Free energy surfaces for the interaction of D-glucose with planar aromatic groups in aqueous solution

Multidimensional potentials of mean force for the interactions in aqueous solution of both anomers of D-glucopyranose with two planar aromatic molecules, indole and para-methyl-phenol, have been calculated using molecular dynamics simulations with umbrella sampling and were subsequently used to estimate binding free energies. Indole and para-methyl-phenol serve as models for the side chains of the amino acids tryptophan and tyrosine, respectively. In all cases, a weak affinity between the glucose molecules and the flat aromatic surfaces was found. The global minimum for these interactions was found to be for the case when the pseudoplanar face of β-D-glucopyranose is stacked against the planar surfaces of the aromatic residues. The calculated binding free energies are in good agreement with both experiment and previous simulations. The multidimensional free energy maps suggest a mechanism that could lend kinetic stability to the complexes formed by sugars bound to sugar-binding proteins.     

Capillary zone electrophoresis of neutral organic molecules in organic – aqueous solution

Very good separations of non-ionic organic compounds have been achieved by capillary zone electrophoresis (CZE) using a tetrahep-tylammonium salt as an additive in aqueous acetonitrile as solvent. A systematic study was undertaken to determine the effect of experimental parameters on electroosmotic mobility and electro-phoretic mobility. It was found that pH, acetonitrile concentration, and the type and concentration of quaternary ammonium salt are important experimental variables. Under appropriate conditions, the separation window was enlarged and a broad range of electrically neutral organics, including very hydrophobic compounds (e. g. poly-cyclic aromatic hydrocarbons) and fairly hydrophilic compounds were separated in a relatively short time. By adjusting the separation conditions, high resolution CZE of a specific group of neutral organic compounds could be achieved. A method for calculation of capacity factor was proposed and capacity factors for a variety of non-lonic organic compounds were calculated.     

DNA interaction with synthetic polymers in solution

DNA complexes with cationic polymers (polyvinylamine (PVA), polyallylamine (PAA), polydimethylaminoethylmethacrylate (PDMAEM), poly-(N,N,N-trimethylammonio)ethyl methacrylate chloride (PTMAEM), poly-l-lysine (PLL)) were investigated. It was shown that volume and persistent length of DNA do not change essentially at low cationic polymer concentration in a solution. DNA packaging in 0.005 M NaCl was observed at charge ratio N/P ≈ 1. Secondary DNA structure in complexes was not disrupted, and DNA was protected from protonation. The comparison between DNA packaging in complexes with polycations and DNA condensation induced by trivalent ions was made.     

Molecular simulation studies on the interaction between different polymers in aqueous solution

A molecular dynamics simulation was performed to investigate the aggregates of mixing and the interaction between different polymers in aqueous solution. These polymers include partially hydrolyzed polyacryamide (HPAM), hydroxyethylcellulose (HEC) and polyvinylpyrrolidone (PVP). The structures of mixed aggregates were analyzed from the dihedral angle distribution of: (1) pure HPAM; (2) HPAM in aqueous solution; (3) HPAM with small segments of PVP or HEC in aqueous solution. At the same time, the simulated IR spectra and the calculated interaction parameters were used to distinguish the different interactions between HPAM and PVP or HEC. In order to confirm the validity of the simulated predictions, experimental IR spectra of polymer systems were made, and the specific viscosity of the HPAM and PVP or HEC system was measured using capillary viscometry. It can be seen from the viscosity measurements that the viscosity of the HPAM/PVP system in aqueous solution decreases linearly with an increase in concentration of PVP, whereas a maximum viscosity value appears with the increase in concentration of HEC in the HPAM/HEC system. The conclusion was drawn that the interaction between HPAM and HEC is stronger than the one between HPAM and PVP, and that molecular simulation can be considered as an adjunct to experiments and can provide otherwise inaccessible (or, not easily accessible) microscopic information that experimentalists can use.     

Investigation of the association of aromatic dyestuff molecules in aqueous solution by1H NMR

We have studied experimentally the proton chemical shifts of the molecules of Acridine Orange and proflavine in aqueous solution as a function of the.concentration of the aromatic ligands. A method is proposed for determining the chemical shifts of the protons in associations from the observed concentration dependence of the proton chemical shifts of dyestuff molecules in solution, which can be used at fairly high concentrations of the interacting molecules. The association constants of the dyestuff molecules and the proton chemical shifts in the association have been calculated. The proton shifts obtained have been used toether with a model of the ring currents to determine the most probable structure for the 1:1 molecular complexes of Acridine Organce and proflavine in aqueous solution.Sevastopol Institute for Instrument Construction. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, No. 1, pp. 83–89, January–February, 1991. Original article submitted February 2, 1988.     

The interaction of water-soluble α, ω-disubstituted alkanes (bolaform compounds) with polyvinylpyrrolidone in aqueous solution

Summary The published work on the interactions between polyvinylpyrrolidone (PVP) and small-molecule solutes (cosolutes) in aqueous solution has been briefly surveyed. The further information on these types of interaction which the proposed study of,-disubstituted aliphatic cosolutes, of the general type X(CH₂) _m Y (bolaform compounds), was hoped to yield has been outlined. The technique of equilibrium dialysis (employing two-compartment acrylate cells) has been used to study the interaction between PVP in aqueous solution at 25 °C and the following eight symmetrical (X=Y) bolaform cosolutes: X=SO₄ Na,m=8, 10 and 12 (A, B and C); X=CO₂ K,m=10 (D); X=N(CH₃)₃ Br,m=10 (E); X=NH₃ Cl,m=8, 10 and 12 (F, G and H). The maximum total cosolute concentrations used were between 100 and 230 millimolal. The equilibrated dialysis-cell solutions were assayed by differential refractometry; the data obtained were converted into the values ofa, the concentration of free cosolute, andr, the concurrent number of moles of cosolute bound per base mole (vinylpyrrolidone unit) of the polymer. The limit of detectable binding wasr=0.005 for all cosolutes. The results showed that of the three disulphates (A, B and C), the octamethylene compound (A) was not detectably bound while the deca- and dodecamethylene compounds (B and C) had binding isotherms of the hyperbolic (Langmuir) form:r =nKa/(1 +Ka), withK=65 (±20) molal^–1 andn=0.057 (±0.005) form=10, andK=16 (±2) molal^–1 andn=0.139 (±0.005) form=12; the other anionic cosolute (D) was not detectably bound. Of the cationic cosolutes (E, F, G and H), Decamethonium bromide (E) had a non-hyperbolic binding isotherm with a maximum ata=75 millimolal ofr~0.025; the three di(ammonium chloride)s (F, G and H) were not detectably bound. The refractive index increments and densities of the aqueous solutions of the same eight compounds have also been determined; the results did not show any anomalies which could definitely be attributed to association or micellisation, except in the case of Decamethonium bromide where ion-pairing is known to take place. The marked difference in binding behaviour between the anionic cosolutes and the cationic ones is attributed to the different environments of the positive and negative ends of the dipolar imide groups in the pyrrolidone rings of PVP; the binding data are discussed in terms of the polar (ion-dipole) and non-polar (hydrophobic) forces between cosolute and polymer, and of the relative roles played by the value of the end-to-end distance for the cosolute molecule and by any direct hydrophobic interactions in determining the dependence of binding behaviour upon cosolute chain length,m.     

Interaction of several polymers with p-substituted phenols in aqueous solution

The interactions between several water-soluble polymers, i.e., polyethyleneimine (PEI), poly-(ethylene glycol) (PEG), polyacrylamide (PAM), and poly(methacrylic acid) (PMAA), and p-X-substituted phenols (X = CH₃O, CH₃ C₂H₅, H, Cl, Br and NO₂) in aqueous solution at 30°C were investigated by means of equilibrium dialysis. By applying the Klotz equation to the results obtained, the bonding constants between these polymers, including poly(vinylpyrrolidone) (PVP) which was reported previously, and phenol were determined; they decreased in the following order: PVP > PMAA > PEI > PAM > PEG. This order was in agreement with that reported from the solubility of naphthalene, biphenyl, and alkyl halides in aqueous solution of these polymers caused by a hydrophobic bonding interaction. In this case, therefore, a hydrophobic bonding seemed to be significant. The bonding constants of these polymers with p-substituted phenols were also determined, and they were found to be approximately correlated with Hamett σ constants of the para substituents in phenols. Therefore, it was concluded that the interactions due to electrostatic and hydrogen-bonding forces were also important.     

Multicomponent polyanions. 45. A multinuclear NMR study of vanadate(V)-oxalate complexes in aqueous solution.

The two complexes formed in the aqueous vanadooxalate system, V(Ox)- and V(Ox)2(3-), have been characterized using 51V, 13C and 17O NMR. For the V(Ox)2(3-) complex, two peaks are observed in 13C NMR and four in 17O NMR. This leads to the conclusion that each oxalate ligand has two different distances to the VO2 group. This fact, together with the peak integrals and the chemical shifts, indicates strongly that the hexacoordinate complex [VO2(C2O4)2]3- found in single-crystal X-ray structure determinations persists in aqueous solution. The dependence of the 13C NMR linewidths upon temperature reveals two types of dynamic processes: (1) a rearrangement in which the two different V-Oox switch places and (2) an exchange of the oxalate ligands in the [VO2(C2O4)2]3- complex with free oxalate, probably through a dissociative process. Rate constants and activation parameters for the two dynamic processes involving [VO2(C2O4)2]3- have been calculated from the shape of the 13C NMR signals. For the V(Ox)- complex, only one relatively narrow peak is obtained in 13C NMR and three peaks in 17O. This fact, as well as the relative positions of these peaks, is in accordance with a pentacoordinate complex [VO2(C2O4)H2O]-, where the two V-O distances to the oxalate ligand are equal. We also show that, in the pH range 0.8-6.6, there is no protonation of the studied complexes, in agreement with previous potentiometric results.     

A highly selective fluorescence chemosensor for Pb(II) in neutral buffer aqueous solution

A 3,4-dimethylthieno[2,3-b]thiophene-based fluorogenic probe bearing benzo[d]-thiazole-2-thio unit (sodium 3,4-bis ((benzo[d]thiazol-2-ylthio)methyl) thieno [2, 3-b]thio-phene-2, 5-dicarboxylate) was developed as a novel fluorescent chemosensor with high selectivity towards Pb(II) over other cations tested. The new probe exhibited good water solubility and only sensed Pb(II) among metal ions examined in neutral 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer solution. The selectivity and sensitivity of fluorogenic probe to Pb(II) were discussed on the basis of experimental results.     

On the photolysis of simple anions and neutral molecules as sources of O-/OH, SO(x)- and Cl in aqueous solution

This contribution examines the aqueous phase photolysis processes of simple anions such as nitrate, nitrite, peroxodisulfate and neutral molecules such as H2O2. The review includes new results on absolute effective quantum yields for the photodissociation processes of NO3(-), NO2(-), S2O8(2-), HSO5(-), S2O6(2-), HOCl, and chloroacetone in an aqueous solution. The quantum yields for the photolysis of nitrate and nitrite have also been determined as a function of temperature. Models to interpret the wavelength and the temperature dependencies of the quantum yields for the different systems are discussed and a simple model treatment is developed to quantify the effects of (i) impulse conservation, (ii) electrostatic interaction (e.g., ion-dipole, dipole-dipole and coulomb interaction between the photofragments directly after photolytic fragmentation), and (iii) diffusion and recombination. The combined impulse-interaction-diffusion (IID) model is compared to the experimentally observed effective radical formation quantum yields and reasonable agreement is found for a number of systems. It is shown that the temperature dependencies for effective quantum yields of photolysis processes in aqueous solution are not only governed by the temperature dependence of the viscosity of water but also determined by the temperature dependence of the rate constants of the photofragment recombination reactions.     

Complexation of uranium(VI) with aromatic acids in aqueous solution: a combined computational and experimental study

The complexes of uranium(VI) with salicylhydroxamate, benzohydroxamate, and benzoate have been investigated in a combined computational and experimental study using density functional theory methods and extended X-ray absorption fine structure spectroscopy, respectively. The calculated molecular structures, relative stabilities, as well as excitation spectra from time-dependent density functional theory calculations are in good agreement with experimental data. Furthermore, these calculations allow the identification of the coordinating atoms in the uranium(VI)-salicylhydroxamate complex, i.e. salicylhydroxamate binds to the uranyl ion via the hydroxamic acid oxygen atoms and not via the phenolic oxygen and the nitrogen atom. Carefully addressing solvation effects has been found to be necessary to bring in line computational and experimental structures, as well as excitation spectra.     

Light scattering and fluorescence photobleaching recovery study of poly(amidoamine) cascade polymers in aqueous solution

Diffusion coefficients of polyamidoamine cascade polymers (PAMAMs) were measured in aqueous solutions by dynamic light scattering and, after labeling with fluorescein isothiocyanate, by fluorescence photobleaching recovery. The dynamic light scattering results depended weakly on pH at a high salt concentration, but varied strongly with the concentration of added salt in the low-salt limit. The fluorescence photobleaching recovery values were almost independent of salt concentration. The difference between the two techniques is that thermodynamic nonideality strongly affects light scattering at the concentrations that are experimentally accessible. The hydrodynamic sizes from fluorescence photobleaching recovery were somewhat smaller than those from dynamic light scattering in the high-salt limit, despite attachment of the dye. Nevertheless, fluorescently tagged PAMAMs should make suitable markers and diffusion probes. © 1996 John Wiley & Sons, Inc.     

Pulsed radiolysis of perrhenate in neutral aqueous solution: Comparison with pertechnetate

Reaction of perrhenate with the aquated electron in neutral aqueous solution yields ReO₄^2? (k_f 1.3 × 10¹⁰ M^?1 s^?1), with an absorption maximum at 290 nm (ε 1700). This decays by a second-order path (k_d 1.5 × 10⁹ M^?1 s^?1) at a rate ～ 100-fold faster than the decay of TcO₄^2? under similar conditions.     

Substituent interaction effects in aromatic molecules in RP-HPLC with acetonitrile-water and tetrahydrofuran-water eluents

Summary Retention volumes of monosubstituted benzenes, benzoic acids, phenols and anilines have been measured in reversed-phase liquid chromatography. Buffered acetonitrile-water and tetrahydrofuran-water eluents were used with an octadecylsilylsilica adsorbent. From the net retention volumes a substituent interaction effect was calculated and described with the linear free energy relationship developed by Taft. The positive sign of the values of the -parameters, figuring in this relationship, was interpreted in terms of hydrogen bonding between the solutes and the eluent.     

Studies on the interaction of poly(4-vinylpyridiniumchloride) with poly(sodiumphosphate) in an aqueous solution by conductometry

A reaction between poly(4-vinylpyridiniumchloride) and poly(sodiumphosphate) in the presence and absence of NaCl and NaBr salts was studied in aqueous solution by conductometry. The interaction of polycation and polyanion gave insoluble polyelectrolyte complex which contained polycation and polyanion in unit mole ratio in a salt-free solution. A deviation from stoichiometry was observed at high polyion concentration and in the presence of NaCl and NaBr salts. The resultant complex showed swelling property in different solvent mixtures. A maximum degree of swelling was obtained in the solvent mixture of NaBr + water and NaBr + water + acetone. Furthermore, polyelectrolyte complex sorbed salts from aqueous electrolyte solutions. The sorption of salts increased with increasing salt concentration. © 1996 John Wiley & Sons, Inc.     

The kinetics of cobalt(II) complexation with o-phthalate in aqueous solution

The rate constants for the formation and dissociation of cobalt(II) complexes with o-phthalate in aqueous solution have been determined by a pressure-jump technique. The forward and reverse rate constants obtained are k_f = 1.1 x 10⁶ M⁻¹ s⁻¹ and k_r = 1.6 x 1O³ s⁻ at 25°C, respectively. The activation parameters of the reaction were also obtained from the temperature variation study. The results indicate that chelate ring closure and rupture are affected by the rigidity of the benzene ring of the ligand, and thus the rate determining step of the reaction is the chelate ring closure process.