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PubMed2001

Relation between the degree of acetylation and the electrostatic properties of chitin and chitosan.

A series of chitosan/chitin samples with DA's varying between 5.2 and 89% was prepared from the reacetylation under soft conditions of a unique chitosan sample allowing the preservation of the chain distribution. The study of the variation of pH for the same concentration of amine groups, at different ionic strengths, on the scale of DA's allows us to extrapolate the variation of pKa at dissociation degrees (alpha) 0 and 1. A modeling of all the curves was obtained by means of only one equation. Then, for given concentration of chitosan and ionic strength, it is possible to predict the pH of the solution whatever the DA and alpha. The role of DA through the participation of hydrophobic interactions and hydrogen bondings on the electrostatic parameters is discussed. The results allow a better understanding of some physicochemical and biological properties of chitosan and chitin.

Acetylation↗
PubMedDate not supplied

Electrostatic adsorption of heme proteins alternated with polyamidoamine dendrimers for layer-by-layer assembly of electroactive films.

A novel thin film of heme proteins, including hemoglobin (Hb), myoglobin (Mb), and catalase (Cat), was successfully assembled layer by layer with polyamidoamine (PAMAM) dendrimers on different solid surfaces. At pH 7.0, protonated PAMAM possesses positive surface charges, whereas the proteins have net negative surface charges at pH above their isoelectric points. Thus, layer-by-layer {PAMAM/protein}(n)() films were assembled with alternate adsorption of oppositely charged PAMAM and proteins from their aqueous solutions mainly by electrostatic interaction. The assembly process was monitored by quartz crystal microbalance (QCM), UV-vis spectroscopy, and cyclic voltammetry (CV). The growth of the protein multilayer films was regular and linear, whereas the electroactivity of the films was only extended to a few bilayers. CVs of {PAMAM/protein}(n)() films showed a pair of well-defined and nearly reversible peaks characteristic of the protein heme Fe(III)/Fe(II) redox couples. Although {PAMAM/Hb}(n)() and {PAMAM/Mb}(n)() films showed very similar properties, {PAMAM/Cat}(n)() films displayed different and unique characters. The substrates with biological or environmental significance, such as oxygen, hydrogen peroxide, trichloroacetic acid, and nitrite, were catalytically reduced at {PAMAM/protein}(n)() film electrodes, showing the potential applicability of the films as new types of biosensors or bioreactors based on direct electrochemistry of the proteins. Both the electrochemical and electrocatalytic activity of {PAMAM/protein}(n)() films can be tailored precisely by controlling the number of bilayers or the film thickness.

Adsorption↗
PubMed2006

Macromolecular assemblies based on coupled inclusion complex and electrostatic interactions.

Macromolecular assemblies are elaborated by mixing beta-cyclodextrin-containing polymer (polybetaCD), dextran sulfate polyanion (NaDxS), and cationic amphiphiles which are adamantane derivatives (Ada1 or Ada2) in aqueous medium. These components are assembled via coupled inclusion complex interactions (adamantyl group with cyclodextrin cavity) and electrostatic attractive interactions (positive charges of Ada with negative charge of NaDxS). The structural properties are studied by viscometry and small angle neutron scattering. Ternary aggregates with larger size and lower compacities are observed as the cation concentration is increased, until phase separation occurs. The results are in good agreement with a core-shell association mechanism, the core being made of one polybetaCD chain, the shell of NaDxS chains, and the Ada amphiphiles being distributed more or less homogeneously inside the cyclodextrin cavities. The nature of the Ada counterions has a strong influence on the association as Ada1 with I(-) counterions give smaller and less compact aggregates than Ada2 with Br(-) counterions.

Binding, Competitive↗
PubMedDate not supplied

Electrostatic potential derived charges for enzyme cofactors: methods, correlations, and scaling for organic cofactors.

The correlations between electrostatic potential (ESP) derived atomic charges for a wide range of different quantum mechanical approaches and basis sets have been investigated for both small and large organic structures including several enzyme cofactors. The previously observed linear correlation between ESP charges calculated by different approaches has been verified to extend to many different basis sets and procedures including effective core potential (ECP) basis sets, density functional theory (DFT) approaches, a hybrid Hartree-Fock (HF)/DFT approach, and inclusion of electron correlation corrections. Above a threshold level of complexity, most procedures and basis sets give results that correlate very well (linear correlation coefficients > 0.99), including several procedures that have reasonable computational costs for large molecules. These procedures have been used to calculate ESP charges for five different types and forms of enzyme cofactors: biotin, pyridoxal-5'-phosphate, pyridoxamine-5'-phosphate, and the flavin mononucleotide in two different oxidation states.

Biotin↗
PubMedDate not supplied

Correlating the molecular electrostatic potentials of some organic peroxides with their antimalarial activities.

The molecular electrostatic potentials (MEPs) of artemisinin (also known as qinghaosu), yingzhaosu A, and some synthetic analogues have been calculated and studied as a means of distinguishing between high and low antimalarial activity. To facilitate comparison, the dimensionality of the MEP was reduced by Kohonen Neural Network transforms. The reduction revealed that peroxides exhibiting high antimalarial activity are characterized by a continuous strip of negative electric potential surrounding the molecule, whereas peroxides of lesser activity show a broken strip.

Animals↗
PubMed2004

Particle collection efficiency and particle re-entrainment of an electrostatic precipitator in a sewage sludge incineration plant.

In several recent studies it was shown that high atmospheric loads of submicrometer particles in the size range below 500 nm have strong impact on human health. Therefore, extensive research concerning the reduction of fine particle emissions is needed to further improve air quality. Regarding health effects, especially the emission characteristics of fine and ultrafine particles emerging from anthropogenic sources such as combustion processes are of special interest. This study shows that the emission characteristic of an electrostatic precipitator (ESP) due to re-entrainment of fine particles and their subsequent release into the atmosphere can be significantly lowered by application of different operating conditions. For this purpose the particle collection efficiency of an ESP was studied in a municipal sewage sludge incineration plant. Particles were sampled under different operating conditions upstream and downstream from the ESP, and the particle number concentrations were measured simultaneously with aerodynamic particle sizers. In addition, the size distribution of the particles downstream from the ESP was measured with high time resolution by an electrical low-pressure impactor to investigate the particle re-entrainment into the flue gas. To determine the influence of operating conditions, different rapping cycles were investigated regarding their impact on the collection efficiency and the subsequent particle re-entrainment.

Air Pollution↗
PubMed2006

Mass transfer within electrostatic precipitators: in-flight adsorption of mercury by charged suspended particulates.

Electrostatic precipitation is the dominant method of particulate control used for coal combustion, and varying degrees of mercury capture and transformation have been reported across ESPs. Nevertheless, the fate of gas-phase mercury within an ESP remains poorly understood. The present analysis focuses on the gas-particle mass transfer that occurs within a charged aerosol in an ESP. As a necessary step in gas-phase mercury adsorption or transformation, gas-particle mass transfer-particularly in configurations other than fixed beds-has received far less attention than studies of adsorption kinetics. Our previous analysis showed that only a small fraction of gas-phase mercury entering an ESP is likelyto be adsorbed by collected particulate matter on the plate electrodes. The present simplified analysis provides insight into gas-particle mass transfer within an ESP under two limiting conditions: laminar and turbulent fluid flows. The analysis reveals that during the process of particulate collection, gas-particle mass transfer can be quite high, easily exceeding the mass transfer to ESP plate electrodes in most cases. Decreasing particle size, increasing particle mass loading, and increasing temperature all result in increased gas-particle mass transfer. The analysis predicts significantly greater gas-particle mass transfer in the laminar limitthan in the turbulent limit; however, the differences become negligible under conditions where other factors, such as total mass of suspended particulates, are the controlling mass transfer parameters. Results are compared to selected pilot- and full-scale sorbent injection data.

Adsorption↗
PubMed2006

Electrostatic precipitator performance and trace element emissions from two Kraft recovery boilers.

Fine particle emissions from combustion sources have gained attention recently due to their adverse effects on human health. The emission depends on the combustion process, fuel, and particulate removal technology. Particle concentrations at Kraft recovery boiler exits are very high, and the boilers are typically equipped with electrostatic precipitators (ESP). However, little data are available on the ESP performance in recovery boilers. Particle concentrations and size distributions were determined at two modern, operating recovery boilers. In addition, we determined the fractional collection efficiency of the ESPs by simultaneous measurements at the ESP inlet and outlet and the particulate emissions of trace metals. The particle mass concentration atthe ESP inlet was 11-24 g/Nm3 at the two boilers. Particle emissions were 30-40 mg/ Nm3 at boiler A and 12-15 mg/Nm3 at boiler B. The particle size distributions had a major particle mode at around 1 microm. These fume particles contained most of the particle mass. The main components in the particles were sodium and sulfate with minor amounts of chloride, potassium, and presumably some carbonate. The ESP collection efficiency was 99.6-99.8% at boiler A and 99.9% at boiler B. The particle penetration through the ESP was below 0.6% in the entire fume particle size range of 0.3-3 microm. Trace element emissions from both boilers were well below the limit values set by EU directive for waste incineration.

Static Electricity↗
PubMed2006

Anion adsorption on oxide surfaces: inclusion of the water dipole in modeling the electrostatics of ligand exchange.

Adsorption of aqueous anions, such as sulfate, arsenite, and oxalate, to oxide surfaces is important in the retardation of toxic species in the environment, but predicting the surface speciation as a function of environmental parameters is a major challenge. Recent laboratory spectroscopic studies defining surface speciation must be integrated with surface complexation models. However, the latter have neglected the electrostatic work of desorption of water dipoles in treating anion adsorption by ligand exchange. Taking this effect into account permits close quantitative description of anion adsorption and the prediction of anion surface speciation as a function of pH, ionic strength, and surface coverage in agreement with spectroscopic results.

Adsorption↗
PubMed2006

Particle size distribution effects on gas-particle mass transfer within electrostatic precipitators.

Varying degrees of mercury capture and transformation have been reported across electrostatic precipitators (ESPs). Previous analyses have shown that the dominant mass transfer mechanism responsible for mercury capture within ESPs is gas-particle mass transfer during particulate collection. Whereas previous analyses assumed dispersions of uniform size, the present analysis reveals the effects of polydispersity on both gas-particle mass transfer and particle collection within an ESP. The analysis reveals that the idealized monodisperse particle size distribution provides the highest gas-particle mass transfer but results in the lowest particle collection efficiency (% mass). As the particle size distribution broadens, gas-particle mass transfer decreases and particle collection efficiency increases. The results suggest that more than just reporting mean particle diameter provided by the sorbent manufacturer, pilot- and field-tests of sorbent injection for mercury emissions control need to experimentally measure the particle size distribution of the sorbent as it is injected in order to facilitate interpretation of their results.

Air Pollutants↗
PubMed2006

Impact of natural organic matter on monochloramine reduction by granular activated carbon: the role of porosity and electrostatic surface properties.

Steady-state monochloramine reduction in fixed-bed reactors (FBRs) was quantified on five types of granular activated carbon (GAC) using two background waters-one natural source water (LAW) containing 2.5-3.5 mg/L organic carbon and one synthetic organic-free water (NW). While more monochloramine was reduced at steady-state using NW compared to LAW for each GAC and empty-bed contact time studied, the differences in removal varied considerably among the GACs tested. Physical characterization of the GACs suggested that the degree of interference caused by natural organic matter (NOM) increased with increasing GAC surface area contained within pores greater than 2 nm in width. Acid/base and electrostatic properties of the GACs were not found to be significant in terms of NOM uptake, which indicated that size exclusion effects of the GAC pores overwhelmed the impact of the GAC surface chemistry. Therefore, selection of GAC to limit the impact of NOM on monochloramine reduction in FBRs should be based on pore size distribution alone, with the impact of NOM decreasing with decreasing mesoporosity and macroporosity.

Carbon↗
PubMed2001

An extremely bent cyanide bridge in crystals of [(CN)3Pt(mu-CN)Cu(NH3)4]. Influence of electrostatic forces on the nature and geometry of bridging cyanides in the solid state.

The dinuclear heterometallic complex [(CN)3Pt(mu-CN)Cu(NH3)4] has been obtained in the solid state, and its structure has been determined by X-ray diffraction techniques at two temperatures (102 and 233 K). C4H12CuN8Pt crystallizes in the orthorhombic system, with a = 14.554(3) A, b = 7.1901(12) A, c = 10.369(2) A, and V = 1085.1(3) A3 at 102 K (a = 14.5091(10) A, b = 7.2739(4) A, c = 10.4570(7) A, and V = 1103.61(12) A3 at 233 K), in space group Pnma, with Z = 4. The Pt and Cu atoms are linked by a CN bridge that presents a very bent C identical to N-Cu angle (120.1(6) degrees at 102 K). The C identical to N and N-Cu distances at 102 K are 1.147(10) and 2.394(7) A, respectively. [(CN)3Pt(mu-CN)Cu(NH3)4] is embedded in an extensive electrostatic net formed by (N)H...N(C) interactions which, it is concluded, play an important role in the extreme deviation from linearity observed for the C identical to N-Cu angle.

Journal Article↗
PubMed2002

Quantifying the electronic effect of substituted phosphine ligands via molecular electrostatic potential.

Values of the molecular electrostatic potential minimum (V(min)) corresponding to the lone pair region of several substituted phosphine ligands (PR(3)) have been determined at the DFT level. The V(min) value is proposed as a quantitative measure of the electronic effect of the PR(3) ligands. Good linear correlation between V(min) and Tolman electronic parameter of PR(3) has been obtained. V(min) is also proportional to the pK(a) values of the conjugate acids of PR(3), viz., [PR(3)H](+). Further, the DeltaE values of the reaction Ni(CO)(3) + PR(3) --> Ni(CO)(3)PR(3) and ScH(3) + PR(3) --> ScH(3)PR(3) are also linearly proportional to the V(min) values. However, if there is a strong metal to phosphorus pi-back-bonding, the DeltaE and V(min) do not fit to a line. It is also found that the standard reduction potential as well as the enthalpy change corresponding to the electrochemical couple eta-Cp(CO)(PR(3))(COMe)Fe(+)/eta-Cp(CO)(PR(3))(COMe)Fe(0) is linearly proportional to the V(min) values of PR(3). These correlations suggest that V(min) is a quantitative measure of the sigma-donating ability of the phosphine. It is hoped that, in phosphine-metal coordination chemistry, the V(min) based electronic parameter could be more advantageous than nu-CO and pK(a) based electronic parameters as it solely represents the inherent electronic property of the ligand.

Journal Article↗
PubMed2003

Comments on the electrostatic minimum potential (V(min)) as a measure of the electron-donating power of PZ(3).

In a recent publication, Suresh and Koga suggested that the molecular electrostatic potential minimum (V(min)) could be used as a quantitative measure of the "sigma donating power" of substituted phosphorus(III) ligands. In support of this assertion, they showed linear correlations between a number of physicochemical properties and V(min). Our examination of the analyses of the sets of experimental data used to support this assertion shows unequivocally that the variations in the properties cannot be described by a single parameter. Therefore, the utility of V(min) as a measure of the "sigma donating power" of substituted phosphorus(III) ligands remains unsubstantiated.

Comment↗
PubMed2004

Structure, reactivity and aromaticity of acenes and their BN analogues: a density functional and electrostatic investigation.

Density functional calculations have been carried out on a series of linearly annelated acenes and their BN analogues. Even though borazine shows aromatic and reactivity behavior parallel with that of benzene, its condensed derivatives show patterns different from those of their hydrocarbon analogues. Nucleus independent chemical shift (NICS) values in acenes suggest that the aromaticity of the inner rings is more than that of benzene, whereas in BN-acenes there is no substantial change in the aromaticity of the individual rings. Molecular electrostatic potential (MESP) is employed to obtain further insights into the bonding and reactivity trends for these systems. The MESP topography patterns of acenes and BN-acenes are substantially different, with BN-acenes showing more localized pi electron features compared to those of acenes. The MESP values at the critical points (CPs) indicate overall lowering of aromaticity in these annelated systems. However, this change is gradual among the BN-acenes.

Journal Article↗
PubMed2006

Molecular electrostatic potential approach to determining the steric effect of phosphine ligands in organometallic chemistry.

A two-layer ONIOM(B3LYP/6-31G(d,p):UFF) quantum mechanics (QM)-molecular mechanics (MM) optimization of PR3 ligands, where the QM layer is always constructed as PH3, followed by molecular electrostatic potential (MESP) analysis of the QM layer is suggested as a simple and effective method for evaluating the steric effect of PR3 ligands. The subtle variations in the electron distribution that arise as a result of the steric bulkiness as well as the conformational changes in the substituent groups is well reflected in the value of the MESP minimum (Vmin) located in the QM region. In general, a sterically bulky group has always shown a more negative Vmin than a sterically less bulky group. The difference between the Vmin value of free PH3 and the Vmin value at the QM region of PR3 is used as a measure of the steric effect of the PR3 ligand. This value, designated as MESPsteric, showed a good linear correlation with the cone angle values as well as the average of the intervalence HPH angles found in the QM layer. Further, the difference between the Vmin value at the QM region of PR3 (an indicator of the steric effect) and the Vmin value of the completely optimized PR3 ligand (an indicator of the combined effect of steric and electronic effects) showed a good linear correlation with the Hammett Sigmasigmap constants, which further confirmed the present approach to understanding the steric effect separately from the electronic effect.

Journal Article↗
PubMed2001

Direct determination of the permanent dipole moments and structures of Al-CH(3)CN and Al-NH(3) by using a 2-m electrostatic hexapole field.

The supersonic beams of the (1-1) metal-ligand complexes of Al-CH(3)CN and Al-NH(3) were produced by a laser evaporation method. Nondestructive structure selection of the complexes and the dipole moment determination were performed by using a 2-m electrostatic hexapole field. The experimentally determined permanent dipole moments are 1.2 +/- 0.1 D for Al-CH(3)CN and 2.7 +/- 0.2 D for Al-NH(3). We find that the dipole moment of Al-NH(3) becomes larger than that of neat NH(3), while the formation of the Al-CH(3)CN complex produces a smaller dipole moment than that of neat CH(3)CN on the other hand. We performed the ab initio calculations to draw out plausible complex structures and to clarify the bonding character after formation of the complex, and we made comparisons with the computational results done by several groups. The Mulliken population analysis suggests the Al-->CH(3)CN charge flow, but on the other hand the Natural population analysis indicates very little charge flow. For the Al-NH(3) complex, the polarization effect of NH(3) and the N-->Al sigma donation would enhance the dipole moment strength. However, there still remains a controversial disagreement between the theoretical predictions and the experimental results. Further experimental determination using the hexapole method for various metal-ligand complexes and clusters could reveal the basic nature of interaction in the complex systems in general, and this method would complement theoretical calculations.

Journal Article↗