Ion Bridges Research Articles

The role of strong electrostatic interactions at the dimer interface of human glutathione synthetase.

The obligate homodimer human glutathione synthetase (hGS) provides an ideal system for exploring the role of protein-protein interactions in the structural stability, activity and allostery of enzymes. The two active sites of hGS, which are 40 Å apart, display allosteric modulation by the substrate γ-glutamylcysteine (γ-GC) during the synthesis of glutathione, a key cellular antioxidant. The two subunits interact at a relatively small dimer interface dominated by electrostatic interactions between S42, R221, and D24. Alanine scans of these sites result in enzymes with decreased activity, altered γ-GC affinity, and decreased thermal stability. Molecular dynamics simulations indicate these mutations disrupt interchain bonding and impact the tertiary structure of hGS. While the ionic hydrogen bonds and salt bridges between S42, R221, and D24 do not mediate allosteric communication in hGS, these interactions have a dramatic impact on the activity and structural stability of the enzyme.

Hydration of gaseous m-aminobenzoic acid: ionic vs neutral hydrogen bonding and water bridges.

Hydration of a protonated amine and a neutral carboxylic acid were investigated for protonated m-aminobenzoic acid (MABAH(+)) with up to 15 water molecules attached using infrared photodissociation spectroscopy, laser-induced dissociation kinetics, and computational chemistry. A free COO-H stretch in the spectra of MABAH(+)·(H2O)1-5 indicates that water does not bind to the carboxylic acid H atom. This band is absent in the spectrum of MABAH(+) with six or more water molecules attached, and there is a hydrogen-bonded (HB) COO-H stretch indicating that water hydrogen bonds to the carboxylic acid H atom for these larger clusters. Photodissociation kinetic data for MABAH(+)·(H2O)6 indicate that greater than 74 ± 13% of the ion population consists of the HB COO-H isomer, consistent with this isomer being ≥0.5 kJ mol(-1) lower in energy than isomers where the carboxylic acid H atom does not donate a hydrogen bond. Calculations at the B3LYP/6-31+G** and MP2/6-31+G**//B3LYP/6-31+G** levels of theory indicate that this energy difference is 3-5 kJ mol(-1), in agreement with the experimental results. Lower effective ion heating rates, either by attenuation of the laser power or irradiation of the ions at a lower frequency, result in more time for interconversion between the free and HB COO-H isomers. These data suggest that the barrier to dissociation for the free COO-H isomer is less than that for the HB COO-H isomer but greater than the barrier for interconversion between the two isomers. These results show the competition between hydration of a primary protonated amine vs that of a neutral carboxylic acid and the effect of water bridging between the two functional groups, which provide valuable insight into the hydration of protonated amino acids and establish rigorous benchmarks for theoretical modeling of water-biomolecule interactions.

Influence of Ca2+ on the performance of poly(acrylic acid)-g-poly(ethylene glycol) comb-like copolymers in cement suspensions

Poly(acrylic acid)-g-poly(ethylene glycol) (PAA-g-PEG) comb-like copolymer (PCE) is one of the best kind of superplasticizers for cement suspensions. But PCE can be considered as polyelectrolyte which is easily affected by the ions in cement suspensions. In cement system, calcium ions are the most abundant cations which would have great influence on the performance of polymer superplasticizer. Here, the effects of calcium ion on the surface charge, adsorption and dispersion in cement system with PCE as dispersant are investigated in detail. And the changes in the conformation of PCE before and after adding calcium ions are studied by methods of dynamic light scattering and transmission electron microscopy. Calcium ions increase the zeta potential of cement particles and make the molecular chains of PCE curlier, which induces the changes in adsorption and dispersion behavior. The adding of calcium ions makes the ion bridging between calcium and PCE to take place and the ion bridging makes the PCE molecules shrink to smaller size. In this paper, it tells that the molecular structure of PCE polymer could be changed based on the conclusion to adapt the cement with different content of calcium. In cement suspensions, the effect of calcium ions on the performance of PCE polymer would be weakened by decreasing the content of carboxyl groups in the molecular chain.

Mixed-valent diruthenium diphosphonate containing zigzag chain structure of {Ru2(hedp)2(SCN)}n4n−

This paper reports the crystal structure and magnetism properties of a novel mixed-valent diruthenium(II,III) complex, [C(NH2)3]4[Ru2(hedp)2(SCN)]·4H2O (1), where hedp represents 1-hydroxyethylidenediphosphonate. It shows a zigzag chain structure in which the paddlewheel diruthenium(II,III) dimers of Ru2(hedp)23− linked by SCN− bridges. The chains are stacked along the [011] direction with strong intra- and interchain hydrogen bonds. Magnetic studies show that significant antiferromagnetic exchanges are mediated between the [Ru2(hedp)2]3− (S=3/2) units through thiocyanate ion bridges. Structural analysis proves that the RuRu bond weakening ability is H2O<Br−<Cl−<μ-NCS−<μ-NC−<μ-SCN−.

Molecular Adhesion between Cartilage Extracellular Matrix Macromolecules

In this study, we investigated the molecular adhesion between the major constituents of cartilage extracellular matrix, namely, the highly negatively charged proteoglycan aggrecan and the type II/IX/XI fibrillar collagen network, in simulated physiological conditions. Colloidal force spectroscopy was applied to measure the maximum adhesion force and total adhesion energy between aggrecan end-attached spherical tips (end radius R ≈ 2.5 μm) and trypsin-treated cartilage disks with undamaged collagen networks. Studies were carried out in various aqueous solutions to reveal the physical factors that govern aggrecan–collagen adhesion. Increasing both ionic strength and [Ca2+] significantly increased adhesion, highlighting the importance of electrostatic repulsion and Ca2+-mediated ion bridging effects. In addition, we probed how partial enzymatic degradation of the collagen network, which simulates osteoarthritic conditions, affects the aggrecan–collagen interactions. Interestingly, we found a significant increase in aggrecan–collagen adhesion even when there were no detectable changes at the macro- or microscales. It is hypothesized that the aggrecan–collagen adhesion, together with aggrecan–aggrecan self-adhesion, works synergistically to determine the local molecular deformability and energy dissipation of the cartilage matrix, in turn, affecting its macroscopic tissue properties.

Bovine α1-acid glycoprotein, a thermostable version of its human counterpart: Insights from Fourier transform infrared spectroscopy and in silico modelling

α1-Acid glycoprotein (AGP) is a plasma protein and a member of the acute phase response. AGP is known to bind and carry several biologically active compounds, as well as to down-modulate the immune system activities. In this work, the structure of bovine AGP has been investigated by Fourier-Transform infrared spectroscopy. A model structure has been obtained on the basis of human AGP and refined by molecular dynamics. In spite of the similar structure, bovine AGP shows an unexpectedly higher (∼20 °C) thermostability than its human counterpart. Inspection of the model structure has pointed out the presence of 12 ionic bridges and 2 sulphur-aromatic interactions, whereas only 6 ionic bridges were detected in human AGP. The high number (9) of glutamic acid residues involved in the ionic interactions might explain the significantly decreased thermostability measured at pH 5.5 (Tm ∼ 71 °C) with respect to pH 7.4 (Tm ∼ 81 °C), whereas thermostability of human AGP was only slightly affected by lowering the pH. As in human AGP and several other lipocalins, a temperature-induced molten globule state has been observed in the denaturation pathway of bovine AGP.

Heuristic consequences of a load of oxygen in microtubules

The current cell oxygen paradigm shows some major gaps that have not yet been resolved. Something seems to be lacking for the comprehensive statement of the oxygen distribution in the cell, especially the low cytoplasmic oxygen level. The entrapment of oxygen in microtubules (MTs) resolves the latter observation, as well as the occurrence of an extensive cytoplasmic foam formation. It leads to a novel oxygen paradigm for cells. During the steady-state treadmilling, the mobile cavity would absorb oxygenated cytoplasm forward, entrap gas nuclei and concentrate them. A fluorescence method is described to confirm the in vitro load of oxygen in MTs during their periodic growths and shrinkages. The latter operating mechanism is called the gas dynamic instability (GDI) of MTs. Several known biosystems could rest on the GDI. (1) The GTP-cap is linked with the gas meniscus encountered in a tube filled with gas. The GTP hydrolysis is linked to the conformational change of the GTPase domain according to the bubble pressure, and to the shaking of protofilaments with gas particles (soliton-like waves). (2) The GDI provides a free energy water pump because water molecules have to escape from MT pores when foam concentrates within the MT. Beside ATP hydrolysis in motor proteins, the GDI provides an additional driving force in intracellular transport of cargo. The water streams flowing from the MT through slits organize themselves as water layers between the cargo and the MT surface, and break ionic bridges. It makes the cargo glide over a water rail. (3) The GDI provides a universal motor for chromosome segregation because the depolymerization of kinetochorial MTs is expected to generate a strong cytoplasmic foam. Chromosomes are sucked up according to the pressure difference (or density difference) applied to opposite sides of the kinetochore, which is in agreement with Archimedes' principle of buoyancy. Non-kinetochorial MTs reabsorb foam during GDI. Last, the mitotic spindle is imagined as a gas recycler. (4) The luminal particles within MTs (called MIPs) are imagined as a foam organizer, the luminal proteins being part of the borders and edges of identical bubbles. (5) Last, volatile anesthetics could destabilize MTs through anesthetic-induced bubble nucleation between protofilaments, and therefore causing shear stress and the opening of MT. The load of oxygen in MTs might provide a major advance in this area of research.

Comparison of the π-stacking properties of purine versus pyrimidine residues. Some generalizations regarding selectivity

Aromatic-ring stacking is pronounced among the noncovalent interactions occurring in biosystems and therefore some pertinent features regarding nucleobase residues are summarized. Self-stacking decreases in the series adenine>guanine>hypoxanthine>cytosine~uracil. This contrasts with the stability of binary (phen)(N) adducts formed by 1,10-phenanthroline (phen) and a nucleobase residue (N), which is largely independent of the type of purine residue involved, including (N1)H-deprotonated guanine. Furthermore, the association constant for (phen)(A)(0/4-) is rather independent of the type and charge of the adenine derivative (A) considered, be it adenosine or one of its nucleotides, including adenosine 5'-triphosphate (ATP(4-)). The same holds for the corresponding adducts of 2,2'-bipyridine (bpy), although owing to the smaller size of the aromatic-ring system of bpy, the (bpy)(A)(0/4-) adducts are less stable; the same applies correspondingly to the adducts formed with pyrimidines. In accord herewith, [M(bpy)](adenosine)(2+) adducts (M(2+)is Co(2+), Ni(2+), or Cu(2+)) show the same stability as the (bpy)(A)(0/4-) ones. The formation of an ionic bridge between -NH3 (+) and -PO3 (2-), as provided by tryptophan [H(Trp)(±)] and adenosine 5'-monophosphate (AMP(2-)), facilitates recognition and stabilizes the indole-purine stack in [H(Trp)](AMP)(2-). Such indole-purine stacks also occur in nature. Similarly, the formation of a metal ion bridge as occurs, e.g., between Cu(2+) coordinated to phen and the phosphonate group of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA(2-)) dramatically favors the intramolecular stack in Cu(phen)(PMEA). The consequences of such interactions for biosystems are discussed, especially emphasizing that the energies involved in such isomeric equilibria are small, allowing Nature to shift such equilibria easily.

Synergy of photocatalysis and adsorption for simultaneous removal of Cr(VI) and Cr(III) with TiO2 and titanate nanotubes

An one-step efficient simultaneous removal of Cr(VI) and Cr(III) was achieved with mixture of TiO2 and titanate nanotubes (TNTs). Unlike the conventional two-step Cr removal with a first photocatalytic reduction of Cr(VI) and a subsequent adsorption of Cr(III), the proposed single process significantly reduced reaction time (over 50%). The synergy of photocatalysis and adsorption played an important role in enhancing Cr removal process. The synergetic mechanism was interpreted and indirectly confirmed with H2O2 variation during photocatalysis. The instant transfer of the reduced Cr from TiO2 surface to TNTs interlayer greatly promoted the release of photocatalytic sites of TiO2, which in turn considerably enhanced photocatalytic activity of TNTs by inhibiting electron–hole pairs recombination. The optimum condition for the whole process was at pH 5. Adsorption of Cr(III) was primarily in the interlayer of TNTs at pH ≤ 5. However, higher pH would lead to precipitation of Cr(OH)3 onto TNTs as observed by X-ray photoelectron spectroscopy (XPS). Addition of Ca2+ could promoted photocatalysis owing to its ionic bridging function and form of ≡TiOH+-Cr(VI)–Ca2+-Cr(VI) linkages, while SO42− only slightly inhibited photo-reduction of Cr(VI), indicating good synergy of photocatalysis and adsorption even at high ionic strength of electrolyte. Besides, the desorbed TNTs could be easily regenerated by remedying the damaged tubular structure and reused for Cr removal with excellent performance. The outstanding synergetic effects with essential explanation of the mechanism make this study not only fundamentally important but also potentially practical applicable.

Molecular dynamics simulations reveal the assembly mechanism of polysaccharides in marine aerosols.

The high Arctic marine environment has recently detected polymer gels in atmospheric aerosol particles and cloud water originating from the surface microlayer of the open leads within the pack ice area. These polysaccharide molecules are water insoluble but water solvated, highly surface-active and highly hydrated (99% water). In order to add to the understanding and to complement missing laboratory characterization of marine polymer gels we have in this work performed an atomistic study of the assembly process and interfacial properties of polysaccharides. Our study reveals a number of salient features of the microscopic process behind polysaccharide assembly into nanogels. With three- and four-repeating units the polysaccharides assemble into a cluster in 50 ns. The aggregates grow quicker by absorbing one or two polymers each time, depending on the unit length and the type of inter-bridging cation. Although both the hydrophobic and hydrophilic domains are contracted, the latter dominates distinctly upon the contraction of solvent accessible surface areas. The establishment of inter-chain hydrogen-bonds is the key to the assembly while ionic bridges can further promote aggregation. During the assembly of the more bent four-unit polymers, intra-chain hydrogen bonds are significantly diminished by Ca(2+). Meanwhile, the percentage of Ca(2+) acting as an ionic bridge is more eminent, highlighting the significance of Ca(2+) ions for longer-chain polysaccharides. The aggregates are able to enhance surface tension more in the presence of Ca(2+) than in the presence of Na(+) owing to their more compact structure. These conclusions all demonstrate that studies of the present kind provide insight into the self-assembly process and interfacial properties of marine gels. We hope this understanding will keep up the interest in the complex and the fascinating relationship between marine microbiology, atmospheric aerosols, clouds and climate.

Ultraviolet Photofunctionalization of Titanium Implants

In the face of growing demands and challenges in implant therapy, implant surfaces with improved biologic capabilities are required. This review paper summarizes the findings of recent in vitro and in vivo studies related to ultraviolet (UV) photofunctionalization of titanium. UV photofunctionalization is defined as an overall phenomenon of modification of titanium surfaces occuring after UV treatment, including the alteration of physicochemical properties and the enhancement of biologic capabilities. Bone morphogenesis around UV-treated titanium implants is distinctly improved compared with that seen around untreated control implants, leading to rapid and complete establishment of osseointegration with nearly 100% bone-to-implant contact in an animal model, as opposed to less than 55% for untreated implants. A series of in vitro studies demonstrated considerable enhancement of attachment, retention, and subsequent functional cascades of osteogenic cells derived from animals and humans after UV treatment. UV treatment converts titanium surfaces from hydrophobic to superhydrophilic and removes unavoidably contaminated hydrocarbons. UV-treated titanium surfaces also manifest a unique electrostatic status and act as direct cell attractants without the aid of ionic and organic bridges, which imparts a novel physicochemical functionality to titanium, which has long been understood as a bioinert material. UV treatment is simple and low in cost, and it has been proven effective for all types of titanium surfaces tested. These data suggest that UV photofunctionalization can be a novel, effective measure to improve implant therapy in the dental and orthopedic fields. Future research will focus on validating these findings in clinical studies.

Dermcidin, an anionic antimicrobial peptide: influence of lipid charge, pH and Zn2+on its interaction with a biomimetic membrane

The mechanism of membrane permeabilization by dermcidin (DCD-1L), an antimicrobial peptide present in human sweat, was investigated at a mercury-supported monolayer of dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylserine (DOPS) and at a mercury-supported tethered bilayer lipid membrane (tBLM) consisting of a thiolipid (DPTL) with a DOPC or DOPS monolayer self-assembled on top of it. In an unbuffered solution of pH 5.4, DCD-1L is almost neutral and permeabilizes a DPTL/DOPS tBLM at transmembrane potentials, ϕtrans, which are physiological. In a pH 7 buffer solution DCD-1L bears two negative charges and has no effect on a DPTL/DOPC tBLM, whereas it permeabilizes a DPTL/DOPS tBLM only outside the physiological ϕtrans range; however, the presence of zinc ion induces DCD-1L to permeabilize the DPTL/DOPS tBLM at physiological ϕtrans values. The effect of zinc ions suggests a DCD-1L conformation with its positive N-terminus embedded in the lipid bilayer and the negative C terminus floating on the membrane surface. This conformation can be stabilized by a zinc ion bridge between the His(38) residue of the C terminus and the carboxyl group of DOPS. Chronocoulometric potential jumps from ϕtrans ≅ +160 mV to sufficiently negative values yield charge transients exhibiting a sigmoidal shape preceded by a relatively long "foot". This behavior is indicative of ion-channel formation characterized by disruption of DCD-1L clusters adsorbed on top of the lipid bilayer, incorporation of the resulting monomers and their aggregation into hydrophilic pores by a mechanism of nucleation and growth.

Overcoming Target-Mediated Quinolone Resistance in Topoisomerase IV by Introducing Metal-Ion-Independent Drug–Enzyme Interactions

Quinolones, which target gyrase and topoisomerase IV, are the most widely prescribed antibacterials worldwide. Unfortunately, their use is threatened by the increasing prevalence of target-mediated drug resistance. Greater than 90% of mutations that confer quinolone resistance act by disrupting enzyme-drug interactions coordinated by a critical water-metal ion bridge. Quinazolinediones are quinolone-like drugs but lack the skeletal features necessary to support the bridge interaction. These compounds are of clinical interest, however, because they retain activity against the most common quinolone resistance mutations. We utilized a chemical biology approach to determine how quinazolinediones overcome quinolone resistance in Bacillus anthracis topoisomerase IV. Quinazolinediones that retain activity against quinolone-resistant topoisomerase IV do so primarily by establishing novel interactions through the C7 substituent, rather than the drug skeleton. Because some quinolones are highly active against human topoisomerase IIα, we also determined how clinically relevant quinolones discriminate between the bacterial and human enzymes. Clinically relevant quinolones display poor activity against topoisomerase IIα because the human enzyme cannot support drug interactions mediated by the water-metal ion bridge. However, the inclusion of substituents that allow quinazolinediones to overcome topoisomerase IV-mediated quinolone resistance can cause cross-reactivity against topoisomerase IIα. Therefore, a major challenge in designing drugs that overcome quinolone resistance lies in the ability to identify substituents that mediate strong interactions with the bacterial, but not the human, enzymes. On the basis of our understanding of quinolone-enzyme interactions, we have identified three compounds that display high activity against quinolone-resistant B. anthracis topoisomerase IV but low activity against human topoisomerase IIα.

Quantification of the interactions between Ca2+, Hg2+ and extracellular polymeric substances (EPS) of sludge

The interactions between metals (Ca2+ and Hg2+) and extracellular polymeric substances (EPS) extracted from the aerobic and anaerobic sludge in wastewater treatment reactors were investigated using a combination of zeta potential measurement and 3-dimensional excitation–emission matrix (EEM) fluorescence spectroscopy with parallel factor (PARAFAC) analysis. Results show that Ca2+ had no substantial effects on the EEM fluorescence spectra of the EPS, but their zeta potentials increased with the increasing Ca2+ dosage. However, Hg2+ had a significant effect on the EEM fluorescence spectra of the EPS, while their zeta potentials seemed not to be affected by the dose of Hg2+. The interactions between Hg2+ and EPS were elucidated using the fluorescence quenching with PARAFAC analysis, while the interactions between Ca2+ and EPS were evaluated by the zeta potential technique. The binding constants for Hg2+ and EPS were two orders of magnitude higher than those for Ca2+ and EPS, suggesting that the binding mechanisms between Ca2+ and EPS were different from those between Hg2+ and EPS. The results might be useful for understanding the roles of EPS in bacterial self-protection against heavy metals and the aggregate formation mechanisms through ionic bridging interactions.

Interactions of tetracycline with Cd (II), Cu (II) and Pb (II) and their cosorption behavior in soils

Tetracycline (TC), a common antibiotic, can behave as an efficient ligand with cations, but the effect of its interaction with heavy metal cations on the mobility of both species in soils has not been well evaluated. In this study, the complexation affinities of TC with Cd (II), Cu (II) and Pb (II) were examined using potentiometric titration and spectroscopic methods. The cosorption behavior of TC and metal ions onto three selected Chinese soils was evaluated using batch adsorption experiments. The presence of metal cations promoted TC adsorption through an ion bridging effect in the order Cu (II) > Pb (II) > Cd (II), which is in accordance with their complexation ability with TC. The addition of TC affects metal adsorption differently depending on the solution pH and metal type. Therefore, it is necessary to consider the complexation ability of TC and divalent metal cations when evaluating their mobility in soils.

Comparison on aggregation and sedimentation of titanium dioxide, titanate nanotubes and titanate nanotubes-TiO2: Influence of pH, ionic strength and natural organic matter

Aggregation and sedimentation are of significant importance to the safe applications of nanomaterials. In this study, aggregation and sedimentation of three homologous nanomaterials, titanium dioxide (TiO2), titanate nanotubes (TNs), and titanate nanotubes–TiO2 (TNs–TiO2) were systematically studied under various aqueous conditions. The influences of pH, ionic strength (Na+ and Ca2+) and natural organic matter (humic acid, HA) on aggregate size, zeta potential and sedimentation rate were totally investigated. It was found that maximum aggregation invariably occurred at pH close to point of zero charge. However, the three nanomaterials underwent different aggregation processes, even though aggregation was always greatly enhanced with increasing ionic strength in either Na+ or Ca2+ solutions. In Ca2+ solutions, the aggregate size of TiO2 tended to increase solely with increasing ionic strength, while the size of TNs and TNs–TiO2 increased initially before decreasing, due to the surface charge changing from negative to positive at high ionic strengths. The aggregation energy barrier was enhanced considerably by HA, thus inhibiting aggregation. TNs and TNs–TiO2 exhibited a greater energy barrier transition than TiO2 due to their higher adsorption capacity for HA, and moreover, the co-existence of Ca2+ also exhibited ion bridge effects on aggregation. Different microstructure and chemical composition of the nanomaterials resulted in different aggregation and sedimentation behaviors, and the micro-heterogeneity of TNs–TiO2 led to smaller size but higher sedimentation rate than TNs.

LT1002 Metalloantibody Uses Ca2+ Cofactor

The biologically‐active lipid, Sphingosine‐1‐phosphate (S1P), is a signaling molecule present in cell membranes that promotes heart disease, and tumor growth. Murine, Immunoglobulin G (IgG) LT1002 and human IgG LT1009 specifically bind S1P and inhibit its effects. X‐ray crystallography revealed that two Ca2+ bridge LT1009 and its S1P antigen. This was the first direct structural evidence of metal ion bridging in an antibody: antigen complex. While the role of Ca2+ in the complex is clear, it is unknown to what extent Ca2+ binds in the absence of antigen and whether other biologically significant metals interact with the antibody. To this end, we have conducted quantitative analysis comparing the binding enthalpy, dissociation constant, and stoichiometry of LT1002 and nonmetal‐binding IgG binding to biologically relevant, divalent cations. Isothermal titration calorimetry (ITC) revealed 300uM [LT1002] bound 20mM [Ca2+] with a KD= 238uM with a LT1002:Ca2+ stoichiometry of 1:3. Ba2+, another metal ion that was observed to bind free LT1002 in previous studies, showed no binding to LT1002 by ITC and Ca2+ failed to bind to control murine IgG1. ITC data, in conjunction with the crystal structure of an antigen‐free antibody: Ca2+ complex, indicate that Ca2+ specifically binds to the epitope region of LT1002.Supported by NIH MBRS‐IMSD Grant 5R25GM058906

Topoisomerase IV-quinolone interactions are mediated through a water-metal ion bridge: mechanistic basis of quinolone resistance

Although quinolones are the most commonly prescribed antibacterials, their use is threatened by an increasing prevalence of resistance. The most common causes of quinolone resistance are mutations of a specific serine or acidic residue in the A subunit of gyrase or topoisomerase IV. These amino acids are proposed to serve as a critical enzyme-quinolone interaction site by anchoring a water-metal ion bridge that coordinates drug binding. To probe the role of the proposed water-metal ion bridge, we characterized wild-type, GrlAE85K, GrlAS81F/E85K, GrlAE85A, GrlAS81F/E85A and GrlAS81F Bacillus anthracis topoisomerase IV, their sensitivity to quinolones and related drugs and their use of metal ions. Mutations increased the Mg2+ concentration required to produce maximal quinolone-induced DNA cleavage and restricted the divalent metal ions that could support quinolone activity. Individual mutation of Ser81 or Glu85 partially disrupted bridge function, whereas simultaneous mutation of both residues abrogated protein–quinolone interactions. Results provide functional evidence for the existence of the water-metal ion bridge, confirm that the serine and glutamic acid residues anchor the bridge, demonstrate that the bridge is the primary conduit for interactions between clinically relevant quinolones and topoisomerase IV and provide a likely mechanism for the most common causes of quinolone resistance.

Molecular interactions of Alzheimer amyloid-β oligomers with neutral and negatively charged lipid bilayers

Interaction of p3 (Aβ(17-42)) peptides with cell membranes is crucial for the understanding of amyloid toxicity associated with Alzheimer's disease (AD). Such p3-membrane interactions are considered to induce the disruption of membrane permeability and integrity, but the exact mechanisms of how p3 aggregates, particularly small p3 oligomers, induce receptor-independent membrane disruption are not yet completely understood. Here, we investigate the adsorption, orientation, and surface interaction of the p3 pentamer with lipid bilayers composed of both pure zwitterionic POPC (palmitoyl-oleoyl-phosphatidylcholine) and mixed anionic POPC-POPG (palmitoyl-oleoyl-phosphatidylglycerol) (3 : 1) lipids using explicit-solvent molecular dynamics (MD) simulations. MD simulation results show that the p3 pentamer has much stronger interactions with mixed POPC-POPG lipids than pure POPC lipids, consistent with experimental observation that Aβ adsorption and fibrillation are enhanced on anionic lipid bilayers. Although electrostatic interactions are main attractive forces to drive the p3 pentamer to adsorb on the bilayer surface, the adsorption of the p3 pentamer on the lipid bilayer with C-terminal β-strands facing toward the bilayer surface is a net outcome of different competitions between p3 peptides-lipid bilayer and ions-p3-bilayer interactions. More importantly, Ca(2+) ions are found to form ionic bridges to associate negatively charged residues of p3 with anionic headgroups of the lipid bilayer, resulting in Aβ-Ca(2+)-PO4(-) complexes. Intensive Ca(2+) bound to the lipid bilayer and Ca(2+) ionic bridges may lead to Ca(2+) hemostasis responsible for neuronal dysfunction and death. This work provides insights into the mutual structure, dynamics, and interactions of both Aβ peptides and lipid bilayers at the atomic level, which expand our understanding of the complex behavior of amyloid-induced membrane disruption.

Uptake of Sulfadiazine Sulfonamide from Water by Clinoptilolite

The interactions between sulfadiazine (SDZ), a sulfonamide antibiotic, and clinoptilolite, a hydrophilic zeolite, were investigated under batch experimental conditions. The uptake of SDZ on the zeolite followed a linear sorption isotherm under neutral pH conditions. Higher SDZ uptake on the zeolite was observed when solution pH was below the or above the values of SDZ, while minimal SDZ uptake was observed when the solution pH was between the and values of SDZ. These observations suggested that hydrophobic interaction between SDZ and the zeolite was minimal due to the hydrophilic nature of the substrate. Electrostatic interactions and ion bridging were attributed to the elevated SDZ uptake under low and high pH conditions. As SDZ had a low value, the hydrophilic nature of the substrate prevented extensive uptake of SDZ, which could contribute to its extensive detection in the environment, including surface water and wastewater.