Positive Electrostatic Field Research Articles

Thermodynamic Driving Forces for Divalent Cations Binding to Zwitterionic Phospholipid Membranes.

We calculated the free energies for calcium, magnesium, and zinc ions binding to a zwitterionic phospholipid bilayer by using molecular dynamics simulations and the enhanced umbrella sampling technique. By decomposing the free energy into entropic and enthalpic contributions, we found that Ca2+ has the highest binding affinity and that the overall process is endothermic combined with a secondary exothermic process at higher ion concentrations. The relatively low dehydration free energy of Ca2+ allows it to release coordinated water upon binding to the membrane. The dehydrated Ca2+ further coordinates with lipids, resulting in a weaker influence on the water orientation and increased entropy. However, when sufficient Ca2+ ions are adsorbed, the concentrated cation layer induces a positive electrostatic field, which enhances the energy barrier for further ion binding and orients the adjacent water, resulting in decreased entropy. In contrast, binding of Mg2+ and Zn2+ is exothermic and less favored because they remain fully hydrated when interacting with lipids.

Magnetic Field Must Be Accompanied by Electric Field

 Not only the changing magnetic field produces an electric field, but also the static magnetic field is accompanied by an electric field. It is considered in the superposition principle of electrostatic field and the special relativity that the electric field around the moving electrons in permanent magnet still exists and presents length contraction in the direction of movement. Through experimental analysis, it is also considered that the motion states of two electric fields having same direction and opposite direction of motion cancel out each other. Based on the two above-mentioned basic concepts, it can be inferred that negative electrostatic field exists in the vicinity of cubic permanent magnet N and S pole surface edge while positive electrostatic field exists in the vicinity of side edge. In addition, the results of experimental verification are also introduced.

Impact of a charge sheet on neoclassical transport

Perpendicular electron heating in cyclotron waves accumulates resonant particles on the low magnetic field side, where an electron charge sheet may result (Hsu et al 1984 Phys. Rev. Lett. 53 564). Electrostatic fields are produced on both sides of the charge sheet. The impact of a charge sheet on transport is investigated based on neoclassical theory. A full particle simulation is performed using the Boris algorithm. The electrostatic field in the R-direction traps circulating particles or makes trapped particles circulate depending on the field direction. Transport coefficients are rigorously derived for such an electrostatic field. The electrostatic trapping or de-trapping changes the trapped particle population, bootstrap current and thermal conductivity. The positive electrostatic field corresponding to the negative biasing reduces transport, and then a thermal barrier is produced. Therefore, a strong positive electrostatic field can improve confinement. The transport behavior qualitatively agrees with a polarity study of biasing in TEXTOR, where the particle confinement time is three times lower in positive biasing.

Electric-Field-Driven Molecular Recognition Reactions of Guanine with 1,2-Dipalmitoyl-sn-glycero-3-cytidine Monolayers Deposited on Gold Electrodes

Monolayers of 1,2-dipalmitoyl-sn-glycero-3-cytidine were incubated with guanine in a 0.1 M NaF electrolyte at the surface of a Langmuir trough and transferred to gold (111) electrodes using the Langmuir-Schaefer technique. Chronocoulometry and photon polarization modulation infrared reflection absorption spectroscopy were employed to investigate the influence of the static electric field on the orientation and conformation of the cytidine nucleolipid molecules on the metal surface in the presence of guanine and to monitor the molecular recognition of guanine with the cytosine moiety. When the monolayer is exposed to guanine solutions, the cytosine moiety binds to the guanine residue in either a Watson-Crick complex at positively charged electrode surfaces or a noncomplexed state at negative surface charges. The positive electrostatic field causes the cytosine moiety and the cytosine-guanine complex to adopt a nearly parallel orientation with respect to the plane of the monolayer with a measured tilt angle of ∼10°. The parallel orientation is stabilized by the interactions between the permanent dipole of the cytosine moiety or the Watson-Crick complex and the static electric field. At negative charge densities, the tilt of the cytosine moiety increases by ∼15-20°, destabilizing the complex. Our results demonstrate that the static electric field has an influence on the molecular recognition reactions between nucleoside base pairs at the metal-solution interface and can be controlled by altering the surface charge at the metal.

An Insight into Electrostatic Field Effects on SO3 Adsorption by CaO with CO2, SO2 and H2O: A DFT Approach

ABSTRACT Because of the compulsory installation of electrostatic precipitators in coal-fired power plants, SO3 and particles in flue gas inevitably pass through areas with electric field and electric charge distributed. CaO is the highest alkaline content in fly ash and has strong interactions with SO3. Therefore, it is important to understand the effects of the electric field on the binding energies of SO3 and CaO. Density functional theory calculations were applied to examine the electrostatic field dependence of SO3 adsorption on the CaO(100) surface. The energetic, geometric and electronic nature of the modeled systems was analyzed. Trends of the adsorption energy variation in a positive or negative electrostatic field presented great differences. When the electrostatic field was small, it was observed that the required adsorption energy was reduced regardless of the positive or negative electrostatic field. With increased electrostatic field strength, the adsorption energy showed an increasing trend in a positive field, while the adsorption energy decreased in a negative field. The positive field enhanced the charge density changes of the surface O across the Ca around the OCaO, while the negative field enhanced the change of the charge density of the Ca below the OCaO. The SO42–-like structure accumulated electrons as a unit and the charge transfer of the SO42– increased with the incremental adsorption energy in a linear relationship. On the H pre-adsorbed surface, the total increment was much smaller than in previous cases in a positive field, and the total variation was negligible in a negative field.

Electrostatic attraction of weak monoacid anions increases probability for protonation and passage through aquaporins

A positive electrostatic field emanating from the center of the aquaporin (AQP) water and solute channel is responsible for the repulsion of cations. At the same time, however, a positive field will attract anions. In this regard, l-lactate/lactic acid permeability has been shown for various isoforms of the otherwise highly water and neutral substrate selective AQP family. The structural requirements rendering certain AQPs permeable for weak monoacids and the mechanism of conduction have remained unclear. Here, we show by profiling pH-dependent substrate permeability, measurements of media alkalization, and proton decoupling that AQP9 acts as a channel for the protonated, neutral monocarboxylic acid species. Intriguingly, the obtained permeability rates indicate an up to 10 times higher probability of passage via AQP9 than given by the fraction of the protonated acid substrate at a certain pH. We generated AQP9 point mutants showing that this effect is independent from properties of the channel interior but caused by the protein surface electrostatics. Monocarboxylic acid-conducting AQPs thus employ a mechanism similar to the family of formate-nitrite transporters for weak monoacids. On a more general basis, our data illustrate semiquantitatively the contribution of surface electrostatics to the interaction of charged molecule substrates or ligands with target proteins, such as channels, transporters, enzymes, or receptors.

Mechanism of formate-nitrite transporters by dielectric shift of substrate acidity.

Bacterial formate-nitrite transporters (FNTs) regulate the metabolic flow of small, weak mono-acids. Recently, the eukaryotic PfFNT was identified as the malaria parasite's lactate transporter and novel drug target. Despite crystal data, central mechanisms of FNT gating and transport remained unclear. Here, we show elucidation of the FNT transport mechanism by single-step substrate protonation involving an invariant lysine in the periplasmic vestibule. Opposing earlier gating hypotheses and electrophysiology reports, quantification of total uptake by radiolabeled substrate indicates a permanently open conformation of the bacterial formate transporter, FocA, irrespective of the pH Site-directed mutagenesis, heavy water effects, mathematical modeling, and simulations of solvation imply a general, proton motive force-driven FNT transport mechanism: Electrostatic attraction of the acid anion into a hydrophobic vestibule decreases substrate acidity and facilitates protonation by the bulk solvent. We define substrate neutralization by proton transfer for transport via a hydrophobic transport path as a general theme of the Amt/Mep/Rh ammonium and formate-nitrite transporters.

Energetics and dynamics of laser-assisted field evaporation: Time-dependent density functional theory simulations

High positive electrostatic (dc) field can break the bonds in molecules and strip away atoms from the solid surfaces. The dynamics of this field evaporation under laser pulse is studied for a ${\mathrm{H}}_{2}$ molecule and a ${\mathrm{Si}}_{4}$ cluster using time-dependent density functional theory combined with molecular dynamics. This allows us to monitor the position and charge state of the evaporated atom in real time. Our simulations demonstrate that the critical dc field for the evaporation is lower if the molecule/cluster is illuminated by a laser pulse. The behavior of the evaporation threshold as a function of laser intensity and dc field is in qualitative agreement with experiments and provides important insights into the mechanisms of laser-assisted field evaporation. Additionally, the laser-assisted field evaporation is found to be sensitive to the laser energy according to the photoabsorption spectra that demonstrate a pronounced redshift in the lower energy region at higher dc field values.

Destructive Effect of an α‐Particle on TEX

AbstractBy computational studies it was found that interaction of an α‐particle with TEX (an explosive molecule having a cage structure) disturbs the molecular skeleton dramatically. As the α‐particle exerts its positive electrostatic field, the C–C bond between the dioxalane rings present in TEX breaks down forming the dication of TEX. The calculation were done within the constraints of the density functional theory at the level of B3LYP using various bases sets, mainly 6‐31++G(d,p).

Inferring the Fact that Static Magnetic Field Exists Along with Electrostatic Field and Conducting Experimental Verification in Accordance with the Theory of Relativity

It is considered in the superposition principle of electrostatic field and the special relativity that the electric field around the moving electrons in permanent magnet still exists and presents length contraction in the direction of movement. Through experimental analysis, it is also considered that the motion states of two electric fields having same direction and opposite direction of motion cancel out each other. Based on the two above-mentioned basic concepts, it can be inferred that negative electrostatic field exists in the vicinity of cubic permanent magnet N and S pole surface edge while positive electrostatic field exists in the vicinity of side edge. In addition, the results of experimental verification are also introduced.

A Brownian Dynamics Study of the Interaction of Phormidium laminosum Plastocyanin with Phormidium laminosum Cytochrome f

The interaction of Phormidium laminosum plastocyanin (PC) with P. laminosum cytochrome f (cyt f) was studied using Brownian dynamics (BD) simulations. Few complexes and a low rate of electron transfer were observed for wild-type PC. Increasing the positive electrostatic field on PC by the addition of a Zn 2+ ion in the neighborhood of D44 and D45 on PC (as found in crystal structure of plastocyanin) increased the number of complexes formed and the calculated rates of electron transfer as did PC mutations D44A, D45A, E54A, and E57A. Mutations of charged residues on Phormidium PC and Phormidium cyt f were used to map binding sites on both proteins. In both the presence and absence of the Zn 2+ ion, the following residues on PC interact with cyt f: D44, D45, K6, D79, R93, and K100 that lie in a patch just below H92 and Y88 and D10, E17, and E70 located on the upper portion of the PC molecule. In the absence of the Zn 2+ ion, K6 and K35 on the top of the PC molecule also interact with cyt f. Cyt f residues involved in binding PC, in the absence of the Zn 2+ ion, include E165, D187, and D188 that are located on the small domain of cyt f. The orientation of PC in the complexes was quite random in accordance with NMR results. In the presence of the Zn 2+ ion, K53 and E54 in the lower patch of the PC molecule also interact with cyt f and PC interacts with E86, E95, and E123 on the large domain of cyt f. Also, the orientation of PC in the complexes was much more uniform than in the absence of the Zn 2+ ion. The difference may be due to both the larger electrostatic field and the greater asymmetry of the charge distribution on PC observed in the presence of the Zn 2+ ion. Hydrophobic interactions were also observed suggesting a model of cyt f-PC interactions in which electrostatic forces bring the two molecules together but hydrophobic interactions participate in stabilizing the final electron-transfer-active dock.

Specific Nonpeptide Inhibitors of Puromycin‐Sensitive Aminopeptidase with a 2,4(1H,3H)‐Quinazolinedione Skeleton.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Symmetry, Selectivity, and the 2003 Nobel Prize

I thank Rod MacKinnon, Chris Miller, Peter Agre, and Bob Stroud for valuable discussion, Robin Miller for Figure 1Figure 1, Bertil Hille for Figure 2Figure 2, and Andreas Engel for Figure 4BFigure 4B.

Solution NMR Structure of Ribosome-binding Factor A (RbfA), A Cold-shock Adaptation Protein from Escherichia coli

Ribosome-binding factor A (RbfA) from Escherichia coli is a cold-shock adaptation protein. It is essential for efficient processing of 16 S rRNA and is suspected to interact with the 5′-terminal helix (helix I) of 16 S rRNA. RbfA is a member of a large family of small proteins found in most bacterial organisms, making it an important target for structural proteomics. Here, we describe the three-dimensional structure of RbfAΔ25, a 108 residue construct with 25 residues removed from the carboxyl terminus of full-length RbfA, determined in solution at pH 5.0 by heteronuclear NMR methods. The structure determination was carried out using largely automated methods for determining resonance assignments, interpreting nuclear Overhauser effect (NOE) spectroscopy (NOESY) spectra, and structure generation. RbfAΔ25 has an α+β fold containing three helices and three β-strands, α1-β1-β2-α2-α3-β3. The structure has type-II KH-domain fold topology, related to conserved KH sequence family proteins whose βααβ subunits are characterized by a helix-turn-helix motif with sequence signature GxxG at the turn. In RbfA, this βααβ subunit is characterized by a helix-kink-helix motif in which the GxxG sequence is replaced by a conserved AxG sequence, including a strongly conserved Ala residue at position 75 forming an interhelical kink. The electrostatic field distribution about RbfAΔ25 is bipolar; one side of the molecule is strongly negative and the opposite face has a strong positive electrostatic field. A “dynamic hot spot” of RbfAΔ25 has been identified in the vicinity of a β-bulge at strongly conserved residue Ser39 by 15N R 1, R 2 relaxation rate and heteronuclear 15N– 1H NOE measurements. Analyses of these distributions of electrostatic field and internal dynamics, together with evolutionary implications of fold and sequence conservation, suggest that RbfA is indeed a nucleic acid-binding protein, and identify a potential RNA-binding site in or around the conserved polypeptide segment Ser76–Asp100 corresponding to the α3-loop-β3 helix-loop-strand structure. While the structure of RbfAΔ25 is most similar to that of the KH domain of the E. coli Era GTPase, its electrostatic field distribution is most similar to the KH1 domain of the NusA protein from Thermotoga maritima, another cold-shock associated RNA-binding protein. Both RbfA and NusA are regulated in the same E. coli operon. Structural and functional similarities between RbfA, NusA, and other bacterial type II KH domains suggest previously unsuspected evolutionary relationships between these cold-shock associated proteins.

The structure of a new sea anemone toxin: a molecular modeling study of cangitoxin

The pharmacological potential of sea anemone toxins has prompted the improvement of the biochemical and structural knowledge of such toxic proteins. Cangitoxin (CGX) is a new sea anemone toxin found in Bunudosoma cangicum. Long range molecular dynamics in vacuum and also with an explicit solvent representation was used to generate a structural model of CGX. The main structural features, three S–S bridges and anti-parallel β-sheet strands that characterize all known sea anemone toxins, are observed in the final CGX model. The structural analysis of CGX clearly suggests this protein just like a type-I sea anemone toxin. Additionally, the presence of an intensive positive electrostatic field related to positive residues at the vicinity of position 35 and a salt bridge configuration obtained for the Arg14 containing-loop could be directly involved in tissue discrimination and binding affinity of CGX to the Na + channel receptor.

Specific nonpeptide inhibitors of puromycin-sensitive aminopeptidase with a 2,4(1H,3H)-quinazolinedione skeleton.

Potent, specific, chemically stable and non-peptide/small-molecular inhibitors of puromycin-sensitive aminopeptidase, such as 3-(2,6-diethylphenyl)-2,4(1H,3H)-quinazolinedione (PAQ-22, 5), were prepared by the structural development of a potent PSA inhibitor, 2-(2,6-diethylphenyl)-1,2,3,4-tetrahydroisoquinoline-1,3-dione (PIQ-22, 4). The design was carried out partly by applying electrostatic potential field information obtained from PIQ-22 (4) and its derivatives based on thalidomide (2). This information revealed that a positive electrostatic potential field around the benzylic methylene in the tetrahydroisoquinoline ring is necessary for potent activity. Lineweaver-Burk plot analysis showed that PAQ-22 (5) and its derivatives inhibit puromycin-sensitive aminopeptidase (PSA) in a non-competitive manner. These potent and specific PSA inhibitors showed dose-dependent cell invasion-inhibitory activity in a Matrigel assay using mouse melanoma B16F10/L5 cells, in spite of their low cell toxicity.

Escherichia coli GlpE Is a Prototype Sulfurtransferase for the Single-Domain Rhodanese Homology Superfamily

Background: Rhodanese domains are structural modules occurring in the three major evolutionary phyla. They are found as single-domain proteins, as tandemly repeated modules in which the C-terminal domain only bears the properly structured active site, or as members of multidomain proteins. Although in vitro assays show sulfurtransferase or phosphatase activity associated with rhodanese or rhodanese-like domains, specific biological roles for most members of this homology superfamily have not been established. Results: Eight ORFs coding for proteins consisting of (or containing) a rhodanese domain bearing the potentially catalytic Cys have been identified in the Escherichia coli K-12 genome. One of these codes for the 12-kDa protein GlpE, a member of the sn-glycerol 3-phosphate ( glp) regulon. The crystal structure of GlpE, reported here at 1.06 Å resolution, displays α/β topology based on five β strands and five α helices. The GlpE catalytic Cys residue is persulfurated and enclosed in a structurally conserved 5-residue loop in a region of positive electrostatic field. Conclusions: Relative to the two-domain rhodanese enzymes of known three-dimensional structure, GlpE displays substantial shortening of loops connecting α helices and β sheets, resulting in radical conformational changes surrounding the active site. As a consequence, GlpE is structurally more similar to Cdc25 phosphatases than to bovine or Azotobacter vinelandii rhodaneses. Sequence searches through completed genomes indicate that GlpE can be considered to be the prototype structure for the ubiquitous single-domain rhodanese module.

13C NMR analysis of electrostatic interactions between NAD+ and active site residues of UDP-galactose 4-epimerase: implications for the activation induced by uridine nucleotides.

UDP-galactose 4-epimerase contains the coenzyme NAD+ bound tightly at the active site. NAD+ functions as the coenzyme for the interconversion of UDP-galactose and UDP-glucose by reversibly mediating their dehydrogenation to the common intermediate UDP-4-ketohexopyranoside. The epimerase structure and spectrophotometric data indicate that NAD+ may engage in electrostatic interactions with amino acid side chains that may regulate the reactivity of NAD+. In this work, we carried out NMR studies of [nicotinamide-4-13C]NAD+ bound to wild-type epimerase and epimerases mutated at amino acid residues in contact with NAD+. The 4-13C NMR chemical shifts revealed the following: The 4-13C chemical shift in wild-type epimerase is 149.9 ppm; mutation of Ser 124 to Ala changes it slightly by 0.2 ppm to 150.1 ppm; mutation of Tyr 149 to Phe results in a downfield perturbation of 2.7 ppm to 152.6 ppm; and the simultaneous mutation of Ser 124 to Ala and Tyr 149 to Phe also causes a downfield perturbation of 2.8 ppm to 152.7 ppm. Mutation of Lys 153 to Met results in a 13C chemical shift of 150.8 ppm, which is 0.9 ppm downfield from that of wild type and 1.8 ppm upfield from that of Y149F-epimerase. The 13C chemical shifts of nicotinamide C4 of NAD+ in these epimerases are correlated with their respective reactivities with NaBH3CN. In addition, reactivity of NAD+ in wild-type and S124A-epimerases displays pH dependence, with higher rates at lower pH where Tyr 149 in these two enzymes is protonated. The results support an electrostatic model in which repulsion between positively charged Lys 153 and N1 of the nicotinamide ring increases the reactivity of NAD+, while the phenolate of Tyr 149 opposes the positive electrostatic field and attenuates the reactivity of NAD+. Ser 124 has very little effect on the electron distribution within the nicotinamide ring or the reactivity of NAD+. The effects of binding the substrate analogue P1-uridyl-P2-methyl diphosphate (Me-UDP) on the 4-13C chemical shifts are opposite to those induced by the mutations. MeUDP perturbs the 4-13C chemical shift 2.9 ppm downfield in the wild-type and S124A-epimerases but has little or no effect in the cases of Y149F- or K153M-epimerases. The results support the postulate that NAD+ activation induced by uridine nucleotides is brought about by a conformational change of epimerase that repositions Tyr 149 at an increased distance from nicotinamide N1 of NAD+ while maintaining the electrostatic repulsion between Lys 153 and nicotinamide N1 of NAD+.

Electrostatic steering and ionic tethering in the formation of thrombin-hirudin complexes: the role of the thrombin anion-binding exosite-I.

Electrostatic interactions between the thrombin anion-binding exosite-I (ABE-I) and the hirudin C-terminal tail play an important role in the formation of the thrombin-hirudin inhibitor complex and serves as a model for the interactions of thrombin with its many other ligands. The role of each solvent exposed basic residue in ABE-I (Arg(35), Lys(36), Arg(67), Arg(73), Arg(75), Arg(77a), Lys(81), Lys(109), Lys(110), and Lys(149e)) in electrostatic steering and ionic tethering in the formation of thrombin-hirudin inhibitor complexes was explored by site directed mutagenesis. The contribution to the binding energy (deltaG(degrees)b) by each residue varied from 1.9 kJ mol(-)(1) (Lys(110)) to 15.3 kJ mol(-1) (Arg(73)) and were in general agreement to their observed interactions with hirudin residues in the thrombin-hirudin crystal structure [Rydel, T. J., Tulinsky, A., Bode, W., and Huber, R. (1991) J. Mol. Biol. 221, 583-601]. Coupling energies (delta deltaG(degrees) int) were calculated for the major ion-pair interactions involved in ionic tethering using complementary hirudin mutants (h-D55N, h-E57Q, and h-E58Q). Cooperativity was seen for the h-Asp(55)/Arg(73) ion pair (2.4 kJ mol(-1)); however, low coupling energies for h-Asp(55)/Lys(149e) (deltadeltaG(degrees)int 0.6 kJ mol(-1)) and h-Glu(58)/Arg(77a) (deltadeltaG(degrees)int 0.9 kJ mol(-1)) suggest these are not major interactions, as anticipated by the crystal structure. Interestingly, high coupling energies were seen for the intermolecular ion-pair h-Glu(57)/Arg(75) (deltadeltaG(degrees)int 2.3 kJ mol(-1)) and for the solvent bridge h-Glu(57)/Arg(77a) (deltadeltaG(degrees)int 2.7 kJ mol(-1)) indicating that h-Glu(57) interacts directly with both Arg(75) and Arg(77a) in the thrombin-hirudin inhibitor complex. The remaining ABE-I residues that do not form major contacts in tethering the C-terminal tail of hirudin make small but collectively important contributions to the overall positive electrostatic field generated by ABE-I important in electrostatic steering.

Neighboring group effect of pyridazine and pyrazine rings for π-facial selectivity in the reactions of fused isopropylidenenorbornene systems with electrophilic reagents

A series of pyridazine- and pyrazine-fused isopropylidenenorbornenes have been synthesized and the π-facial selectivity of the electrophilic reactions with 4-phenyl-1,2,4-triazole-3,5(4H)-dione (PTAD), m-chloroperbenzoic acid (MCPBA) and N-bromosuccinimide (NBS) has been investigated. The ene reactions with PTAD exhibited exclusive syn selectivity to the heteroaromatic rings except for an isopropylidenenorbornene fused with a pyridazine N-oxide ring. The epoxidations with MCPBA and the ene reactions with NBS afforded mixtures of syn and anti isomers depending on the heteroaromatic rings and substituents. The predominant syn selectivity compared with that of a benzene-fused congener may be attributed to the presence of a strong positive electrostatic potential field over the heteroaromatic ring to stabilize a polar transition state by the electrostatic interaction.