Capillary electrophoretic studies on the migration behavior of cationic solutes and the influence of interactions of cationic solutes with sodium dodecyl sulfate on the formation of micelles and critical micelle concentration

The migration behavior of cationic solutes and influences of the interactions of cationic solutes with sodium dodecyl sulfate (SDS) on the formation of micelles and its critical micelle concentration (CMC) were investigated by capillary electrophoresis at neutral pH. Catecholamines and structurally related compounds, including epinephrine, norepinephrine, dopamine, norephedrine, and tyramine, which involve different extents of hydrophobic, ionic and hydrogen-bonding interactions with SDS surfactant, are selected as cationic solutes. The dependence of the effective electrophoretic mobility of cationic solutes on the concentration of surfactant monomers in the premicellar region provides direct evidence of the formation of ion-pairs between cationic solutes and anionic dodecyl sulfate monomers. Three different approaches, based on the variations of either the effective electrophoretic mobility or the retention factor as a function of surfactant concentration in the premicellar and micellar regions, and the linear relationship between the retention factor and the product of a distribution coefficient and the phase ratio, were considered to determine the CMC value of SDS micelles. The suitability of the methods used for the determination of the CMC of SDS with these cationic solutes was discussed. Depending on the structures of cationic solutes and electrophoretic conditions, the CMC value of SDS determined varies in a wide concentration range. The results indicate that, in addition to hydrophobic interaction, both ionic and hydrogen-bonding interactions have pronounced effects on the formation of SDS micelles. Ionic interaction between cationic solutes and SDS surfactant stabilizes the SDS micelles, whereas hydrogen-bonding interactions weakens the solubilization of the attractive ionic interaction. The elevation of the CMC of SDS depends heavily on hydrogen-bonding interactions between cationic solutes and SDS surfactant. Thus, the CMC value of SDS is remarkably elevated with catecholamines, such as epinephrine and norepinephrine, as compared with norephedrine. In addition, the effect of methanol content in the sample solution of these cationic solutes on the CMC of SDS was also examined.     

Retention in gas-liquid chromatography with a polyethylene oxide stationary phase: molecular simulation and experiment

Configurational-bias Monte Carlo simulations in the isobaric-isothermal Gibbs ensemble were carried out to investigate the partitioning of normal alkanes, primary and secondary alcohols, symmetric alkyl ethers and arenes between a helium vapor phase and a polyethylene oxide stationary phase (M(W)=382 g mol(-1)). The united-atom version of the transferable potentials for phase equilibria force field was used to model all solutes, polyethylene oxide and helium. The Gibbs free energies of transfer and Kovats retention indices of the solutes were calculated directly from the partition constants at two different temperatures, 353 and 393 K. Chromatographic experiments on a Carbowax 20M retentive phase were performed for the same set of solutes and temperatures ranging from 333 to 413 K. The predicted retention indices for alcohols, ethers and arenes are overestimated by about 120, 70 and 20 retention index units, respectively, pointing to an overestimation of the first-order electrostatic interactions in the model system. Molecular-level analysis shows that hydrogen-bonding and dipole-dipole interactions lead to orientational ordering for the alcohol and ether analytes, whereas the weaker dipole-quadrupole interactions for the arene solutes are not sufficient to induce orientational ordering. The retention indices of alcohols and ethers decrease with increasing temperature because of the large entropic cost of hydrogen-bonding and orientational ordering. In contrast, the retention indices for arenes increase with increasing temperature because the entropic cost of cavity formation is smaller for arenes than for comparable alkanes.     

Amino acid side chain interactions in the presence of salts

The effects of salt on the intermolecular interactions between polar/charged amino acids are investigated through molecular dynamics simulations. The mean forces and associated potentials are calculated for NaCl salt in the 0-2 M concentration range at 298 K. It is found that the addition of salt may stabilize or destabilize the interactions, depending on the nature of the interacting molecules. The degree of (de)stabilization is quantified, and the origin of the salt-dependent modulation is discussed based upon an analysis of solvent density profiles. To gain insight into the molecular origin of the salt modulation, spatial distribution functions (sdf's) are calculated, revealing a high degree of solvent structuredness in all cases. The peaks in the sdf's are consistent with long-range hydrogen-bonding networks connecting the solute hydrophilic groups, and that contribute to their intermolecular solvent-induced forces. The restructuring of water around the solutes as they dissociate from close contact is analyzed. This analysis offers clues on how the solvent structure modulates the effective intermolecular interactions in complex solutes. This modulation results from a critical balance between bulk electrostatic forces and those exerted by (i) the water molecules in the structured region between the monomers, which is disrupted by ions that transiently enter the hydration shells, and (ii) the ions in the hydration shells in direct interactions with the solutes. The implications of these findings in protein/ligand (noncovalent) association/dissociation mechanisms are briefly discussed.     

ELECTRONIC SPECTRA OF AZA-AROMATICS IN POLYMER MATRICES†

Abstract— The absorption and fluorescence spectra of acridine, phenazine, their cations and phenazine-di-N-oxide are studied in several polymer matrices. There is a one-to-one correspondence between previously measured spectra in fluid media and those reported here for plastic media. The matrix-induced spectral shifts are dependent upon the polarity and hydrogen-bonding ability of the polymer substrate. Environmental perturbations are also produced by high pressures (0–27 kbar) applied to the polymer matrix. Specific interactions between polymer substrate and the electronically excited solutes are evident from pressure-induced red shifts of the fluorescence and changes in quantum yields under pressure. These interactions between solute and its microscopic environment are subtle and varied for this group of nitrogen hetero-cyclics, although their effects are smaller than those encountered in the adsorption of related dyes.     

Chromatography of aromatic compounds on anion-exchange resins

Complex mixtures of aromatic compounds can be rapidly separated on anion-exchangers in the acetate form with acetic acid as eluent and determined automatically by recording the absorbance in the ultraviolet. Carboxylates are separated by ion-exchange. Hydrogen bonds between non-disso-ciated acids and the counter-ions influence the distribution coefficients. Hydrogen-bonding with the resin has a marked effect on the sorption of solutes containing phenolic protons. Intramolecular hydrogen-bonding depresses their sorption. Hydrophobic interactions have a predominant influence with hydrocarbons and with phenolic compounds containing non-polar aliphatic substituents. The relative importance of these interactions increases with a decreased ion-exchange capacity of the resin.     

Substrate–polymer interactions in liquid exclusion chromatography (GPC) in N,N-dimethylformamide

The importance of nonexclusion effects in the GPC behavior of several stationary phases was investigated with DMF, 0.01M LiBr, as the mobile phase. Various low MW solutes and narrow MWD polymers, encompassing a wide range of polarities, were studied. The elution of the polymers was examined in terms of “universal calibration” behavior. Styragel and silanized glass both exhibit affinity for apolar polymers in DMF; for the former substrate this effect shows a strong inverse dependence on MW. As a consequence, application of polystyrene calibration curves to GPC analysis of more polar polymers with these substrates leads to overestimations of MW parameters. These errors are not corrected when universal calibration procedures are used. Ideal exclusion chromatography is exhibited by a number of polymers on untreated porous glass substrates. However, polymers with strong hydrogen-bonding functionality appear to be susceptible to marked adsorption in this system.     

Competition of hydrophobic and Coulombic interactions between nanosized solutes

The solvation of charged, nanometer-sized spherical solutes in water, and the effective, solvent-induced force between two such solutes are investigated by constant temperature and pressure molecular dynamics simulations of model solutes carrying various charge patterns. The results for neutral solutes agree well with earlier findings, and with predictions of simple macroscopic considerations: substantial hydrophobic attraction may be traced back to strong depletion ("drying") of the solvent between the solutes. This hydrophobic attraction is strongly reduced when the solutes are uniformly charged, and the total force becomes repulsive at sufficiently high charge; there is a significant asymmetry between anionic and cationic solute pairs, the latter experiencing a lesser hydrophobic attraction. The situation becomes more complex when the solutes carry discrete (rather than uniform) charge patterns. Due to antagonistic effects of the resulting hydrophilic and hydrophobic "patches" on the solvent molecules, water is once more significantly depleted around the solutes, and the effective interaction reverts to being mainly attractive, despite the direct electrostatic repulsion between solutes. Examination of a highly coarse-grained configurational probability density shows that the relative orientation of the two solutes is very different in explicit solvent, compared to the prediction of the crude implicit solvent representation. The present study strongly suggests that a realistic modeling of the charge distribution on the surface of globular proteins, as well as the molecular treatment of water, are essential prerequisites for any reliable study of protein aggregation.     

Effects of hydrogen bonding on the acidity of adenine, guanine, and their 8-oxo derivatives

Complexes between ammonia, water, or hydrogen fluoride and adenine, guanine, or their 8-oxo derivatives are investigated using density-functional theory. The binding strengths of the neutral and (N9) anionic complexes are considered for a variety of purine binding sites. The effects of hydrogen-bonding interactions on the (N9) acidity of the purine derivatives are considered as a function of the molecule bound and the binding site. It is found that hydrogen-bonding interactions with one molecule can increase the acidity of purine derivatives by up to 60 kJ mol(-1). The (calculated) simultaneous effects of up to four molecules on the acidity of the purine derivatives are also considered. Our data suggest that the effects of more than one molecule on the acidity of the purines are generally less than the sum of the individual (additive) effects, where the magnitude of the deviation from additivity increases with the number, as well as the acidity, of molecules bound. Nevertheless, the increase in the acidity due to additional hydrogen-bonding interactions is significant, where the effect of two, three, or four hydrogen-bonding interactions can be as large as approximately 95, 115, and 130 kJ mol(-1), respectively. The present study provides a greater fundamental understanding of hydrogen-bonding interactions involving the natural purines, as well as those generated through oxidative DNA damage, which may aid the understanding of important biological processes.     

Sonoluminescence quenching of organic compounds in aqueous solution: frequency effects and implications for sonochemistry

Solute-induced quenching of sonoluminescence (SL) is reported for aqueous solutions of two homologous series of methyl esters and ketones using low (20 kHz) and high (515 kHz) ultrasound frequencies. SL data at 20 kHz from aqueous solutions containing alcohols and carboxylic acids are also presented to compare with previously published results at 515 kHz. In addition to supporting the previous findings on the existence of stable and transient bubbles at 515 and 20 kHz, respectively, the results suggest that the hydrogen-bonding characteristics of the solutes also play a major role in the extent of SL quenching. An increase in the SL intensity at low concentrations for most of the solutes suggests that these solutes increase the number of "active" bubbles by hindering the coalescence of bubbles. It is concluded that the effect of the solutes on the SL signal from aqueous solutions at both frequencies is primarily due to the balance of two factors, namely, the incorporation of solute within the bubble, leading to SL quenching, and the prevention of coalescence of the bubbles, leading to SL enhancement. At the higher frequency, SL quenching by the solutes is the main influence on the emission yield. However, at the lower frequency, hindrance to coalescence by the solutes dominates at lower concentrations and leads to SL enhancement. The implications of these results for optimizing conditions for aqueous sonochemical reactions are discussed.     

杯[8]芳烃键合硅胶固定相的制备、表征及色谱性能

杯芳烃通过疏水Π-Π、氢键和静电等作用能与中性分子及离子形成包合物,在离子选择性电极、催化、分离和酶模拟等领域受到关注.已报道的杯芳烃键合固定相的制备方法^[1～3]都是先合成含杯芳烃硅烷化试剂,然后通过硅胶硅烷化反应制备键合固定相.其制备路线反应过程长,各种有机中间体纯化操作复杂.前文[4]曾以氯丙基键合硅胶为中间体,通过固相连续反应制备了氮杂冠醚键合硅胶固定相.本文采用固相连续反应制备了一种新型的对-叔丁基杯^[8]芳烃键合固定相,通过元素分析、红外光谱等手段获得键合相分子结构信息,以多环芳烃和二取代苯位置异构体为溶质,对固定相的色谱性能及保留机理进行了研究.     

Modeling of tracer diffusion in liquids when solute–solvent interactions are present

The linear solvation energy relationship (LSER) model is employed to correlate the tracer diffusion coefficients of 550 binary systems at 298.15 K. Among the selected solutes and solvents there exist apolar, polar and hydrogen-bonding substances that can interact with themselves (solvent polymerization) or with the other compound (solute–solvent complexes formation). The results of the proposed formulas are compared with those of other predictive equations.     

Effects of Hydrogen-bonding Interaction and Polarity on Emission Spectrum of Naphthalene-Triethylamine in Mixed Solvent

The effects of the protic and aprotic polar solvents on the emission spectrum of the naphthalene-triethyl-amine system in THF were studied under conditions of steady-state illumination. The fluorescence spectrum of the naphthalene-triethylamine system consists of two emission bands, the fluorescence band of naphthalene (band A, 329 nm) and the emission band of the exciplex(band B, 468 nm). The intensities of both the emission bands decrease with increasing the solvent polarity. The intensity of band B also decreases due to the hy-drogen-bonding interaction between triethylamine and protic solvent, while that of band A increases. It is thus suggested that the quenching of naphthalene fluorescence by triethylamine in THF occurs through the charge transfer and electron transfer reactions. The spectral changes upon the increase of solvent polarity can be explained by the dependences of the equilibrium constant between exciplex and ion-pair and the rate constant for the electron transfer reaction from triethyl.amine to the excited naphthalene on the rel.ative permittivity of solvent. It is shown that the formation of intermolecular hydrogen-bonding between triethylamine and protic solvent suppresses the quenching reaction by the decrease in free amine. Acetonitrile has only a polar effect and trichloroacetic acid only a hydrogen-bonding(or protonation) effect, while alcohols have both the effects. The effects of alcohols could be separated into the effects of solvent polarity and intermolecular hy-drogen-bonding interaction quantitatively.     

Solvent extraction of ionic solutes in aqueous solutions

Extraction of ionic solutes in aqueous solutions into various organic solvents is reviewed by showing several examples. The extraction of strong acids into polar organic solvents and nonpolar solvents containing hydrogen-bonding extractants is described as the first example and the extraction of simple metal salts into strongly dielectric or solvating polarsolvents and nonpolar solvents containing solvating extractants is then reported. Finally, the solvent extraction of anionic metal complexes with bulky cations into nonpolar solvents as ion-paris is described and the statistical method for such extraction equilibria is considered.     

3,5-二硝基苯甲酰基高效液相色谱键合硅胶固定相的制备与评价

以N-(β-氨乙基)-γ-氨丙基甲基二甲氧基硅烷为偶联剂,建立了一种制备3,5-二硝基苯甲酰基键合硅胶固定相(DNB)的新方法。采用元素分析、红外光谱和热分析表征了该固定相的结构。根据元素分析碳含量结果,计算出DNB表面配体浓度为2.08 μmol/m2。以芳烃(PAHs)、酚类、芳胺类、硝基苯酚异构体和磺胺类化合物作溶质探针,较系统地研究了该固定相的色谱性能。研究表明,所制备的固定相除了具有弱的疏水性外,还能与溶质发生电荷转移、静电、氢键和偶极-偶极等作用,从而提高对溶质的分离选择性。同时,由于间隔基对硅醇羟基的屏蔽作用,适用于含氮的碱性化合物的分离。     

Theoretical effects of macromolecule concentration and charge on membrane rejection coefficients

A hydrodynamic model for the convection of rigid, spherical solutes through cylindrical pores, which includes both steric and electrostatic interactions between pairs of solute particles and between solutes and the pore wall, has been developed to examine the effects of solute concentration and charge on solute rejection by membrane pores during ultrafiltration. Calculations have been performed for a wide range of charge conditions and the results are presented in terms of the membrane rejection coefficient at infinite dilution and a correction factor which accounts for the first-order effects of concentration. For pores and solutes of like charge, the rejection coefficient is predicted to decrease with increasing feed concentration or ionic strength.     

Molecular rotation as a tool for exploring specific solute-solvent interactions.

Solute-solvent interactions play an important role in determining the physicochemical properties of liquids and solutions. As a consequence, understanding these interactions has been one of the long-standing problems in physical chemistry. This Minireview describes our approach towards attaining this goal, which is to investigate rotational relaxation of a pair of closely related, medium-sized nondipolar solutes in a set of appropriately chosen solvents. Our studies indicate that solute-solvent hydrogen bonding significantly hinders solute rotation. We have also examined the role of solvent size both in the absence and presence of specific interactions and it has been observed that the size of the solvent has a bearing on solute rotation especially in the absence of specific interactions. Our results point to the fact that only strong solute-solvent hydrogen bonds have the ability to impede the rotation of the solute molecule because, in such a scenario, hydrogen-bonding dynamics and rotational dynamics transpire on comparable time scales. This aspect has been substantiated by measuring the reorientation times of the chosen solutes in solvents such as ethanol and trifluoroethanol, which have distinct hydrogen-bond donating and accepting abilities, and correlating them with solute-solvent interaction strengths. As an alternative treatment, it has been shown that specific interactions between the solute and the solvent can be modeled as dielectric friction with the extended charge distribution model. This approach is not unrealistic considering the fact that specific as well as non-specific interactions are electrostatic by nature and the differences between them are subtle.     

Hydrogen-bonding effects,electrostatic potential,and the antitumor activity of flavone acetic acid and related compounds. II. Ab initio studies on the second stable conformations

We previously studied the first stable conformations of flavone acetic acid and related compounds. In this article, a similar investigation was carried out on the second stable conformation of the same compounds. Emphasis is on the conformation dependence of the hydrogen-bonding effects, the molecular electrostatic potential (MEP), and the antitumor activity shown by these compounds. The results show that the second conformation is about 7.0 kcal mol⁻¹ higher in energy and possibly is an inactive conformation as no correlation has been found between the antitumor activity and the MEP features. In addition, a detailed comparison with the first conformation, which is probably the active conformation, has been made of the geometry, the total energy, the Mulliken charges on some important atoms, hydrogen-bonding effects, and the MEP minima and isosurfaces. The role of the hydrogen-bonding effects, which was unclear in our previous work, is clarified in this work. The possible molecular basis of the antitumor activity is suggested. © 1996 John Wiley & Sons, Inc.     

Thermal oxidation of blends of poly(vinyl methyl ether) and modified polystyrenes

Thermal oxidation of blends of poly(vinyl methyl ether) and styrene copolymers containing hydroxyl groups as hydrogen-bond donors was studied. The hydrogen-bonding interaction provide for improved miscibility between the component polymers and more extensive crosspropagation/cross-termination reactions. In addition to their contribution to improved miscibility, phenol groups in the copolymers exhibited apparent antioxidant and prooxidant effects.     

姜黄素键合硅胶固定相的制备、表征及色谱性能

通过γ-［(2,3)-环氧丙氧］丙基三甲氧基硅烷(KH-560)偶联剂将具有抗菌功能的植物有效成分姜黄素键合到硅胶上,制备了姜黄素液相色谱键合硅胶固定相(CCSP)。采用元素分析、红外光谱和热分析对该固定相结构进行了表征。以甲醇和水为二元流动相,不同的中性、酸性和碱性化合物为溶质探针,并用ODS柱作参比,对固定相的色谱性能及保留机理进行了研究。研究结果表明,姜黄素键合固定相不仅具有良好的反相色谱性能,同时由于配体结构中所含有的基团形成了含芳环的共轭体系,从而引入了n-π和π-π作用位点,所含的羟基和β-二羰基与溶质之间存在偶极-偶极和氢键作用,与ODS相比,该固定相在极性化合物分离中占优势,且分析速度较快。