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At least 1,657 records · Page 92Linked to original sources
PubMed2005

Conformationally induced electrostatic stabilization of persulfoxides: a new suggestion for inhibition of physical quenching of singlet oxygen by remote functional groups.

1,5-Dithiacyclooctane is shown to chemically react more efficiently and to remove singlet oxygen from solution more rapidly than either thiane or 1,4-dithiane. These unusual characteristics of the 1,5-dithiacyclooctane reaction were explored using ab initio quantum chemical methods. A large number of persulfoxides, thiadioxiranes, and hydroperoxy sulfonium ylides were located and their structures analyzed. The unusual efficiency of the reaction was attributed to a conformational change that electrostatically stabilized the persulfoxide and increased the potential energy barrier for physical quenching.

Computer Simulation↗
PubMed2006

Direct observation of salt effects on molecular interactions through explicit-solvent molecular dynamics simulations: differential effects on electrostatic and hydrophobic interactions and comparisons to Poisson-Boltzmann theory.

Proteins and other biomolecules function in cellular environments that contain significant concentrations of dissolved salts and even simple salts such as NaCl can significantly affect both the kinetics and thermodynamics of macromolecular interactions. As one approach to directly observing the effects of salt on molecular associations, explicit-solvent molecular dynamics (MD) simulations have been used here to model the association of pairs of the amino acid analogues acetate and methylammonium in aqueous NaCl solutions of concentrations 0, 0.1, 0.3, 0.5, 1, and 2 M. By performing simulations of 500 ns duration for each salt concentration properly converged estimates of the free energy of interaction of the two molecules have been obtained for all intermolecular separation distances and geometries. The resulting free energy surfaces are shown to give significant new insights into the way salt modulates interactions between molecules containing both charged and hydrophobic groups and are shown to provide valuable new benchmarks for testing the description of salt effects provided by the simpler but faster Poisson-Boltzmann method. In addition, the complex many-dimensional free energy surfaces are shown to be decomposable into a number of one-dimensional effective energy functions. This decomposition (a) allows an unambiguous view of the qualitative differences between the salt dependence of electrostatic and hydrophobic interactions, (b) gives a clear rationalization for why salt exerts different effects on protein-protein association and dissociation rates, and (c) produces simplified energy functions that can be readily used in much faster Brownian dynamics simulations.

Acetates↗
PubMed2006

Electrostatically controlled hierarchical arrangement of monocationic silver(I) and dicationic mercury(II) ions between disk-shaped template ligands.

This paper describes hierarchical arrangement of three Ag+ and three Hg2+ ions using two disk-shaped hexa-monodentate template ligands in which three oxazolyl rings are arranged each on the two concentric circles. The hierarchical manner of ligand arrangement allows the selective binding of monocationic Ag+ and dicationic Hg2+ ions to the inner first and the outer second generation coordination sites, respectively. This hierarchically well-balanced metal assembly should arise primarily from a minimized electrostatic repulsion between positively charged six metal ions arranged within the heteronuclear complex.

Journal Article↗
PubMed2005

Increasing the oxidative stability of liquid and dried tuna oil-in-water emulsions with electrostatic layer-by-layer deposition technology.

Omega-3 Fatty acids have numerous health benefits, but their addition to foods is limited by oxidative rancidity. Engineering the interfacial membrane of oil-in-water emulsion droplets to produce a cationic and/or thick interface is an effective method to control lipid oxidation. Cationic and thick emulsion droplet interfacial membranes can be produced by an electrostatic layer-by-layer deposition technique resulting in droplets that are coated by multiple layers of emulsifiers. Tuna oil-in-water emulsion droplets coated by lecithin and chitosan produce cationic emulsion droplets that are more oxidatively stable than emulsions coated by lecithin alone. Ethylenediaminetetraacetic acid (EDTA) was able to increase the oxidative stability of emulsions stabilized with lecithin and chitosan more effectively than mixed tocopherols. The combination of EDTA and mixed tocopherols was not more effective than EDTA alone suggesting that control of prooxidant metals was the most important antioxidant technology. The production of emulsion droplets coated with lecithin and chitosan could be an excellent technology for stabilization of oxidatively unstable lipids for use in a variety of food products.

Animals↗
PubMed2005

Kinetics of formation and functional properties of conjugates prepared by dry-state incubation of beta-lactoglobulin/acacia gum electrostatic complexes.

The formation of conjugates between beta-lactoglobulin and acacia gum based on electrostatic complexes formed at pH 4.2 was investigated upon dry-state incubation for up to 14 days at 60 degrees C and 79% relative humidity (RH). By means of SEC-HPLC and RP-HPLC, it was shown that the beta-lactoglobulin incubated alone was able to form polymers with molecular masses higher than 200 kDa until 50% of the initial monomeric protein disappeared after 14 days. In the presence of acacia gum at initial protein to polysaccharide weight mixing ratios of 2:1 and 1:2, only 35% of the initial beta-lactoglobulin monomers disappeared after 14 days. Using RP-HPLC, an apparent reaction order of 2 was found for the disappearance of monomeric beta-lactoglobulin both in the presence or absence of acacia gum. However, the reaction rate was faster in the absence of acacia gum. SDS-PAGE electrophoresis with silver staining confirmed the formation of beta-lactoglobulin/acacia gum conjugates. The solubility curves of the incubated beta-lactoglobulin showed a minimum around pH 4-5. By contrast, the minimum of solubility of the beta-lactoglobulin/acacia gum incubated mixtures shifted to lower pH values compared to initial mixtures. The conjugates exhibited higher foam capacity than the incubated protein as well as lower equilibrium air/water surface tension. Conjugation at ratio 1:2 led to increased interfacial viscosity (300 mN s m(-1) at 0.01 Hz) compared to beta-lactoglobulin alone (100 mN s m(-1) at 0.01 Hz), but similar interfacial elasticity (30-40 mN m(-1)). The foam capacity of the conjugates was significantly higher than that of the incubated beta-lactoglobulin as well as foam expansion and drainage time, especially at pH 5.3, i.e., higher than the pH of formation of the conjugates.

Chemical Phenomena↗
PubMed2006

Stabilization of model beverage cloud emulsions using protein-polysaccharide electrostatic complexes formed at the oil-water interface.

The potential of utilizing interfacial complexes, formed through the electrostatic interactions of proteins and polysaccharides at oil-water interfaces, to stabilize model beverage cloud emulsions has been examined. These interfacial complexes were formed by mixing charged polysaccharides with oil-in-water emulsions containing oppositely charged protein-coated oil droplets. Model beverage emulsions were prepared that consisted of 0.1 wt % corn oil droplets coated by beta-lactoglobulin (beta-Lg), beta-Lg/alginate, beta-Lg/iota-carrageenan, or beta-Lg/gum arabic interfacial layers (pH 3 or 4). Stable emulsions were formed when the polysaccharide concentration was sufficient to saturate the protein-coated droplets. The emulsions were subjected to variations in pH (from 3 to 7), ionic strength (from 0 to 250 mM NaCl), and thermal processing (from 30 or 90 degrees C), and the influence on their stability was determined. The emulsions containing alginate and carrageenan had the best stability to ionic strength and thermal processing. This study shows that the controlled formation of protein-polysaccharide complexes at droplet surfaces may be used to produce stable beverage emulsions, which may have important implications for industrial applications.

Alginates↗
PubMed2007

Impact of electrostatic interactions on formation and stability of emulsions containing oil droplets coated by beta-lactoglobulin-pectin complexes.

Interfacial protein-polysaccharide complexes can be used to improve the physical stability of oil-in-water emulsions. The purpose of this study was to examine the impact of ionic strength on the formation and stability of oil-in-water emulsions containing polysaccharide-protein-coated droplets. Emulsions were prepared that contained 0.1 wt % corn oil, 0.05 wt % beta-lactoglobulin, and 0.02 wt % pectin at pH 7. The emulsions were then adjusted to pH 4 to promote electrostatic deposition of the pectin molecules onto the surfaces of the protein-coated droplets. The salt concentration of the aqueous phase (0 or 50 mM NaCl) was adjusted either before or after deposition of the pectin molecules onto the droplet surfaces. We found that stable emulsions containing polysaccharide-protein-coated droplets could be formed when the salt was added after pectin adsorption but not when it was added before pectin adsorption. This phenomenon was attributed to the ability of NaCl to promote droplet flocculation in the protein-coated droplets so that the pectin molecules adsorbed onto the surfaces of flocs rather than individual droplets when salt was added before pectin adsorption. We also found that polysaccharide-protein-coated droplets had a much improved stability to salt-induced flocculation than protein-coated droplets with the same droplet charge (zeta-potential). Theoretical predictions indicated that this was due to the ability of the adsorbed polysaccharide layer to strongly diminish the van der Waals attraction between the droplets.

Adsorption↗
PubMed1995

Molecular similarity from atomic electrostatic multipole comparisons. Application to anti-HIV drugs.

A procedure is presented for the rapid calculation of the similarity between a pair of molecules based on atomic electrostatic multipole comparison. The multipoles are derived from semiempirical SCF wave functions, and the results obtained compare favorably with ab initio results. The method is illustrated by correlating the similarity and anti-HIV-1 activity of a series of azo compounds. Some generalizations are presented on the structure-activity relationships which are based on the atomic multipole distribution in the azo compounds.

Antiviral Agents↗
PubMed1992

Antiandrogenic steroidal sulfonyl heterocycles. Utility of electrostatic complementarity in defining bioisosteric sulfonyl heterocycles.

Complementarity of electrostatic potential surface maps was utilized in defining bioisosteric steroidal androgen receptor antagonists. Semiempirical and ab initio level calculations performed on a series of methanesulfonyl heterocycles indicated the requirement for a partial negative charge at the heteroatom attached to C-3 of the steroid nucleus to attain androgen receptor affinity. Synthesis and testing of six heterocycle A-ring-fused dihydroethisterone derivatives support this hypothesis, and we have identified two new androgen receptor antagonists of this class.

Androgen Antagonists↗
PubMed1988

Modeling of beta-adrenoceptors based on molecular electrostatic potential studies of agonists and antagonists.

The molecular electrostatic potential (MEP) of 32 beta-adrenoceptor ligands, mainly antagonists, was calculated by the STO-3G ab initio quantum mechanical method. The MEP of phenylethanolamines (PEAs) features a negative minimum in the meta region (designated M1) which is topographically equivalent to a minimum (designated M2) found in the vicinity of the aromatic ring in all (aryloxy)propanolamines (AOPAs). In these compounds, a second negative zone located beyond the meta position and designated M3 is found in all beta 1-selective antagonists and in some nonselective and beta 2-selective antagonists. The beta 1-selective antagonists feature in the para position an additional zone which is positive (P4) in the full antagonists and negative (M4) in the antagonists displaying intrinsic sympathomimetic activity (ISA). The MEP-based pharmacophoric models of PEAs, AOPAs, and oxime ethers show common elements and lead to a proposed general model for beta-adrenoceptor ligands.

Adrenergic beta-Agonists↗
PubMed1986

Molecular electrostatic potential of orthopramides: implications for their interaction with the D-2 dopamine receptor.

The electronic properties of orthopramides, a group of selective D-2 dopamine receptor antagonists, were investigated by calculating molecular electrostatic potentials (MEP) of model compounds with the ab initio STO-3G MO method. The various substitution patterns of the aromatic ring are characterized by a positive region comprising the H-bonded 2-methoxy group and ring positions 2, 3, and 4 and a negative region comprising the CONH group, 5-substituent, and ring positions 5 and 6. The regions of positive and negative potential are separated by a "curtain" running along the longitudinal axis of the molecule. At shorter distances from the plane of the aromatic ring (1.75 and 2.0 A), this "curtain" is quite sinuous, but at greater distances (2.5 and 3.0 A) it tends toward rectilinearity. We postulate that this longitudinal separation, together with the single positive maximum and the three negative minima perceptible at 3.0 A, constitute a distance pharmacophore responsible for the recognition and proper alignment of the ligand. The more complex MEP at 1.75 and 2.0 A are equated with a contact pharmacophore. Comparison of the MEP of orthopramides and dopamine reveals some analogies and suggests a possible mode of binding of these antagonists to the D-2 receptor.

Benzamides↗
PubMed1986

Dopaminergic pharmacophore of ergoline and its analogues. A molecular electrostatic potential study.

Spatial correspondence between apomorphine, a prototype dopaminergic (DA) drug, and ergoline and some of its (partial) analogues were derived by matching their molecular electrostatic potential (MEP) patterns surrounding the aromatic moieties with respect to the coincident aliphatic N atoms. The MEP patterns were calculated from ab initio wave functions of model molecules. The congruent superimpositions of the molecular frameworks obtained between apomorphine and DA active ergoline analogues might corroborate the hypothesis that they bind with the same receptor sites when activating certain subtypes of the DA receptor.

Apomorphine↗
PubMed1982

Correlation between affinity toward adrenergic receptors and approximate electrostatic potentials of phenylethylamine derivatives. 1. Effects of the side chain.

The molecular electrostatic potential (VN) in the region of the nitrogen lone pair of a series of substituted propylamines is used in a correlation with the dissociation constants of parent phenylethylamine-type ligands obtained on beta-adrenoceptors by Bilezikian et al. It is shown that VN is a more effective index for quantitative structure-activity relationship studies than an optimal set of substituent constants used in additive, linear models. No significant correlation between the total electronic charge on the nitrogen and the binding potencies was obtained in the examined series. Protonation energies of the propylamines have been computed, but no meaningful correlation with the dissociation constants was obtained.

Chemical Phenomena↗
PubMed1988

Structural factors of importance for 5-hydroxytryptaminergic activity. Conformational preferences and electrostatic potentials of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and some related agents.

The conformational characteristics of two series of 5-hydroxytryptamine (5-HT) receptor agonists, monophenolic N,N-dialkylated 2-aminotetralins and trans-2-phenylcyclopropylamines, have been studied by a combination of experimental (NMR spectroscopy) and theoretical (molecular mechanics and MNDO calculations) methods. In addition, molecular electrostatic potentials have been calculated for selected conformations and the absolute configuration of the potent 5-HT-receptor agonist (+)-cis-8-hydroxy-1-methyl-2-(di-n-propylamino)tetralin has been determined, by X-ray crystallography of the synthetic precursor, to be 1S,2R. Results obtained are discussed in terms of conformational, steric, and electronic requirements for 5-HT-receptor activation. It is suggested that different conformations of the 5-HT-receptor agonists (1R,2S)-2-(2-hydroxyphenyl)-N,N-di-n-propylcyclopropylamine [(1R,2S)-4] and its 3-hydroxy isomer (1R,2S)-5 are able to activate 5-HT receptors. The strongly increased stereoselectivity of 2, 4, and 5 as compared to that of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 1) is rationalized on the basis of steric factors. Conformational factors appear to be responsible for the inability of the trans-C1-methyl-substituted derivative of 1 to activate 5-HT receptors.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed2005

Autocorrelation of molecular electrostatic potential surface properties combined with partial least squares analysis as new strategy for the prediction of the activity of human A(3) adenosine receptor antagonists.

The combination of molecular electrostatic potential (MEP) surface properties (autocorrelation vectors) with the conventional partial least squares (PLS) analysis has been used for the prediction of the human A(3) receptor antagonist activities. Three-hundred-fifty-eight structurally diverse human A(3) receptor antagonists have been utilized to generate a novel ligand-based three-dimensional structure-activity relationship. Remarkably, our chemical library includes all 21 important chemical classes of human A(3) antagonists currently discovered, and it represents the largest molecular collection used to generate a general human A(3) antagonist structure-activity relationship. A robust quantitative model has been obtained as described by both cross-validated correlation coefficient (r(cv) = 0.81) and prediction capability (r(pred) = 0.82). The proposed MEP/PLS approach can be considered as an alternative hit identification tool in virtual screening applications.

Adenosine A3 Receptor Antagonists↗
PubMed2001

Revisiting Markovnikov addition to alkenes via molecular electrostatic potential.

Molecular electrostatic potentials (MESP) surrounding the pi-region of several substituted ethylenes (CH(2)CHR) have been characterized by locating the most negative-valued point (V(min)) in that region. The substituents have been classified as electron donating and withdrawing on the basis of the increase or decrease in the negative character of V(min) in these systems as compared to ethylene. The values of V(min) show a good linear correlation with the Hammett sigma(p) constants, suggesting that the substituent electronic effects in substituted ethylenes and substituted benzenes are basically similar. With electron-donating substituents, the position of MESP minimum is closer to the unsubstituted carbon facilitating the pi-complex formation of it with HCl at this site. Such a regiospecific pi-complex formation is found to favor the formation of Markovnikov-type transition state for the addition of HCl to CH(2)CHR. For the electron-withdrawing substituents, the V(min) location is almost equidistant and farther from the ethylenic carbon atoms. This and the less negative V(min) values account for the less regiospecific CH(2)CHR...HCl pi-complexes as well as the transition states for the HCl addition to CH(2)CHR when R is an electron-withdrawing group. The interaction energy (E(int)) between CH(2)CHR and HCl for the formation of the CH(2)CHR...HCl pi-complex shows a good linear correlation with the corresponding V(min) value.

Journal Article↗
PubMed2001

Synthesis and biological activity of N-sulfonylphosphoramidates: probing the electrostatic preferences of alkaline phosphatase.

N-Sulfonylphosphoramidates have been synthesized to investigate the electrostatic requirements for binding to alkaline phosphatase. Alkyl- and aryl N-benzylated sulfonamides were phosphorylated with bromophosphates or synthesized via phosphoramidite chemistry in moderate yields (44-77%.) The resulting tribenzylated N-sulfonylphosphoramidates may be deprotected in one step to give the free acids in quantitative yields. Physical data of N-sulfonylphosphoramidates, including pK(a)'s and stability toward hydrolysis, were determined. Inhibition data suggests that AP does not bind trianionic N-sulfonylphosphoramidates better than dianionic N-sulfonylphosphoramidates, although N-sulfonylphosphoramidates are bound tighter than N-phenylphosphoramidate. k(cat) for the hydrolysis of N-sulfonylphosphoramidates by bovine and E. coli alkaline phosphatases is 10-60% that of p-nitrophenyl phosphate.

Alkaline Phosphatase↗
PubMed2002

Electrostatic interactions between substituents as regioselectivity control elements in Diels-Alder cycloadditions. A DFT study of cycloadditions of 1-methoxy-4-trimethylsiloxy dienes with acrylonitrile.

The regioselectivities of Diels-Alder reactions of 1-methoxy-4-trimethylsiloxy-1,3-butadiene and the corresponding o-xylylene with acrylonitrile were explored with density functional theory. The transition state of the reaction of the diene with acrylonitrile was studied in both the gas phase and in a low dielectric (epsilon = 2.247) solvent. The regioselectivities of these reactions are predicted to be controlled by the direct electrostatic interactions between the diene and dienophile substituents. The electron-donating capacities of methoxy and trimethylsiloxy substituents are shown to be very similar and to not contribute to regioselectivity control. A prediction is made that the cycloadditions of the o-xylylene will be unselective, while 1-methoxy-4-trimethylsiloxy-1,3-butadiene will give a small preference ( approximately 5:1) in favor of the proximal methoxy and cyano groups. The thermochromic behavior of trans-disubstituted disiloxy benzocyclobutene was also explored.

Acrylonitrile↗