Electrostatic Interaction Forces Research Articles

Binding and release studies of a cationic drug from a star-shaped four-arm poly(ethylene oxide)-b-poly(methacrylic acid)

Star-shape polymers possess higher densities of terminal functional groups and three-dimensional tetrahedron structure that induce significantly different association and interactions with drug compared to linear structure of identical molecular weights. Four-arm poly(ethylene oxide)-b-poly(methacrylic acid) block copolymer was synthesized by atom transfer radical polymerization technique, and it self-assembled into core-shell micelles and extended unimers at low and high pH respectively. The negatively charged carboxylate groups on the polymer chains interacted with a cationic drug through electrostatic interaction forming polymer/drug complexes stabilized by biocompatible hydrophilic PEO segments. The hydrodynamic radius (Rh) of the polymeric aggregates and polymer/drug complexes ranged from 46 to 84 nm and 32 to 55 nm at pH of 4.6 and 8.0 respectively, making them suitable for drug delivery applications. The thermodynamic parameters and interactions between polymer and drug were determined by isothermal titration calorimetric technique. The electrostatic force, hydrogen bonding and hydrophobic interactions controlled the characteristics of polymer/drug formation and complexes when the molar ratios of drug and polymer were varied. Drug selective electrode system was used to measure the dynamic release of imipramine hydrochloride (IPH) from multi-arm PEO-b-PMAA star polymer. The release exponent n was greater than 0.5 indicating a non-Fickian type diffusion behavior, where the release behavior was dominated by chain relaxation induced by ion exchange that was dependent on pH. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:782–793, 2010

Pegylated Phospholipids-Based Self-Assembly with Water-Soluble Drugs

To investigate the self-assembly of polyethylene glycol (PEG)-phosphatidylethanolamine (PE) conjugate with water-soluble drugs (doxorubicin hydrochloride, vinorelbine tartrate and vincristine sulfate) and give insight into the mechanism of formation and mode of interaction of the drug with PEG-PE as well as the general principles of self-assembly using pegylated lipid micelles. One-step self-assembly method to prepare drug-loaded micelles was developed. The micelles were characterized by dynamic light scattering, transmission electron microscopy, encapsulation efficiency, and release study. NMR was used to study molecular assembly of PEG-PE with doxorubicin. Doxorubicin hydrochloride and vinorelbine tartrate were entrapped into micelles with high efficiency of >99.0% at molar ratios of 1:1 and 2:1 of PEG-PE to drugs, respectively. Drug loading did not measurably perturb either the geometry or the size. It was found that electrostatic interaction and hydrophobic forces are responsible for the intercalation of drugs into PEG-PE micelles. NMR data revealed that the anthracycline ring of doxorubicin was inserted between PE phospholipids, and its amino sugar located in the outer shell of micelle between PEG chains. Based on our results, the structure and self-assembly mechanism of water-soluble drugs encapsulated in PEG-PE micelles were proposed.

Structure of a biologically active binuclear palladium complex in the compound bis(bis[(μ2-2-chloroethylammonium) (1,10-phenanthroline)-palladium(I)]) · octanitrates · dihydrates

The structure of a biologically active binuclear palladium complex, namely, [(1,10-phenanthroline) Pd(μ2-2-chloroethylammonium) 2 4+ · 4NO 3 − · H2O, has been determined using X-ray diffraction analysis. The compound crystallizes in the monoclinic system, space group of symmetry Cc, with the unit cell parameters a = 24.527(3) A, b = 13.097(1) A, c = 22.651(3) A, β = 104.23(1)°, V = 7052(2) A3, Z = 8, and ρ = 1.88 g/cm3. The final discrepancy factor is R1 = 0.0316 for 15581 symmetrically nonequivalent reflections with I ≥ 2σ(I), wR2 = 0.0513, and GooF = 1.047. The palladium atoms reside in the oxidation state of 1+, which is rarely encountered in organometallic compounds. The asymmetric part of the unit cell contains two chemically equivalent but crystallographically independent positively charged binuclear palladium complexes, eight NO 3 − anions, and two water molecules. The π-π stacking interaction between the nearest 1,10-phenanthroline rings of the neighboring layers takes place. Moreover, the interlayer and intralayer interactions occur through electrostatic interaction forces and a complex three-dimensional system of hydrogen bonds, which are formed by water molecules and N 3 + groups.

Adhesion of Spores of Bacillus thuringiensis on a Planar Surface

Adhesion of spores of Bacillus thuringiensis (Bt) and spherical silica particles on surfaces was experimentally and theoretically investigated in this study. Topography analysis via atomic force microscopy (AFM) and electron microscopy indicates that Bt spores are rod shaped, approximately 1.3 mum in length and approximately 0.8 mum in diameter. The adhesion force of Bt spores and silica particles on gold-coated glass was measured at various relative humidity (RH) levels by AFM. It was expected that the adhesion force would vary with RH because the individual force components contributing to the adhesion force depend on RH. The adhesion force between a particle and a planar surface in atmospheric environments was modeled as the contribution of three major force components: capillary, van der Waals, and electrostatic interaction forces. Adhesion force measurements for Bt spore (silica particle) and the gold surface system were comparable with calculations. Modeling results show that there is a critical RH value, which depends on the hydrophobicity of the materials involved, below which the water meniscus does not form and the contribution of the capillary force is zero. As RH increases, the van der Waals force decreases while the capillary force increases to a maximum value.

Measurement of Molecular Diffusion Based on Optoelectrofluidic Fluorescence Microscopy

This technical note reports a method for measuring the diffusion coefficient of molecules based on an optoelectrofluidic platform. Optoelectrofluidic fluorescence microscopy, which is constructed with an optoelectrofluidic device and a conventional fluorescence microscope, is a useful tool for controlling and detecting local molecular concentration in a fluid with a single light source. When we project a light for fluorescence excitation and apply an ac signal of a few hundred Hertz frequency around 100 Hz to the optoelectrofluidic device, a sudden decay of molecular concentration occurs within the illuminated area due to several mechanisms, including optically induced ac electroosmosis, electrostatic interaction forces among the polarized molecules, and interactions among the molecules and between the molecules and the electric double layer. After the applied voltage was turned off, the dispersed molecules diffuse into the molecular depletion area and the fluorescence signal is recovered. On the basis of these phenomena, we successfully measured the diffusion coefficient of various dextran molecules. A statistical analysis to determine the significance of our experimental values compared to the previously reported values measured using fluorescence recovery after photobleaching technology was also performed. This new technique demonstrates the first analytical measurement of diffusion based on the optoelectrofluidic platform and can be a useful tool for measuring the mobility of molecules in a simple and easy way.

Spectroscopic Studies on the Binding of Cobalt(II) 1,10-Phenanthroline Complex to Bovine Serum Albumin

The binding interaction of the cobalt(II) 1,10-phenanthroline complex (Co(phen) (3) (2+) , phen = 1,10-phenanthroline) with bovine serum albumin (BSA) was investigated by fluorescence spectroscopy combined with UV-Vis absorption and circular dichroism measurements under simulative physiological conditions. The experiment results showed that the fluorescence intensity of BSA was dramatically decreased owing to the formation of Co(phen) (3) (2+) -BSA complex. The corresponding association constants (K (a)) between Co(phen) (3) (2+) and BSA at four different temperatures were calculated according to the modified Stern-Volmer equation. The enthalpy change (DeltaH degrees ) and entropy change (DeltaS degrees ) were calculated to be -2.73 kJ mol(-1) and 82.27 J mol(-1) K(-1), respectively, which suggested that electrostatic interaction and hydrophobic force played major roles in stabilizing the Co(phen) (3) (2+) -BSA complex. Site marker competitive experiments indicated that the binding of Co(phen) (3) (2+) to BSA primarily took place in site I of BSA. A value of 4.11 nm for the average distance r between Co(phen) (3) (2+) (acceptor) and tryptophan residues of BSA (donor) was derived from Förster's energy transfer theory. The conformational investigation showed that the presence of Co(phen) (3) (2+) resulted in the change of BSA secondary structure and induced the slight unfolding of the polypeptides of protein, which confirmed the microenvironment and conformational changes of BSA molecules.

Electrostatic force microscopy and potentiometry of realistic nanostructured systems

We investigate the dependency of electrostatic interaction forces on applied potentials in electrostatic force microscopy (EFM) as well as in related local potentiometry techniques such as Kelvin probe microscopy (KPM). The approximated expression of electrostatic interaction between two conductors, usually employed in EFM and KPM, may loose its validity when probe-sample distance is not very small, as often realized when realistic nanostructured systems with complex topography are investigated. In such conditions, electrostatic interaction does not depend solely on the potential difference between probe and sample, but instead it may depend on the bias applied to each conductor. For instance, electrostatic force can change from repulsive to attractive for certain ranges of applied potentials and probe-sample distances, and this fact cannot be accounted for by approximated models. We propose a general capacitance model, even applicable to more than two conductors, considering values of potentials applied to each of the conductors to determine the resulting forces and force gradients, being able to account for the above phenomenon as well as to describe interactions at larger distances. Results from numerical simulations and experiments on metal stripe electrodes and semiconductor nanowires supporting such scenario in typical regimes of EFM investigations are presented, evidencing the importance of a more rigorous modeling for EFM data interpretation. Furthermore, physical meaning of Kelvin potential as used in KPM applications can also be clarified by means of the reported formalism.

The Major Surface-Associated Saccharides of Klebsiella pneumoniae Contribute to Host Cell Association

Analysing the pathogenic mechanisms of a bacterium requires an understanding of the composition of the bacterial cell surface. The bacterial surface provides the first barrier against innate immune mechanisms as well as mediating attachment to cells/surfaces to resist clearance. We utilised a series of Klebsiella pneumoniae mutants in which the two major polysaccharide layers, capsule and lipopolysaccharide (LPS), were absent or truncated, to investigate the ability of these layers to protect against innate immune mechanisms and to associate with eukaryotic cells. The capsule alone was found to be essential for resistance to complement mediated killing while both capsule and LPS were involved in cell-association, albeit through different mechanisms. The capsule impeded cell-association while the LPS saccharides increased cell-association in a non-specific manner. The electrohydrodynamic characteristics of the strains suggested the differing interaction of each bacterial strain with eukaryotic cells could be partly explained by the charge density displayed by the outermost polysaccharide layer. This highlights the importance of considering not only specific adhesin:ligand interactions commonly studied in adherence assays but also the initial non-specific interactions governed largely by the electrostatic interaction forces.

Micellization of Surfactin and Its Effect on the Aggregate Conformation of Amyloid β(1-40)

The aggregation of amyloid beta-peptide (Abeta(1-40)) into fibrils is a key pathological process associated with Alzheimer's disease. This work has investigated the micellization process of biosurfactant surfactin and its effect on the aggregation behavior of Abeta(1-40). The results show that surfactin has strong self-assembly ability to form micelles and the micelles tend to form larger aggregates. Surfactin adopts a beta-turn conformation at low micelle concentration but a beta-sheet conformation at high micelle concentration. The effect of surfactin on the Abeta(1-40) aggregation behavior exhibits a strong concentration-dependent fashion. Below the critical micelle concentration of surfactin, the electrostatic binding of surfactin monomers on Abeta(1-40) causes Abeta(1-40) molecules to unfold. Assisted by the hydrophobic interaction among surfactin monomers on the Abeta(1-40) chain, the conformation of Abeta(1-40) transfers to the beta-sheet structure, which promotes the formation of fibrils. At low surfactin micelle concentration, besides the electrostatic force and hydrophobic interaction, hydrogen bonds formed between surfactin micelles and adjacent Abeta(1-40) peptide chains may promote the ordered organization of these Abeta(1-40) peptide chains, thus leading to the formation of beta-sheets and fibrils to a great extent. At high surfactin micelle concentration, the separating of Abeta(1-40) chains by the excessive surfactin micelles and the aggregation of the complexes of Abeta(1-40) with surfactin micelles inhibit the formation of beta-sheets and fibrils.

Thermodynamic Analysis of the Interaction of Lipopolysaccharides with Cationic Compounds

AbstractBacterial lipopolysaccharides (LPS) from Gram‐negative bacteria, located in the outer leaflet of their outer membrane, are called endotoxins due to their ability to induce a variety of biological effects in mammals. Their lipid moiety, lipid A, is called “the endotoxic principle” and is responsible for the toxic effects of LPS. As a result of the polyanionic character of LPS, the study of the interaction with divalent cations Mg2+ and Ca2+ and polycationic peptides such as polymyxin B (PMB) and its nonapeptide PMBN is of considerable interest, and therefore the authors have investigated the interaction of LPS/lipid A with cationic compounds by applying isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC). The data indicate a clear binding of the divalent cations with the anionic glycolipids, leading to calorimetric reactions, such as an increase in the phase transition temperature, Tm, of the gel to the liquid crystalline phase of LPS, indicating a stabilization of the gel phase. The peptides react quite differently as assessed by DSC. In contrast to the interaction of divalent cations with the glycolipids, a destabilization of the gel phase is observed, accompanied by a decrease in the gel to liquid crystalline phase transition enthalpy for the peptide‐glycolipid interaction. The extent of this effect is peptide‐concentration‐dependent. Using ITC for the analysis of the binding reaction of the cations and the peptides with the glycolipid in the liquid crystalline phase, strong exothermic effects are observed. These are indicative of the dominance of electrostatic attractions between the reaction partners. Interestingly, Ca2+ binding to LPS leads to a slightly exothermic reaction, whereas Mg2+ binding leads to an endothermic reaction (some kJ/mol). The observed highly endothermic binding reactions for the lipid‐peptide interaction in the gel phase are mainly driven by a gain in entropy. This is explained by the fact that during binding, water molecules from the hydration shells of the components are liberated. Although the electrostatic attraction is still the driving force of the interaction, it is quantitatively of minor importance for the interaction in the gel phase. The binding results are discussed in terms of competition between electrostatic interaction and hydration forces. These data are of importance for the understanding of the reaction mechanisms of cationic compounds with LPS under physiological conditions.

Considering the First Steps toward a Stable and Orderly Way of Bacterial Life

Bacteria are small unicellular organisms who could well enjoy a bohemian life—moving independently wherever and whenever they want to and existing with no regard for conventional rules of behavior. In spite of this apparent freedom, most bacteria abandon their footloose lifestyle as soon as they come into contact with a surface. Irrespective of whether the surface is of biotic or abiotic origin, they clinch to it, forgoing independence in favor of settling down. Similar to animals that gather in flocks and people who live in societies, surface-attached microbes can form networks as multicellular communities called biofilms. Bacterial biofilms are heterogeneous structures of increasing complexity that consist of differently specialized cells enclosed in a self-produced polymeric matrix associated with the surface [1]. Depending on the setting and the composition of biofilms, they may have either beneficial or detrimental effects on our environment and health. One of the most serious concerns about biofilms is their high antibiotic tolerance, which makes the treatment of infections difficult and contributes to the spread of antibiotic resistance among pathogenic bacteria [2]. A high antibiotic tolerance in biofilm bacteria can partly be explained by a surface-induced change in gene expression, but how does this happen? What are the first critical steps towards an orderly life on a surface and how are these controlled? These questions have puzzled scientists for a considerable amount of time, and still do. Attempts to solve these questions can be roughly divided into two major approaches: one focusing on physicochemical aspects of cell–surface interactions, the other aiming at elucidating the expression of adhesion-specific genes. When bacteria approach a surface they encounter an energy barrier, and a balance of repulsive and attractive forces determines whether adhesion occurs. Several theoretical models originally developed for colloidal particles have been used to describe this process, including the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [3], the thermodynamic approach [4], and an extended DLVO theory [5]. However, the predictions made for cell–surface interactions in terms of electrostatic interaction forces or interfacial energy have only limited success, because they do not take into account the heterogeneity of bacterial cell surfaces [6]. Bacterial cells not only have a variety of cell surface structures, they also sense and respond to changes in their environment by immediately adjusting their gene expression, which results in dynamic cell surface alterations. Among surface-induced cellular changes, an altered expression of cell envelope components is of particular interest because it directly affects the mode of intimate cell–surface contact. To understand the genetic basis of the decisions bacteria make upon surface contact, many studies have focused either on the characterization of adhesion factors or on the isolation of biofilm-deficient mutants. During the past decade, a huge interest in biofilm research has resulted in amazing insights into how various cell surface structures affect the rate and extent of attachment, e.g., [7–10], how biofilms grow and develop in coordinated steps, e.g., [11–16], how the extracellular matrix is produced, e.g., [17,18], and how bacteria inside biofilms communicate via signal molecules, e.g., [17,19]. However, the very first steps that actually cause bacteria to stick to a surface and that are required to trigger reprogramming of gene expression are still not well understood. To a large extent, this lack of knowledge is due to a lack of appropriate methodology.

Coupled Electrostatic, Hydrodynamic, and Mechanical Properties of Bacterial Interfaces in Aqueous Media

The interactions of bacteria with their environment are governed by a complex interplay between biological and physicochemical phenomena. The main challenge is the joint determination of the intertwined interfacial characteristics of bacteria such as mechanical and hydrodynamic softness, interfacial heterogeneity, and electrostatic properties. In this study, we have combined electrokinetics and force spectroscopy to unravel this intricate coupling for two types of Shewanella bacterial strains that vary according to the nature of their outer, permeable, charged gel-like layers. The theoretical interpretation of the bacterial electrokinetic response allows for the estimation of the hydrodynamic permeability, degree of interfacial heterogeneity, and volume charge density for the soft layer that constitutes the outer permeable part of the bacteria. Additionally, the electrostatic interaction forces between an AFM probe and the bacteria were calculated on the basis of their interfacial properties obtained from advanced soft particle electrokinetic analysis. For both bacterial strains, excellent agreement between experimental and theoretical force curves is obtained, which highlights the necessity to account for the interfacial heterogeneity of the bioparticle to interpret AFM and electrokinetic data consistently. From the force profiles, we also derived the relevant mechanical parameters in relation to the turgor pressure within the cell and the nature of the bacterial outer surface layer. These results corroborate the heterogeneous representation of the bacterial interface and show that the decrease in the turgor pressure of the cell with increasing ionic strength is more pronounced for bacteria with a thin surface gel-like layer.

Activity of Membrane Proteins Immobilized on Surfaces as a Function of Packing Density

A systematic study of the influence of the packing density of proteins on their activity is performed with cytochrome c oxidase (CcO) from R. sphaeroides as an example. The protein was incorporated into a protein-tethered bilayer lipid membrane and CcO was genetically engineered with a histidine-tag, attached to Subunit II, and then tethered by an interaction with functionalized thiol compounds bound to a gold electrode. The packing density was varied by diluting the functionalized thiol with a nonfunctionalized thiol that does not bind to the enzyme. After attaching the CcO to the gold surface, a lipid bilayer was formed to incorporate the tethered proteins. The reconstituted protein-lipid bilayer was characterized by surface enhanced infrared reflection absorption spectroscopy (SEIRAS), electrochemical impedance spectroscopy, surface plasmon resonance, and atomic force microscopy. The activity of the proteins within the reconstituted bilayer was probed by direct electrochemical electron injection and was shown to be very sensitive to the packing density of protein molecules. At low surface density of CcO, the bilayer did not effectively form, and protein aggregates were observed, whereas at very high surface density, very little lipid is able to intrude between the closely packed proteins. In both of these cases, redox activity, measured by the efficiency to accept electrons, is low. Redox activity of the enzyme is preserved in the biomimetic structure but only at a moderate surface coverage in which a continuous lipid bilayer is present and the proteins are not forced to aggregate. Electrostatic and other interaction forces between protein molecules are held responsible for these effects.

Interaction of rac-[Cu(diimine)3]2+ and rac-[Zn(diimine)3]2+ complexes with CT DNA: effect of fluxional Cu(ii) geometry on DNA binding, ligand–promoted exciton coupling and prominent DNA cleavage

The complexes [Cu(phen)(3)](ClO(4))(2) 1, [Cu(5,6-dmp)(3)](ClO(4))(2) 2, [Cu(dpq)(3)](ClO(4))(2) 3, [Zn(phen)(3)](ClO(4))(2) 4, [Zn(5,6-dmp)(3)](ClO(4))(2) 5 and [Zn(dpq)(3)](ClO(4))(2) 6, where phen = 1,10-phenanthroline, 5,6-dmp = 5,6-dimethyl-1,10-phenanthroline and dpq = dipyrido[3,2-d:2',3'-f]quinoxaline, have been isolated, characterized and their interaction with calf thymus DNA studied by using a host of physical methods. The X-ray crystal structures of rac-[Cu(5,6-dmp)(3)](ClO(4))(2) and rac-[Zn(5,6-dmp)(3)](ClO(4))(2) have been determined. While 2 possesses a regular elongated octahedral coordination geometry (REO), 5 possesses a distorted octahedral geometry. Absorption spectral titrations of the Cu(II) complexes with CT DNA reveal that the red-shift (12 nm) and DNA binding affinity of 3 (K(b), 7.5 x 10(4) M(-1)) are higher than those of 1 (red-shift, 6 nm; K(b), 9.6 x 10(3) M(-1)) indicating that the partial insertion of the extended phen ring of dpq ligand in between the DNA base pairs is deeper than that of phen ring. Also, 2 with a fluxional Cu(II) geometry interacts with DNA (K(b), 3.8 x 10(4) M(-1)) more strongly than 1 suggesting that the hydrophobic forces of interaction of 5,6 methyl groups on the phen ring is more pronounced than the partial intercalation of phen ring in the latter with a static geometry. The DNA binding affinity of 1 is lower than that of its Zn(ii) analogue 4, and, interestingly, the DNA binding affinity 2 of with a fluxional geometry is higher than that of its Zn(II) analogue 5 with a spherical geometry. It is remarkable that upon binding to DNA 3 shows an increase in viscosity higher than that the intercalator EthBr does, which is consistent with the above DNA binding affinities. The CD spectra show only one induced CD band on the characteristic positive band of CT DNA upon interaction with the phen (1,4) and dpq (3,6) complexes. In contrast, the 5,6-dmp complexes 2 and 5 bound to CT DNA show exciton-coupled biphasic CD signals with 2 showing CD signals more intense than 5. The Delta-enantiomer of rac-[Cu(5,6-dmp)(3)](2+) 2 binds specifically to the right-handed B-form of CT DNA at lower ionic strength (0.05 M NaCl) while the Lambda-enantiomer binds specifically to the left-handed Z-form of CT DNA generated by treating the B-form with 5 M NaCl. The complex 2 is stabilized in the higher oxidation state of Cu(II) more than its phen analogue 1 upon binding to DNA suggesting the involvement of electrostatic forces in DNA interaction of the former. In contrast, 3 bound to DNA is stabilized as Cu(I) rather than the Cu(II) oxidation state due to partial intercalative interaction of the dpq ligand. The efficiencies of the complexes to oxidatively cleave pUC19 DNA vary in the order, 3> 1 >> 2 with 3 effecting 100% cleavage even at 10 microM complex concentration. However, interestingly, this order is reversed when the DNA cleavage is performed using H(2)O(2) as an activator and the highest cleavage efficiency of 2 is ascribed to its electrostatic interaction with the exterior phosphates of DNA.

Protein‐responsive imprinted polymers with specific shrinking and rebinding

Stimuli-responsive protein imprinted polymers were obtained via a combination of molecular imprinting and reversible stimuli-responsive polymer using lysozyme or cytochrome c as template, N-isopropylacrylamide (NIPA) as major monomer, methacrylic acid (MAA) and acrylamide (AAm) as functional co-monomers, and N,N-methylenebisacrylamide (MBAAm) as crosslinker. The molecularly imprinted polymers (MIPs) can respond not only to external stimuli such as temperature and salt concentration, but also to the corresponding template protein with significant specific volume shrinking. This specific shrinking behavior was attributed to the synergistic effect of multiple-site weak interactions (electrostatic force, hydrogen bonding and hydrophobic interaction) and the cavity effect. The MIPs showed highly selective adsorption of template proteins with specific shrinking compared with the non-imprinted polymers. The results indicated that the MIPs seemed to change shape to accommodate the conformation of the template protein leading to the formation of a shape complementary cavity.

Protein patterning on self-assembled polyelectrolyte thin films

This study demonstrated a simple micropatterning method using engineered surface and microcontact printing (μCP). For the selection of appropriate surface, several representative surfaces modified with various functional materials including aldehyde, epoxide, poly- l-lysine, amine, and self-assembled polyelectrolyte multilayers (PEL) were investigated. The PEL coated surface endowing electrostatic interaction force showed most high functionality in point of homogeneous patterning of proteins with high density and preservation of inherent three-dimensional structure of proteins. The printed sizes of protein micropatterns were almost identical to the used microstamp, which proves the μCP method is independent of diffusion of the transferring proteins. At last, the immunoassay as a model system of protein–protein interaction showed good linearity over a range from 100 to 25 μg/ml, indicating the feasibility of a quantitative measurement of the concentration of target proteins in sample. Our proposed approach will be useful for the development of biomedical microdevices because of green chemistry, simple process, and high potential of various applications.

Variational treatment of electrostatic interaction force in atomic force microscopy

In this paper we introduce the mathematical model for the electrostatic interaction force between an atomic force microscope (AFM) tip and a sample surface. We formulate the electrostatic potential problem in Sobolev spaces and find the corresponding weak solution in terms of the integral potential, which can be approximated numerically by generalized Fourier series and used to find the interaction force between an AFM tip and a sample surface. The formulation of the problem in a weak (Sobolev) space setting allows us to determine the force for AFM tips of arbitrary shape. Efficiency of the method is illustrated using numerical examples for the spherical and tetrahedral AFM tips.

Mechanics of Force Propagation in TonB-Dependent Outer Membrane Transport

For the uptake of scarce yet essential organometallic compounds, outer membrane transporters of Gram-negative bacteria work in concert with an energy-generating inner membrane complex, thus spanning the periplasmic space to drive active transport. Here, we examine the interaction of TonB, an inner membrane protein, with an outer membrane transporter based upon a recent crystal structure of a TonB-transporter complex to characterize two largely unknown steps of the transport cycle: how energy is transmitted from TonB to the transporter and how energy transduction initiates transport. Simulations of TonB in complex with BtuB reveal that force applied to TonB is transmitted to BtuB without disruption of the very small connection between the two, supporting a mechanical mode of coupling. Based on the results of different pulling simulations, we propose that the force transduction instigates a partial unfolding of the pore-occluding luminal domain of the transporter, a potential step in the transport cycle. Furthermore, analysis of the electrostatic potentials and salt bridge interactions between the two proteins during the simulations hints at involvement of electrostatic forces in long-range interaction and binding of TonB and BtuB.

Shielding and Interaction of Dust Particles in Non‐Equilibrium Plasma

AbstractThis paper investigates the interaction of two dust particles in non‐equilibrium plasma at elevated pressures. On the basis of asymptotic shielding theory resulting in two‐exponential shielding the electrostatic energy of two charged fine particles is determined. The electrostatic energy dependence on interparticle distance proves to have a minimum as in an equilibrium plasma. The electrostatic interaction force of the particles is calculated. This force is proved to be non‐symmetric: for different charges the force acting on one dust particle was not equal to the force acting on the other dust particle. This is due to non‐symmetric charge separation in the vicinity of different charged dust particles. It is stated that likely charged particles repel each other and at the closest approach the attraction is only possible. Relationships to determine the modified nonideality parameter for the interaction potential, which consisted of two exponents with different shielding lengths, were obtained. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Role of Solution Chemistry and Ion Valence on the Adhesion Kinetics of Groundwater and Marine Bacteria

The role of solution chemistry on bacterial adhesion has been investigated using a radial stagnation point flow (RSPF) system. This experimental system utilized an optical microscope and an image-capturing device to directly observe the deposition kinetics of a groundwater bacterium, Burkholderia cepacia G4g, and a marine bacterium, Halomonas pacifica g. Experiments were carried out under well-controlled hydrodynamic and solution chemistry conditions, allowing for the sensitivity of bacterial adhesion behavior to be examined under a range of ionic strength and valence (KCl vs CaCl2) simulating groundwater and marine environments. Complimentary cell characterization techniques were conducted to evaluate the electrophoretic mobility, hydrophobicity, surface charge density, and viability of the bacteria under the same range of conditions. Solution chemistry was found to have a marked effect on the electrokinetic and surface properties of bacteria and the quartz collector, as well as on the resulting rate of bacterial deposition. Comparable adhesion trends were observed for B. cepacia G4g and H. pacifica g. Specifically, the deposition rates of the two bacteria species in both KCl and CaCl2 solutions increased with ionic strength, a trend consistent with traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which considers the combination of van der Waals and electrostatic double-layer interaction forces. However, in some cases, experimental results showed bacterial deposition behavior to deviate from DLVO predictions. On the basis of the systematic investigation of bacterial cell characteristics, it was found that Ca2+ ions play a distinct role on bacterial surface charge, hydrophobicity, and deposition behaviors. It is further suggested that bacterial adhesion is determined by the combined influence of DLVO interactions, electrosteric interactions associated with solution chemistry, and the hydrodynamics of the deposition system.