sensing region formed by two RCK domains. The RCK2 domain harbors a Ca2+-binding site of clustered negatively charged residues, known as the Ca2+-bowl, while the RCK1 domain is believed to contain another Ca2+-binding site (RCK1-site). Recent X-ray crystallographic structures of the Ca2+-free and -bound intracellular regions revealed an interesting location of the Ca2+-bowl near the tertrameric intersubunit assembly interface. We investigated the role of the intersubunit assembly interface on Ca2+-gated BK channel activation through mutational analyses of charged residues Glu955, Glu956, Arg786, and Arg790 which may electrostatically stabilize RCK2 intersubunit interactions by formation of intersubunit salt-bridges. We observed that mutations E955Q, E956Q, and E955Q/E956Q caused ∼50%, ∼30%, and ∼70% loss, respectively, of Ca2+-sensitivity as determined at 0, 7.3, and 90.4 μM internal free Ca2+. Combined analyses with Ca2+-bowl or RCK1-site null mutations indicated that E955Q mutation eliminated most Ca2+-sensitivity from the Ca2+-bowl and ∼50% sensitivity from the RCK1-site. The charge neutralization mutation R786A/R790A on the interacting subunits also led to ∼50% loss of Ca2+-sensitivity, suggesting involvement of intersubunit salt-bridge(s) between residues E955 and R786 or R790. Furthermore, intersubunit charge reversal mutations induced recovery of Ca2+-sensitivity. In contrast to the drastic effects of these charged residue mutations, which are located distant (≥20A) to the bound Ca2+, mutation of the Asn449 residue, which may coordinate the RCK2 Ca2+ from a different subunit, displayed little effect on Ca2+-sensitivity. We conclude that the intersubunit interfacial electrostatic interactions play an important role in Ca2+-dependent BK channel gating. We propose that such interactions are unlikely to be involved in direct Ca2+-binding but may provide intersubunit supporting points to mediate Ca2+-gated RCK gating ring expansion or conformational coupling to channel activation.

Synthesis of Anionic Sulfonate-functionalized Conducting Polymer Nanotubes and Selective Confinement of Cationic Gold Nanoparticles in Their Inner Cavities via Electrostatic Interaction

Abstract New sulfonate-functionalized conducting polythiophene nanotubes were synthesized and easily dispersed in water without them aggregating. The positively charged Au nanoparticles with viologen were selectively attached to the inner surfaces of their negatively charged polythiophene nanotubes via an interfacial electrostatic interaction.

A unifying motif of intermolecular cooperativity in protein associations

At the molecular level, most biological processes entail protein associations which in turn rely on a small fraction of interfacial residues called hot spots. Our theoretical analysis shows that hot spots share a unifying molecular attribute: they provide a third-body contribution to intermolecular cooperativity. Such motif, based on the wrapping of interfacial electrostatic interactions, is essential to maintain the integrity of the interface. Thus, our main result is to unravel the molecular nature of the protein association problem by revealing its underlying physics and thus by casting it in simple physical grounds. Such knowledge could then be exploited in rational drug design since the regions here indicated may serve as blueprints to engineer small molecules disruptive of protein-protein interfaces.

TiO2 as an electrostatic template for epitaxial growth of EuO on MgO(001) by reactive molecular beam epitaxy

We investigate the initial growth modes and the role of interfacial electrostatic interactions of EuO epitaxy on MgO(001) by reactive molecular beam epitaxy. A TiO2 interfacial layer is employed to produce high quality epitaxial growth of EuO on MgO(001) with a 45° in plane rotation. For comparison, direct deposition of EuO on MgO, without the TiO2 layer shows a much slower time evolution in producing a single crystal film. Conceptual arguments of electrostatic repulsion of like-ions are introduced to explain the increased EuO quality at the interface with the TiO2 layer. It is shown that ultrathin EuO films in the monolayer regime can be produced on the TiO2 surface by substrate-supplied oxidation and that such films have bulk Curie temperatures.

Critical micelle concentration of some surfactants and thermodynamic parameters of their micellization

Measurements of density, viscosity, conductivity and light scattering of aqueous solutions of sodium dodecylsulfate (SDDS), sodium hexadecylsulfonate (SHS), sodium N-lauryl sarcosinate (SDSa), cetyltrimethylammonium bromide (CTAB), cetylpyridinium bromide (CPyB), dodecyldimethyethylammonium bromide (DDEAB), tetradecyltrimethylammonium bromide (TTAB), benzyldimethyldodecylammonium bromide (BDDAB) and Triton X-100 (TX-100), Triton X-114 (TX-114), Triton X-165 (TX-165) were carried out at different temperatures. On the basis of the results obtained from these measurements critical micelle concentration, aggregation number, apparent and partial molar volume, standard Gibbs energy, enthalpy and entropy were calculated. In the case of ionic surfactants the degree of surfactant dissociation in micelle was taken into account. There was also determined the standard Gibbs energy of micellization using hydrophobic tail-water, hydrophilic head-water interfacial free energy and electrostatic intermolecular interactions. Then the results were compared with those obtained by other methods and the literature data. The presence of micelles at the concentration of aqueous surfactant solutions determined by the above mentioned methods was confirmed by the light scattering measurements.

Hb S-São Paulo: A new sickling hemoglobin with stable polymers and decreased oxygen affinity

Hb S-São Paulo (SP) [HBB:c.20A>T p.Glu6Val; c.196A>G p.Lys65Glu] is a new double-mutant hemoglobin that was found in heterozygosis in an 18-month-old Brazilian male with moderate anemia. It behaves like Hb S in acid electrophoresis, isoelectric focusing and solubility testing but shows different behavior in alkaline electrophoresis, cation-exchange HPLC and RP-HPLC. The variant is slightly unstable, showed reduced oxygen affinity and also appeared to form polymers more stable than the Hb S. Molecular dynamics simulation suggests that the polymerization is favored by interfacial electrostatic interactions. This provides a plausible explanation for some of the reported experimental observations.

First-principles study of the adsorption of Au atoms and Au2and Au4clusters on FeO/Pt(111)

Adsorption of Au atoms and Au-2 and Au-4 clusters on Pt(111)-supported bilayer FeO film were studied using first-principles density functional theory. For atom adsorption on the FeO/Pt(111) coincidence lattice, two types of adsorption were found, in agreement with previous studies. One is that the Au atom binds with one Fe cation lifted from underneath to the top of the oxygen layer (flipped adsorption), and the other is that the Au atom adsorbs directly on the oxygen layer (direct adsorption). Flipped adsorption was found to be energetically most favored in all domains of the Moire superstructure, while direct adsorption was either metastable or unstable. The charge of the Au adatom is negative for the flipped adsorption and positive for the direct adsorption. For the Au2 and Au4 clusters, the global most stable adsorption configurations on the FeO/Pt(111) were explored, and the results indicated that one-dimensional (dimer) or two-dimensional (Au-4) upright configurations of direct adsorption were energetically preferred over the flat or three-dimensional ones throughout the Moire superstructure. Moreover, it was found that the configurations of Au-2 dimers and Au-4 clusters on FeO/Pt(111) is tightly related to the interfacial interaction including two effects: the direction-dependent reactivity of a planar Au cluster and the interfacial electrostatic interaction between the Au clusters and the FeO/Pt(111) support. The role of relativistic effects in the configuration of Au clusters and the interaction with FeO/Pt(111) were illustrated. The evolution of small Au cluster configuration on FeO/Pt(111) was discussed.

Complement Inhibition by Staphylococcus aureus: Electrostatics of C3d–EfbC and C3d–Ehp Association

Virulence factors EfbC and Ehp from Staphylococcus aureus are potent inhibitors of complement activation. Both are excessively charged and bind to complement protein C3d at an acidic interface. We computationally generated single-alanine mutants of charged residues in the C3d–EfbC and C3d–Ehp complexes, and utilized electrostatic clustering and Poisson–Boltzmann free energy calculations to evaluate the role of electrostatics in association. Our results indicate that both interfacial electrostatic interactions and electrostatic potential distribution are crucial for C3d–EfbC and C3d–Ehp association. The results presented herein serve as a predictive tool in the selection of mutants with desired binding and immunological activity, and will narrow the search for viable candidates for further computational and experimental analyses. This study serves as the foundation for development of an inhibitor of the C3d–EfbC and C3d–Ehp interactions to combat bacterial infection and design of a complement inhibitor using EfbC and Ehp as a model.

Novel Zn–Al layered double hydroxide/carbon nanotube nanocomposite for electrochemical determination of catechol and hydroquinone

A novel nanocomposite of Zn–Al layered double hydroxide (LDH) and poly acrylic acid-functionalized multi-walled carbon nanotubes was assembled successfully through interfacial electrostatic interaction between the positively-charged layers of LDH nanocrystallites and the negatively-charged functional groups on the surface of functionalized carbon nanotubes. The resulting glassy carbon electrode modified by as-assembled nanocomposite exhibited excellent electrochemical activity toward the oxidation of catechol. The oxidation peak potential difference of catechol and hydroquinone was achieved at modified electrode, which could effectively distinguish catechol from hydroquinone. The present work provided a new method for electrochemical determination of dihydroxybenzene isomers with high sensitivity.

Weakly Coupled Lipid Bilayer Membranes on Multistimuli-Responsive Poly(N-isopropylacrylamide) Copolymer Cushions

Polymer-cushioned lipid bilayers are frequently used to mimic the native environment of cellular membranes in respect to the extracellular matrix and intracellular structures. With the aim to actively tune lipid membrane characteristics, we pursue the approach to use temperature and pH responsive polymer thin films of poly(N-isopropylacrylamide-co-carboxyacrylamide) (PNIPAAm-co-carboxyAAM) as cushions for supported lipid bilayers. A cationic lipid bilayer composed of dioleoylphosphatidylcholine (DOPC) and dioleoyltrimethylammoniumpropane (DOTAP) (9:1) was formed on top of the polymer thin film in a drying/rehydration process. Fluorescence recovery after photobleaching (FRAP) yielded higher lipid diffusion coefficients (6.3-9.6 μm(2) s(-1)) on polymer cushions in comparison to solid glass supports (3.0-5.9 μm(2) s(-1)). No correlation of the lipid mobility was found with the swelling state of (PNIPAAm-co-carboxyAAM), which is ascribed to restrained interfacial electrostatic interactions and dispersion forces. The results revealed a minimal coupling of the lipid bilayer with the polymer cushions, and thus, bilayers supported by (PNIPAAm-co-carboxyAAM) provide interesting opportunities for unperturbed lipid diffusion combined with control of transmembrane protein mobility due to the impact of a tunable frictional drag.

Effect of Sphingomyelin Headgroup Size on Molecular Properties and Interactions with Cholesterol

Sphingomyelins (SMs) and sterols are important constituents of the plasma membrane and have also been identified as major lipid components in membrane rafts. Using SM analogs with decreasing headgroup methylation, we systemically analyzed the effect of headgroup size on membrane properties and interactions with cholesterol. An increase in headgroup size resulted in a decrease in the main phase transition. Atom-scale molecular-dynamics simulations were in agreement with the fluorescence anisotropy experiments, showing that molecular areas increased and acyl chain order decreased with increasing headgroup size. Furthermore, the transition temperatures were constantly higher for SM headgroup analogs compared to corresponding phosphatidylcholine headgroup analogs. The sterol affinity for phospholipid bilayers was assessed using a sterol-partitioning assay and an increased headgroup size increased sterol affinity for the bilayer, with a higher sterol affinity for SM analogs as compared to phosphatidylcholine analogs. Moreover, the size of the headgroup affected the formation and composition of cholesterol-containing ordered domains. Palmitoyl-SM (the largest headgroup) seemed to attract more cholesterol into ordered domains than the other SM analogs with smaller headgroups. The ordering and condensing effect of cholesterol on membrane lipids was also largest for palmitoyl-SM as compared to the smaller SM analogs. The results show that the size of the SM headgroup is crucially important for SM-SM and SM-sterol interactions. Our results further emphasize that interfacial electrostatic interactions are important for stabilizing cholesterol interactions with SMs.

Design of photosensitizer/cyclodextrin nanoassemblies: spectroscopy, intracellular delivery and photodamage

The engineering of multifunctional nanoparticles carrying photosensitizer drugs (PS) and exposing binding groups for cellular receptors is of increasing interest in therapeutics and diagnostics applications. Natural and modified cyclodextrins (CDs) offer useful scaffolds to bind PS guests by supramolecular interactions. In particular, amphiphilic β-CDs, which form nanoaggregates of diverse shape and size according to the polarity of substituent groups on the rims, include in their different compartments as CD cavity, hydrophilic and hydrophobic portion, PS with different physicochemical properties. PS embedded in cationic amphiphilic CD nanoassemblies are effective in inducing photodynamic damage in cancer cells. For a carrier/PS system to be used in photodynamic therapy (PDT) or photodynamic diagnosis (PDD), the appropriate combination of the delivery characteristics with the preservation of the photodynamic activity of the PS is strictly required. Homogeneous multilayer films based on cationic amphiphilic β-CD entrapping anionic porphyrins can be constructed to exploit interfacial electrostatic interactions between the two components. The capability of CDs to generate restricted microenvironments for PS which can facilitate photoinduced energy transfer with suitable donor molecules was investigated for potential application in fluorescence diagnosis. Besides, recent findings suggest that PDT could represent a useful tool for properly addressing an alternative approach for killing pathogens and combating infections at a clinical level. Finally, modified CDs can bind gold nanoparticles, yielding hybrid organic/inorganic nanoparticles which were studied in water solution and after casting on solid substrates. These binary assemblies could further encapsulate PS or other conventional drugs, opening new intriguing routes on multimodal therapy.

Facile synthesis and novel electrocatalytic performance of nanostructured Ni–Al layered double hydroxide/carbon nanotube composites

Nanostructured Ni–Al layered double hydroxide/carbon nanotube (LDH/CNT) composites, where LDH nanocrystallites could highly disperse on the surface of CNTs matrix through the interfacial electrostatic interaction between the positively-charged layers of LDH and the negatively-charged functional groups of modified CNTs, have been successfully synthesized by a simple one-pot coprecipitation method. The materials were characterized by powder X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), thermogravimetry and differential scanning calorimetry (TG-DSC), transmission electron microscopy (TEM), Raman spectra, UV-visible absorption spectra (UV-VIS) and X-ray photoelectron spectra (XPS). The results reveal that the surface coverage of LDH nanoparticles onto CNTs could be tuned easily by changing the mass ratios of CNTs to LDH. The strong electrostatic interaction between LDH and CNTs gave rise to both the weakened affinity between the layers and the interlayer species of LDH and the high positive shift of binding energy for metal and oxygen elements (around 1.8 eV) in LDH/CNT composites. Further, the electrode modified by LDH/CNT nanocomposites exhibits eight times higher electrocatalytic activity for glucose electrooxidation than those modified by either pristine LDH or CNTs, which is attributable to the cooperative effects of the unique hybrid nanostructure and the composition of the as-synthesized nanostructured LDH/CNT composites. The present work provides a simple and feasible approach for incorporating two types of functional CNT and LDH materials into nanocomposites, which can result in the enhanced performance to meet advanced applications.

Dual‐Function Multilayers for the Photodelivery of Nitric Oxide and Singlet Oxygen

In this contribution we report the design, preparation and characterization of bichromophoric Langmuir-Schäfer multilayer films which enable the simultaneous generation of nitric oxide (NO) and singlet oxygen (1)O(2) ((1)Delta(g)). They are achieved by exploiting effective interfacial electrostatic interactions between the floating film of a cationic amphiphilic nitric oxide (NO) photodonor and an anionic porphyrin dissolved in the water subphase. The morphology and the spectroscopic features of the floating films at the water-air interface are inspected by Brewster angle microscopy and UV/Vis reflection spectroscopy, respectively. Multilayer films containing both the chromogenic centres are obtained by horizontal lifting deposition onto hydrophobized quartz substrates and are characterized by steady-state UV/Vis absorption, fluorescence and laser flash photolysis. The anionic porphyrin units are entangled into the films network mainly in their monomeric forms and no significant intra and inter-chromophoric interaction occur in the excited state, preserving the photochemical independency of the two photoactive units in the multilayers. The capability of the bichromophoric films to delivery NO and singlet oxygen, (1)O(2) ((1)Delta(g)), under light excitation is unambiguously demonstrated by the direct and in real time monitoring of these transient species using an ultrasensitive electrode and a time-resolved infrared luminescence apparatus, respectively.

Epitaxial Growth of Peptide Nanofilaments on Inorganic Surfaces: Effects of Interfacial Hydrophobicity/Hydrophilicity

Taking a stand: Self-assembled peptide aggregates adopt interfacial orientations in which they either lie horizontally (on highly ordered pyrolytic graphite, hydrophobic) or are oriented upright (on mica, hydrophilic) to form nanofilaments with different heights. This process is controlled by the interfacial hydrophobic or electrostatic interactions.

Adhesion of epoxy to hydrotalcite conversion coatings: II. Surface modification with ionic surfactants

Abstract Hydrotalcites (HT) are naturally occurring compounds being developed as environmentally benign conversion coatings for aluminum aerospace alloys. A successful conversion coating should provide corrosion resistance and act as a base for adhesion with subsequent organic layers. In the first paper of this series, the effect of interlayer anion substitution on the adhesion of epoxy primer was investigated. Interfacial tension and electrostatic interactions were found to influence adhesion between epoxy and conversion coatings. It was believed that increased adhesion could be attained by modification of these properties. This investigation examines the effect of ionic surfactants on the corrosion resistance and epoxy adhesion for HT-based conversion coatings. Post-treatment of a NO 3 HT conversion coating with a trialkyl ammonium surfactant (Adogen®-464) increased the pull-off tensile strength (POTS) of an epoxy coating by 55%. However, treatment with a linear anionic surfactant, sodium lauryl sulfate (SLS); a linear cationic surfactant, cetyltrimethylammonium bromide (CTAB); or a divalent cation (Ca 2+ ) had no effect on the epoxy POTS. Electrophoresis and the rising height method were employed to investigate the effect of the Adogen on electrostatic interactions and interfacial tension, respectively. Adogen decreased the isoelectric point (IEP) of NO 3 HT powders from 11.3 to 10.5. This was attributed to a shift in the Outer Helmholtz Plane (OHP) by the Adogen. A model epoxy molecule, N -methylethanolamine (NMEA), was used for contact angle measurements on conversion coating powders by the rising height method. The contact angle data was found to correlate with the epoxy dry adhesion rankings reported in Part 1 of this series. Adogen adsorption on NO 3 HT powders decreased the NMEA contact angle from 67.9 to 24.1°. Fourier Transform Infrared (FTIR) spectroscopy did not indicate primary bonding between NO 3 HT and Adogen. However, covalent bonding between NO 3 HT and NMEA (and, thus, epoxy) was suggested by FTIR. The improvement of epoxy adhesion on NO 3 HT by Adogen is believed to be due to the increased wettability of the HT.

Reduction potentials of vesicle-bound viologens

Thermodynamic reduction potentials have been determined by using spectroelectrochemical and cyclic voltammetric methods for a homologous series of amphiphilic ciologens (N-methyl-N{prime}-alkyl-4,4{prime}-bipyridinium ions, C{sub n}MV{sup 2+}) in a variety of media, including dihexadecyl phosphate (DHP), dioctadecylimethylammonium, and phosphatidylcholine small unilamellar vesicles. In general, potentials for both one-electron steps, i.e., C{sub n}MV{sup 2+} + e{sup {minus}} {yields} C{sub n}MV{sup +} and C{sub n}MV{sup +} + e{sup {minus}} {yields} C{sub n}MV{sup 0}, were insensitive to the alkyl chain length, which was varied over the range n = 6{minus}20. The single exception was a large decrease ({approximately}100 mV) in the first reduction potential for DHP-bound viologens when the chain length was increased from n = 10 to n = 12; this effect was attributed to a change in binding topography. The magnitudes of the reduction potentials were highly dependent upon the vesicle charge; the pattern observed indicated that interfacial electrostatic interactions between the surfactant headgroups and bipyridinium rings were the dominant factors determining the potentials. As discussed in the text, the data allow resolution of several heretofore puzzling observations concerning viologen reactivities in microphase suspensions.

Interfacial electrochemistry of cytochrome c at tin oxide, indium oxide, gold, and platinum electrodes

Cyclic voltammetry has been used to study the heterogeneous electron transfer kinetics of horse heart cytochrome c in pH 7 tris/cacodylate media at several electrode surfaces. Reversible voltammetric responses (formal heterogeneous electron transfer rate constant>10−2 cm/s) were observed at bare gold electrodes and at tin-doped indium oxide semiconductor electrodes for certain experimental conditions. Quasireversible voltammetric responses were more typically observed at fluorine-doped tin oxide semiconductor electrodes, bare platinum electrodes, and at the indium oxide electrodes. Reaction rates at bare metal electrodes were strongly dependent on pretreatment procedures and experimental protocol. Reaction rates at metal oxide electrodes were strongly dependent on solution conditions, pretreatment procedures, and on the hydration state of the electrode surface. A general mechanistic scheme involving both interfacial electrostatic and chemical interactions is proposed for cytochrome c electrode reactions. The asymmetric distribution of surface charges on cytochrome c appears to play a dominant role in controlling electron transfer rates by its interaction with the electric field at the electrode surface. Electron transfer distances are also considered, and it is concluded that electron transfer between an electrode surface and the exposed heme edge of properly oriented cytochrome c molecules involves maximum distances of ca. 0.6–0.9 nm.

Interfacial electrochemistry of cytochrome c at tin oxide, indium oxide, gold, and platinum electrodes

Cyclic voltammetry has been used to study the heterogeneous electron transfer kinetics of horse heart cytochrome c in pH 7 tris/cacodylate media at several electrode surfaces. Reversible voltammetric responses (formal heterogeneous electron transfer rate constant>10 −2 cm/s) were observed at bare gold electrodes and at tin-doped indium oxide semiconductor electrodes for certain experimental conditions. Quasireversible voltammetric responses were more typically observed at fluorine-doped tin oxide semiconductor electrodes, bare platinum electrodes, and at the indium oxide electrodes. Reaction rates at bare metal electrodes were strongly dependent on pretreatment procedures and experimental protocol. Reaction rates at metal oxide electrodes were strongly dependent on solution conditions, pretreatment procedures, and on the hydration state of the electrode surface. A general mechanistic scheme involving both interfacial electrostatic and chemical interactions is proposed for cytochrome c electrode reactions. The asymmetric distribution of surface charges on cytochrome c appears to play a dominant role in controlling electron transfer rates by its interaction with the electric field at the electrode surface. Electron transfer distances are also considered, and it is concluded that electron transfer between an electrode surface and the exposed heme edge of properly oriented cytochrome c molecules involves maximum distances of ca. 0.6–0.9 nm.