Adhesion of Pseudomonas fluorescens (ATCC 17552) to nonpolarized and polarized thin films of gold.

The adhesion of Pseudomonas fluorescens (ATCC 17552) to nonpolarized and negatively polarized thin films of gold was studied in situ by contrast microscopy using a thin-film electrochemical flow cell. The influence of the electrochemical potential was evaluated at two different ionic strengths (0.01 and 0.1 M NaCl; pH 7) under controlled flow. Adhesion to nonpolarized gold surfaces readily increased with the time of exposition at both ionic-strength values. At negative potentials (-0.2 and -0.5 V [Ag/AgCl-KCl saturated [sat.]]), on the other hand, bacterial adhesion was strongly inhibited. At 0.01 M NaCl, the inhibition was almost total at both negative potentials, whereas at 0.1 M NaCl the inhibition was proportional to the magnitude of the potential, being almost total at -0.5 V. The existence of reversible adhesion was investigated by carrying out experiments under stagnant conditions. Reversible adhesion was observed only at potential values very close to the potential of zero charge of the gold surface (0.0 V [Ag/AgCl-KCl sat.]) at a high ionic strength (0.1 M NaCl). Theoretical calculations of the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy for the bacteria-gold interaction were in good agreement with experimental results at low ionic strength (0.01 M). At high ionic strength (0.1 M), deviations from DLVO behavior related to the participation of specific interactions were observed, when surfaces were polarized to negative potentials.     

Electrochemical Polarization-Induced Changes in the Growth of Individual Cells and Biofilms of Pseudomonas fluorescens (ATCC 17552)

The effect of surface electrochemical polarization on the growth of cells of Pseudomonas fluorescens (ATCC 17552) on gold electrodes has been examined. Potentials positive or negative to the potential of zero charge (PZC) of gold were applied, and these resulted in changes in cell morphology, size at cell division, time to division, and biofilm structure. At −0.2 V (Ag/AgCl-3 M NaCl), cells elongated at a rate of up to 0.19 μm min⁻¹, rendering daughter cells that reached up to 3.8 μm immediately after division. The doubling time for the entire population, estimated from the increment in the fraction of surface covered by bacteria, was 82 ± 7 min. Eight-hour-old biofilms at −0.2 V were composed of large cells distributed in expanded mushroom-like microcolonies that protruded several micrometers in the solution. A different behavior was observed under positive polarization. At an applied potential of 0.5 V, the doubling time of the population was 103 ± 8 min, cells elongated at a lower rate (up to 0.08 μm min⁻¹), rendering shorter daughters (2.5 ± 0.5 μm) after division, although the duplication times were virtually the same at all potentials. Biofilms grown under this positive potential were composed of short cells distributed in a large number of compact microcolonies. These were flatter than those grown at −0.2 V or at the PZC and were pyramidal in shape. Polarization effects on cell growth and biofilm structure resembled those previously reported as produced by changes in the nutritional level of the culture medium.     

Influence of pH and ionic strength on adhesion of a wild strain of Pseudomonas sp. to titanium

The adherence of an environmental strain of Pseudomonas sp. to titanium was evaluated modifying the pH (2 to 8) and ionic strength (0.1 and 0.6 M NaCl) of the electrolyte solution. Results were analyzed considering the participation of the different interfacial forces under the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. At 0.1 M, maximal bacterial adhesion was at pH 6, in agreement with the point of zero charge of the titanium surface. Similar adhesion values were observed at both sides of this point despite the opposite electrostatic condition of the surface oxide. At 0.6 M an absence of bacterial adhesion was observed throughout the pH range tested. The changes in bacterial adhesion are in agreement with the changes in the number of reinforced H-bond-forming sites on the titanium surface calculated using a simple model for the ionization of OH group adsorbed to the surface. Journal of Industrial Microbiology & Biotechnology (2001) 26, 303–308. Received 25 May 2000/ Accepted in revised form 14 February 2001     

Growth and Nodulation Responses of Rhizobium meliloti to Water Stress Induced by Permeating and Nonpermeating Solutes

Isolates of Rhizobium meliloti, representing antigenically distinct indigenous serogroups 31 and 17, were grown in yeast extract-mannitol broth (YEM) containing NaCl or polyethylene glycol (PEG) to provide external water potentials ranging from −0.15 to −1.5 MPa. Several differences were found between representatives of the two groups in their abilities to adapt to water stress induced by the nonpermeating solute PEG. At potentials below −0.5 MPa, strain 31 had a lower specific growth rate than strain 17 and an irregular cell morphology. In contrast, neither growth nor cell morphology of either strain was affected significantly over the same range of water potentials created by a permeating solute, NaCl. Despite the superior growth of strain 17 at the low water potentials imposed by PEG, upshock of water-stressed cells (−1.0 MPa; PEG) into normal YEM (−0.15 MPa) resulted in a faster recovery of growth by strain 31 than by strain 17. Different responses of the two strains to a water potential increase were also revealed in nodulation studies. Strain 31 required significantly fewer days to nodulate alfalfa than strain 17 did when the strains were transferred from YEM with PEG at −1.0 MPa onto the roots of alfalfa seedlings in plant growth medium (−0.1 MPa). The addition of supplemental calcium (0.1 mM) to growth medium with PEG (−1.0 MPa) reduced the differences between strains in their responses to water stress. The severe growth restriction and morphological abnormalities shown by strain 31 were corrected, and the prolonged recovery time shown by water-stressed cells (−1.0 MPa; PEG) of strain 17 upon transfer to normal YEM was shortened. The latter strain also nodulated earlier and more rapidly after growth in PEG medium at −1.0 MPa in the presence of supplemental calcium ions. These results indicate that the efficacy of osmoregulation can vary among strains of the same species and that the mechanism of osmoregulation may differ depending on the nature of the water stress.     

Enhanced Alcaligenes faecalis Denitrification Rate with Electrodes as the Electron Donor

The utilization by Alcaligenes faecalis of electrodes as the electron donor for denitrification was investigated in this study. The denitrification rate of A. faecalis with a poised potential was greatly enhanced compared with that of the controls without poised potentials. For nitrate reduction, although A. faecalis could not reduce nitrate, at three poised potentials of +0.06, −0.06, and −0.15 V (versus normal hydrogen electrode [NHE]), the nitrate was partially reduced with −0.15- and −0.06-V potentials at rates of 17.3 and 28.5 mg/liter/day, respectively. The percentages of reduction for −0.15 and −0.06 V were 52.4 and 30.4%, respectively. Meanwhile, for nitrite reduction, the poised potentials greatly enhanced the nitrite reduction. The nitrite reduction rates for three poised potentials (−0.06, −0.15, and −0.30 V) were 1.98, 4.37, and 3.91 mg/liter/h, respectively. When the potentials were cut off, the nitrite reduction rate was maintained for 1.5 h (from 2.3 to 2.25 mg/liter/h) and then greatly decreased, and the reduction rate (0.38 mg/liter/h) was about 1/6 compared with the rate (2.3 mg/liter/h) when potential was on. Then the potentials resumed, but the reduction rate did not resume and was only 2 times higher than the rate when the potential was off.     

Characterization of the Bactericidal Effects of Sodium Nitroprusside and Other Pentacyanonitrosyl Complexes on the Food Spoilage Bacterium Clostridium sporogenes

The potent bactericidal activity of sodium nitroprusside {SNP; Na₂[Fe(CN)₅(NO)]} towards Clostridium sporogenes has been investigated. SNP inhibited cell growth in the concentration range of 10 to 40 μM. Concentrations above 80 μM caused irreversible loss of cell viability and cell lysis. Inhibition of cell growth was similar in complex and in defined media. SNP was found to be unreactive towards individual components of the defined medium, with the exception of cysteine. The chemical characteristics responsible for the potency of SNP were investigated by synthesizing analogs of SNP in which the Fe was replaced by different metals. The inhibitory potency of the pentacyanonitrosyl complexes decreased in the order Fe > Cr > V, which correlates with N-O stretching frequency (vNO). In contrast, the Ru complex which had a vNO comparable to that of Fe was a poor inhibitor. Electron paramagnetic resonance spectroscopy showed that SNP was rapidly reduced to the paramagnetic Fe(I) compound [Fe(CN)₄(NO)]²⁻ on contact with cells. Analysis of fractions from SNP-treated cells showed 90% oxidation of thiols in the cell walls compared with those in control cells. The toxicity of SNP involves S-nitrosation and reduction, the lack of toxicity of the Ru analog being consistent with the fact that it has poor reactivity towards thiols. When C. sporogenes cells were exposed to sublethal concentrations of SNP and viewed under the electron microscope, they showed blisters on the surface. These results point to the cell wall surface as a primary point of attack of the nitrosyl complex.     

Fast Inactivation in Shaker K+ Channels : Properties of Ionic and Gating Currents

Fast inactivating Shaker H4 potassium channels and nonconducting pore mutant Shaker H4 W434F channels have been used to correlate the installation and recovery of the fast inactivation of ionic current with changes in the kinetics of gating current known as “charge immobilization” (Armstrong, C.M., and F. Bezanilla. 1977. J. Gen. Physiol. 70:567–590.). Shaker H4 W434F gating currents are very similar to those of the conducting clone recorded in potassium-free solutions. This mutant channel allows the recording of the total gating charge return, even when returning from potentials that would largely inactivate conducting channels. As the depolarizing potential increased, the OFF gating currents decay phase at −90 mV return potential changed from a single fast component to at least two components, the slower requiring ∼200 ms for a full charge return. The charge immobilization onset and the ionic current decay have an identical time course. The recoveries of gating current (Shaker H4 W434F) and ionic current (Shaker H4) in 2 mM external potassium have at least two components. Both recoveries are similar at −120 and −90 mV. In contrast, at higher potentials (−70 and −50 mV), the gating charge recovers significantly more slowly than the ionic current. A model with a single inactivated state cannot account for all our data, which strongly support the existence of “parallel” inactivated states. In this model, a fraction of the charge can be recovered upon repolarization while the channel pore is occupied by the NH₂-terminus region.     

Surface potential and surface charge density of the cerebral-cortex synaptic vesicle and stability of vesicle suspension

Using a microelectrophoresis instrument employing the Lazer-Zee system, the electrophoretic mobility of synaptic vesicles isolated from Guinea-pig brain cortex was measured under several conditions. The mobility was found to depend on both pH and ionic concentration of the solution. The surface of the synaptic vesicle was shown to be negatively charged under physiological conditions. The isoelectric point was observed at pH 4.0 in 0.01 M NaCl solution. Effects of divalent cations were examined and reversal of surface charge was observed in 0.1 M CaCl₂ solution. Interaction of vesicles was also considered on the basis of the DLVO theory of colloid stability by using calculated values of surface charge density and surface potential of the synaptic vesicle.     

Comparison of hard-cylinder and screened coulomb interactions in the modeling of supercoiled DNAs

A 1000 base pair (bp) model supercoiled DNA is simulated using spherical screened Coulomb interactions between subunits on one hand and equivalent hard-cylinder interactions on the other. The amplitudes, or effective charges, of the spherical screened Coulomb electrostatic potentials are chosen so that the electrostatic potential surrounding the middle of a linear array of 2001 subunits (31.8 Å diameter) closely matches the solution of the nonlinear Poisson-Boltzmann equation for a cylinder with 12 Å radius and the full linear charge density of DNA at all distances beyond the 24 Å hard-core diameter. This superposition of spherical screened Coulomb potentials is practically identical to the particular solution of the cylindrical linearized Poisson-Boltzmann equation that matches the solution of the nonlinear Poisson-Boltzmann equation at large distances. The interaction energy between subunits is reckoned from the effective charges according to the standard DLVO expression. The equivalent hard-cylinder diameter is chosen following Stigter's protocol for matching second virial coefficients, but for the full linear charge density of DNA. The electrostatic persistence length of the model with screened Coulomb interactions is extremely sensitive to the (arbitrarily) chosen subunit length at the higher salt concentrations. The persistence length of the hard-cylinder model is adjusted to match that of the screened Coulomb model for each ionic condition. Simulations for a superhelix density σ = -0.05 using a spherical screened Coulomb interaction plus a 24 Å hard-cylinder core (SCPHC) potential indicate that the radius of gyration of this 1000 bp DNA actually undergoes a slight increase as the NaCl concentration is raised from 0.01 to 1.0M. Thus, merely softening the potential from hard-cylinder to screened Coulomb form does not produce a large decrease in radius of gyration with increasing NaCl concentration for DNAs of this size. Radii of gyration, static structure factors, and diffusion coefficients obtained using the equivalent hard-cylinder (EHC) potential agree well with those obtained using the SCPHC potential in 1.0M NaCl, but in 0.1M NaCl the agreement is not as good, and in 0.01M NaCl the agreement is definitely unsatisfactory. These conclusions differ in significant respects from those obtained in previous studies. © 1997 John Wiley & Sons, Inc. Biopoly 42: 455–470, 1997     

Individual and Co Transport Study of Titanium Dioxide NPs and Zinc Oxide NPs in Porous Media

The impact of pH and ionic strength on the mobility (individual and co-transport) and deposition kinetics of TiO₂ and ZnO NPs in porous media was systematically investigated in this study. Packed column experiments were performed over a series of environmentally relevant ionic strengths with both NaCl (0.1−10 mM) and CaCl₂ (0.01–0.1mM) solutions and at pH 5, 7, and 9. The transport of TiO₂ NPs at pH 5 was not significantly affected by ZnO NPs in solution. At pH 7, a decrease in TiO₂ NP transport was noted with co-existence of ZnO NPs, while at pH 9 an increase in the transport was observed. At pH 5 and 7, the transport of ZnO NPs was decreased when TiO₂ NPs was present in the solution, and at pH 9, an increase was noted. The breakthrough curves (BTC) were noted to be sensitive to the solution chemistries; the decrease in the breakthrough plateau with increasing ionic strength was observed under all examined pH (5, 7, and 9). The retention profiles were the inverse of the plateaus of BTCs, as expected from mass balance considerations. Overall, the results from this study suggest that solution chemistries (ionic strength and pH) are likely the key factors that govern the individual and co-transport behavior of TiO₂ and ZnO NPs in sand.     

Influence of Pretreatment and Experimental Conditions on Electrophoretic Mobility and Hydrophobicity of Cryptosporidium parvum Oocysts

Surface properties of Cryptosporidium parvum oocysts were investigated by using electrophoretic mobility and hydrophobicity measurements. Oocysts purified from calf feces by several sucrose flotation steps and deionized water (DI) washes (DIS method) had an electrophoretic mobility (neutral surface charge) near 0.0 m² V⁻¹ s⁻¹ over a pH range of 2 to 10. The mean electrophoretic mobility of oocysts stored in DI containing a mixture of antibiotics had a lower standard deviation (ς = 0.36) than that of oocysts stored in DI without antibiotics (ς = 0.53); their electrophoretic mobility remained unchanged up to 121 days after collection. The electrophoretic mobility of oocysts purified on a cold Percoll-sucrose gradient after the feces was defatted with ethyl acetate (EAPS method) varied linearly with pH from 0.0 m² V⁻¹ s⁻¹ at pH 2.4 to −3.2 × 10⁻⁸ m² V⁻¹ s⁻¹ at pH 10 (ς = 0.52), thus displaying the negative surface charge at neutral pH observed by other researchers. The hydrophobicity of oocysts and two types of polystyrene beads was measured as a function of ionic strength by adhesion to polystyrene. Oocysts were purified by the DIS method. The ionic strength of the suspending solution was varied from 0 to 95 mmol liter⁻¹. Two-week-old oocysts exhibited strong adhesion (~85%) at ionic strengths of 0 to 10 mmol liter⁻¹ and moderate adhesion (~20%) at ionic strengths of 20 to 95 mmol liter⁻¹. Two-month-old oocysts exhibited high adhesion (~60 to 80%) at all ionic strengths. These results show that adhesion properties governed by the electrophoretic mobility of purified C. parvum oocysts can be altered by the method of purification and that hydrophobicity can change as oocysts age.     

Fabrication of an Electrochemical Sensor Based on Gold Nanoparticles/Carbon Nanotubes as Nanocomposite Materials: Determination of Myricetin in Some Drinks

In this paper, the electrochemical behavior of myricetin on a gold nanoparticle/ethylenediamine/multi-walled carbon-nanotube modified glassy carbon electrode (AuNPs/en/MWCNTs/GCE) has been investigated. Myricetin effectively accumulated on the AuNPs/en/MWCNTs/GCE and caused a pair of irreversible redox peaks at around 0.408 V and 0.191 V (vs. Ag/AgCl) in 0.1 mol L⁻¹ phosphate buffer solution (pH 3.5) for oxidation and reduction reactions respectively. The heights of the redox peaks were significantly higher on AuNPs/en/MWNTs/GCE compare with MWCNTs/GC and there was no peak on bare GC. The electron-transfer reaction for myricetin on the surface of electrochemical sensor was controlled by adsorption. Some parameters including pH, accumulation potential, accumulation time and scan rate have been optimized. Under the optimum conditions, anodic peak current was proportional to myricetin concentration in the dynamic range of 5.0×10⁻⁸ to 4.0×10⁻⁵ mol L⁻¹ with the detection limit of 1.2×10⁻⁸ mol L⁻¹. The proposed method was successfully used for the determination of myricetin content in tea and fruit juices.     

Electrochemical and Thermodynamic Properties of Ln(III) (Ln?=?Eu,Sm, Dy,Nd) in 1-Butyl-3-Methylimidazolium Bromide Ionic Liquid

The electrochemical behavior and thermodynamic properties of Ln(III) (Ln = Eu, Sm, Dy, Nd) were studied in 1-butyl-3-methylimidazolium bromide ionic liquid (BmimBr) at a glassy carbon (GC) electrode in the range of 293–338 K. The electrode reaction of Eu(III) was found to be quasi-reversible by the cyclic voltammetry, the reactions of the other three lanthanide ions were regarded as irreversible systems. An increase of the current intensity was obtained with the temperature increase. At 293 K, the cathodic peak potentials of −0.893 V (Eu(III)), −0.596 V (Sm(III)), −0.637 V (Dy(III)) and −0.641 V (Nd(III)) were found, respectively, to be assigned to the reduction of Ln(III) to Ln(II). The diffusion coefficients (D _o), the transfer coefficients (α) of Ln(III) (Ln = Eu, Sm, Dy, Nd) and the charge transfer rate constants (k _s) of Eu(III) were estimated. The apparent standard potential (E ⁰*) and the thermodynamic properties of the reduction of Eu(III) to Eu(II) were also investigated.     

Reversal of Flagellar Rotation Is Important in Initial Attachment of Escherichia coli to Glass in a Dynamic System with High- and Low-Ionic-Strength Buffers

The attachment rates of wild-type, smooth-swimming, tumbly, and paralyzed Escherichia coli to glass was measured at fluid velocities of 0.0044 and 0.044 cms⁻¹ (corresponding to shear rates of 0.34 and 3.4 s⁻¹, respectively), in 0.02 and 0.2 M buffer solutions. At the highest ionic strength, we did not observe a significant difference in the attachment rate of wild-type and paralyzed cells at either fluid velocity. However, when the ionic strength was reduced, paralyzed bacteria attached at rates 4 and 10 times lower than that of the wild type under fluid velocities of 0.0044 and 0.044 cms⁻¹, respectively. This suggested that the rotation of the flagella assisted in attachment. We then compared the attachment rates of smooth-swimming (counterclockwise rotation only) and tumbly (clockwise rotation only) cells to the wild type to determine whether the direction of rotation was important to cell attachment. At 0.0044 cms⁻¹, the smooth-swimming cells attached at rates similar to that of the wild type in both buffer solutions but significantly less at the higher fluid velocity. Tumbly cells attached at much lower rates under all conditions. Thus, the combination of clockwise and counterclockwise flagellar rotation and their coupling appeared to be important in cell attachment. We considered a number of hypotheses to interpret these observations, including a residence time analysis and a comparison of traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to soft-particle theory.     

Analysis of the binding of high mobility group protein 17 to the nucleosome core particle by 1H NMR spectroscopy

The binding of high mobility group (HMG) protein 17 to the nucleosome core particle has been studied in D2O solution using 1H NMR at 500 MHz. Spectra were obtained for purified HMG 17, purified nucleosome core particles, and the reconstituted HMG 17-nucleosome core particle complex at 0.1, 0.2, 0.3, and 0.4 M NaCl. Subtraction of the core particle spectra from spectra of the core particle reconstituted with HMG 17 demonstrated those regions of HMG 17 which interact with the nucleosome at different ionic strengths; the resonance peaks of interacting groups are broadened due to their restricted mobility. At 0.1 M NaCl, the mobility of all the amino acid side chains of HMG 17 was restricted, indicating complete binding of HMG 17 to the much larger nucleosome core particle. At 0.2 M NaCl most of the amino acids were free with the exception of arginine and proline which are confined to or predominant in the basic central region of HMG 17. These amino acids were completely free only at 0.4 M NaCl. We conclude that the entire HMG 17 molecule interacts with the nucleosome core particle at physiological ionic strength. The acidic COOH-terminal region of HMG 17 is released from interaction with the core histones at an NaCl concentration between 0.1 and 0.2 M and so binds weakly at physiological ionic strength. The basic central region binds more strongly to the core particle DNA, being completely released only at much higher ionic strength, between 0.3 and 0.4 M NaCl.     

The effect of sodium chloride on solute potential and proline accumulation in soybean leaves

Two cultivars of soybean (Glycine max [L.] Merr.) were grown in solution with up to 100 millimolar NaCl. Leaf solute potential was −1.1 to −1.2 megapascals in both cultivars without NaCl. At 100 millimolar NaCl leaf solute potential was −3.1 to −3.5 megapascals in Bragg and −1.7 megapascals in Ransom. The decrease in solute potential was essentially proportional to the concentration of NaCl. In both salt susceptible Bragg and salt semitolerant Ransom, leaf proline was no more than 0.4 micromole per gram fresh weight at or below 20 millimolar NaCl. At 40 and 60 millimolar NaCl, Bragg leaf proline levels were near 1.2 and 1.9 micromoles per gram fresh weight, respectively. Proline did not exceed 0.5 micromole per gram fresh weight in Ransom even at 100 millimolar NaCl. Proline accumulated in Bragg only after stress was severe enough to induce injury; therefore proline accumulation is not a sensitive indicator of salt stress in soybean plants.     

Dissimilatory Fe(III) Oxide Reduction by Shewanella alga BrY Requires Adhesion

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was used to examine the relationship between adhesion and dissimilatory Fe(III) oxide reduction. Adhesion of Shewanella alga BrY to hydrous ferric oxide (HFO) was correlated with ionic strength and thus was accurately described by the DLVO theory. Reduction of insoluble HFO was also correlated with KCl concentration. In contrast, there was no correlation between soluble Fe(III) reduction and ionic strength. A correlation between HFO reduction rate and adhesion to HFO was observed. These results provide direct evidence that adhesion is requisite for Fe(III) oxide reduction in the absence of soluble electron shuttles. Received: 26 October 1999 / Accepted: 22 November 1999     

Electrical potentials in stomatal complexes

Guard cells of several species, but predominantly Commelina communis, were impaled by micropipette electrodes and potential differences measured that occurred between cell compartments and the flowing bathing medium. The wall developed a Donnan potential that was between −60 and −70 millivolt in 30 millimolar KCl at pH 7. The density of the fixed charges ranged from 0.3 to 0.5 molar; its dependence on pH was almost identical with the titration curve of authentic polygalacturonic acid. The vacuolar potential of guard cells of Commelina communis L., Zea mays L., Nicotiana glauca Graham, Allium cepa L., and Vicia faba L. was between −40 and −50 millivolt in 30 millimolar KCl when stomata were open and about −30 millivolt when stomata were closed. The vacuolar potential of guard cells of C. communis was almost linearly related to stomatal aperture and responded to changes in the ionic strength in the bathing medium in a Nernstian manner. No specificity for any alkali ion (except Li⁺), ammonium, or choline appeared. Lithium caused hyperpolarization. Calcium in concentrations between 1 and 100 millimolar in the medium led to stomatal closure, also caused hyperpolarization, and triggered transient oscillations in the intracellular potential. Gradients in the electrical potential existed across stomatal complexes with open pores. When stomata closed, these gradients almost disappeared or slightly reverted; all epidermal cells were then at potentials near −30 millivolt in 30 millimolar KCl.     

Interaction of nucleic acids with electrically charged surfaces. VII. The effect of ionic strength of neutral medium on the conformation of dna adsorbed on the mercury electrode

Triangular-wave direct current (d.c.) voltammetry at a hanging mercury drop electrode and phase-selective alternating current (a.c.) polarography at a dropping mercury electrode were used for the investigation of adsorption of double-helical (ds) DNA at mercury electrode surfaces from neutral solutions of 0.05-0.4 M HCOONH4. It was found for the potential region T (from -0.1 V up to ca. -1.0 V) that the height of voltammetric peaks of ds DNA is markedly influenced by the initial potential only at relatively low ionic strength (mu) (from 0.05 up to ca. 0.3). Also a decrease of differential capacity (measured by means of a.c. polarography) in the region T depended markedly on the electrode potential only at relatively low ionic strength. The following conclusions were made concerning the interaction of ds DNA with a mercury electrode charged to potentials of the region T in neutral medium of relatively low ionic strength mu < 0.3). (i) When ds DNA is adsorbed, a significantly higher number of DNA segments is anchored in the positively charged electrode surface than in the surface bearing a negative charge, (ii) In the region T, especially adsorbed labile regions of ds DNA are opened in the electrode surface, which are present in ds DNA already in the bulk of the solution, (iii) In the narrow region of potentials in the Vicinity of the zero charge potential a higher number of ds DNA segments can be opened, probably as a consequence of the strain which could act on the ds DNA molecule in the course of the segmental adsorption/desorption process.     

Stability of Secondary and Tertiary Structures of Virus-Like Particles Representing Noroviruses: Effects of pH,Ionic Strength,and Temperature and Implications for Adhesion to Surfaces

Loss of ordered molecular structure in proteins is known to increase their adhesion to surfaces. The aim of this work was to study the stability of norovirus secondary and tertiary structures and its implications for viral adhesion to fresh foods and agrifood surfaces. The pH, ionic strength, and temperature conditions studied correspond to those prevalent in the principal vehicles of viral transmission (vomit and feces) and in the food processing and handling environment (pasteurization and refrigeration). The structures of virus-like particles representing GI.1, GII.4, and feline calicivirus (FCV) were studied using circular dichroism and intrinsic UV fluorescence. The particles were remarkably stable under most of the conditions. However, heating to 65°C caused losses of β-strand structure, notably in GI.1 and FCV, while at 75°C the α-helix content of GII.4 and FCV decreased and tertiary structures unfolded in all three cases. Combining temperature with pH or ionic strength caused variable losses of structure depending on the particle type. Regardless of pH, heating to pasteurization temperatures or higher would be required to increase GII.4 and FCV adhesion, while either low or high temperatures would favor GI.1 adhesion. Regardless of temperature, increased ionic strength would increase GII.4 adhesion but would decrease GI.1 adhesion. FCV adsorption would be greater at refrigeration, pasteurization, or high temperature combined with a low salt concentration or at a higher NaCl concentration regardless of temperature. Norovirus adhesion mediated by hydrophobic interaction may depend on hydrophobic residues normally exposed on the capsid surface at pH 3, pH 8, physiological ionic strength, and low temperature, while at pasteurization temperatures it may rely more on buried hydrophobic residues exposed upon structural rearrangement.