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1.
Biomacromolecules ; 23(10): 4412-4426, 2022 Oct 10.
Artículo en Inglés | MEDLINE | ID: mdl-36134887

RESUMEN

In the protein purification, drug delivery, food industry, and biotechnological applications involving protein-polyelectrolyte complexation, proper selection of co-solutes and solution conditions plays a crucial role. The onset of (bio)macromolecular complexation occurs even on the so-called "wrong side" of the protein isoionic point where both the protein and the polyelectrolyte are net like-charged. To gain mechanistic insights into the modulatory role of salts (NaCl, NaBr, and NaI) and sugars (sucrose and sucralose) in protein-polyelectrolyte complexation under such conditions, interaction between bovine serum albumin (BSA) and sodium polystyrene sulfonate (NaPSS) at pH = 8.0 was studied by a combination of isothermal titration calorimetry, fluorescence spectroscopy, circular dichroism, and thermodynamic modeling. The BSA-NaPSS complexation proceeds by two binding processes (first, formation of intrapolymer complexes and then formation of interpolymer complexes), both driven by favorable electrostatic interactions between the negatively charged sulfonic groups (-SO3-) of NaPSS and positively charged patches on the BSA surface. Two such positive patches were identified, each responsible for one of the two binding processes. The presence of salts screened both short-range attractive and long-range repulsive electrostatic interactions between both macromolecules, resulting in a nonmonotonic dependence of the binding affinity on the total ionic strength for both binding processes. In addition, distinct anion-specific effects were observed (NaCl < NaBr < NaI). The effect of sugars was less pronounced: sucrose had no effect on the complexation, but its chlorinated analogue, sucralose, promoted it slightly due to the screening of long-range repulsive electrostatic interactions between BSA and NaPSS. Although short-range non-electrostatic interactions are frequently mentioned in the literature in relation to BSA or NaPSS, we found that the main driving force of complexation on the "wrong side" are electrostatic interactions.


Asunto(s)
Sales (Química) , Albúmina Sérica Bovina , Polielectrolitos , Poliestirenos , Albúmina Sérica Bovina/química , Cloruro de Sodio , Sacarosa , Azúcares
2.
Int J Mol Sci ; 23(23)2022 Dec 06.
Artículo en Inglés | MEDLINE | ID: mdl-36499696

RESUMEN

We present here a freely available web-based database, called BioMThermDB 1.0, of thermophysical and dynamic properties of various proteins and their aqueous solutions. It contains the hydrodynamic radius, electrophoretic mobility, zeta potential, self-diffusion coefficient, solution viscosity, and cloud-point temperature, as well as the conditions for those determinations and details of the experimental method. It can facilitate the meta-analysis and visualization of data, can enable comparisons, and may be useful for comparing theoretical model predictions with experiments.


Asunto(s)
Hidrodinámica , Proteínas , Soluciones , Viscosidad , Agua
3.
Molecules ; 27(3)2022 Feb 01.
Artículo en Inglés | MEDLINE | ID: mdl-35164264

RESUMEN

Pharmaceutical design of protein formulations aims at maximum efficiency (protein concentration) and minimum viscosity. Therefore, it is important to know the nature of protein-protein interactions and their influence on viscosity. In this work, we investigated the dependence of the viscosity of BSA in an aqueous 20 mM acetate buffer at pH = 4.3 on protein concentration and on temperature (5-45 °C). The viscosity of the solution increased with protein concentration and was 230% higher than the viscosity of the protein-free formulation at 160 mg/mL. The viscosity decreased by almost 60% in the temperature range from 5 to 45 °C. The agreement of the modified Arrhenius theory with experiment was quantitative, whereas a hard-sphere model provided only a qualitative description of the experimental results. We also investigated the viscosity of a 100 mg/mL BSA solution as a function of the concentration of added low molecular weight salts (LiCl, NaCl, KCl, RbCl, CsCl, NaBr, NaI) in the range of salt concentrations up to 1.75 mol/L. In addition, the particle size and zeta potential of BSA-salt mixtures were determined for solutions containing 0.5 mol/L salt. The trends with respect to the different anions followed a direct Hofmeister series (Cl- > Br- > I-), whereas for cations in the case of viscosity the indirect Hofmeister series was observed (Li+ > Na+ > K+ > Rb+ > Cs+), but the values of particle sizes and zeta potential did not show cation-specific effects. Since the protein is positively charged at pH = 4.3, anions are more attracted to the protein surface and shield its charge, while the interaction with cations is less pronounced. We hypothesize that salt surface charge shielding reduces protein colloidal stability and promotes protein aggregate formation.


Asunto(s)
Sales (Química)/química , Albúmina Sérica Bovina/química , Tampones (Química) , Peso Molecular , Soluciones , Viscosidad
4.
J Mol Liq ; 3532022 May 01.
Artículo en Inglés | MEDLINE | ID: mdl-35273421

RESUMEN

We present a combined computational approach to protein-ligand binding, which consists of two steps: (1) a deep neural network is used to locate a binding region on a target protein, and (2) molecular docking of a ligand is performed within the specified region to obtain the best pose using Autodock Vina. Our in-house designed neural network was trained using the PepBDB dataset. Although the training dataset consisted of protein-peptide complexes, we show that the approach is not limited to peptides, but also works remarkably well for a large class of non-peptide ligands. The results are compared with those in which the binding region (first step) was provided by Accluster. In cases where no prior experimental data on the binding region are available, our deep neural network provides a fast and effective alternative to classical software for its localization. Our code is available at https://github.com/mksmd/NNforDocking.

5.
J Mol Liq ; 3492022 Mar 01.
Artículo en Inglés | MEDLINE | ID: mdl-35082451

RESUMEN

The stability of bovine serum albumin (BSA) solutions against phase separation caused by cooling the system is studied under the combined influence of added poly(ethylene glycol) (PEG) and alkali halide salts in water as solvent. The phase stability of the system depends on the concentration of the added PEG and its molecular mass, the concentration of the low molecular mass electrolyte and its nature, as also on the pH of the solution. More specifically, the addition of NaCl to the BSA-PEG mixture promotes phase separation at pH = 4.0, where BSA carries the net positive charge in aqueous solution, and it increases the stability of the solution at pH=4.6, i.e., near the isoionic point of the protein. Moreover, at pH = 4.6, the cloud-point temperature decreases in the order from NaF to NaI and from LiCl to CsCl. The order of the salts at pH = 4.0 is exactly reversed: LiCl and NaF show the weakest effect on the cloud-point temperature and the strongest decrease in stability is caused by RbCl and NaNO3. An attempt is made to correlate these observations with the free energies of hydration of the added salt ions and with the effect of adsorption of salt ions on the protein surface on the protein-protein interactions. Kosmotropic salt ions decrease the phase stability of BSA-PEG-salt solutions at pH < pI, while exactly the opposite is true at pH = pI.

6.
J Mol Liq ; 367(Pt A)2022 Dec 01.
Artículo en Inglés | MEDLINE | ID: mdl-37790165

RESUMEN

In all biologically relevant media, proteins interact in the presence of surrounding ions, and such interactions are water-mediated. Water molecules play a crucial role in the restructuring of proteins in solution and indeed in their biological activity. Surface water dynamics and proton exchange at protein surfaces is investigated here using NMR relaxometry, for two well-known globular proteins, lysozyme and bovine serum albumin, with particular attention to the role of surface ions. We present a unified model of surface water dynamics and proton exchange, accounting simultaneously for the observed longitudinal and transverse relaxation rates. The most notable effect of salt (0.1 M) concerns the slow surface water dynamics, related to rare water molecules embedded in energy wells on the protein surface. This response is protein-specific. On the other hand, the proton exchange time between labile protein-protons and water-protons at the protein surface seems to be very similar for the two proteins and is insensitive to the addition of salts at the concentration studied.

7.
J Chem Educ ; 99(10): 3595-3600, 2022 Oct 11.
Artículo en Inglés | MEDLINE | ID: mdl-36246424

RESUMEN

Electrochromism encompasses reversible changes of material's optical properties (color, opacity) under the influence of an external electric current or applied voltage. The effect has been known for decades, but its importance continues to grow due to the rapid development of smart systems and the accompanying demand to build devices that consume less power. Most commercial electrochromic devices (ECDs) require sophisticated chemicals and advanced material preparation techniques. Also, the demonstration of electrochromism in chemistry classes mainly uses expensive WO3 films, intrinsically conductive polymers, and/or optically transparent electrodes (OTEs). The aim of this article is to present a simple and fast educational method to build ECDs from household materials without the need for OTEs: unsharpened kitchen knives are used as electrodes, curcumin from turmeric is used as the electrochromic dye, and baking soda is used as the electrolyte. The laboratory experiments presented will help students gain a deeper understanding of the fundamentals of electrochemistry (electrolysis, pH change) and electrochromism (in our case, color changes due to pH-induced keto-enol tautomerism of curcumin).

8.
J Mol Liq ; 3262021 Mar 15.
Artículo en Inglés | MEDLINE | ID: mdl-35082450

RESUMEN

The effect of two disaccharide analogues, sucrose and sucralose, on the phase stability of aqueous lysozyme solutions has been addressed from a mechanistic viewpoint by a combination of experiment and molecular dynamics (MD) simulations. The influence of the added low molecular weight salts (NaBr, NaI and NaNO3) was considered as well. The cloud-point temperature measurements revealed a larger stabilizing effect of sucralose. Upon increasing sugar concentration, the protein solutions became more stable and differences in the effect of sucralose and sucrose amplified. It was confirmed that the addition of either of the two sugars imposed no secondary structure changes of the lysozyme. Enthalpies of lysozyme-sugar mixing were exothermic and a larger effect was recorded for sucralose. MD simulations indicated that acidic, basic and polar amino acid residues play predominant roles in the sugar-protein interactions, mainly through hydrogen bonding. Such sugar mediated protein-protein interactions are thought to be responsible for the biopreserative nature of sugars. Our observations hint at mechanistic differences in sugar-lysozyme interactions: while sucrose does not interact directly with the protein's surface for the most part (in line with the preferential hydration hypothesis), sucralose forms hydrogen bonds with acidic, basic and polar amino acid residues at the lysozyme's surface (in line with the water replacement hypothesis).

9.
Anal Chem ; 92(6): 4527-4534, 2020 03 17.
Artículo en Inglés | MEDLINE | ID: mdl-32075366

RESUMEN

In this study a new method for evaluating the pressure effect on separations of oligonucleotides and proteins on an anion exchange column was developed. The pressure rise of up to 500 bar was attained by coupling restriction capillaries to the column outlet to minimize differences in pressure over the column. Using pH transient measurements it was demonstrated that no shift in ion exchange equilibria occurs due to a pressure increase. Results from isocratic and gradient separations of oligonucleotides (model compounds) were evaluated by stoichiometric displacement and linear gradient elution model, respectively. Both elution modes demonstrated that for smaller oligonucleotides the number of binding sites remained unchanged with pressure rise while an increase for large oligonucleotides was observed, indicating their alignment over the stationary phase. From the obtained model parameters and their pressure dependencies, a thermodynamic description was made and compared between the elution modes. A complementary pattern of a linear increase of partial molar volume change with a pressure rise was established. Furthermore, estimation of the pressure effect was performed for bovine serum albumin and thyroglobulin that required gradient separations. Again, a raise in binding site number was found with pressure increase. The partial molar volume changes of BSA and Tg at the maximal investigated pressure and minimal salt concentration were -31.6 and -34.4 cm3/mol, respectively, indicating a higher rigidity of Tg. The proposed approach provides an insight into the molecule deformation over a surface at high pressures under nondenaturing conditions. The information enables a more comprehensive UHPLC method development.


Asunto(s)
Oligonucleótidos/aislamiento & purificación , Albúmina Sérica Bovina/aislamiento & purificación , Tiroglobulina/aislamiento & purificación , Adsorción , Animales , Bovinos , Cromatografía por Intercambio Iónico , Sustancias Macromoleculares/química , Sustancias Macromoleculares/aislamiento & purificación , Oligonucleótidos/química , Tamaño de la Partícula , Presión , Albúmina Sérica Bovina/química , Propiedades de Superficie , Termodinámica , Tiroglobulina/química
10.
Chem Rev ; 117(19): 12385-12414, 2017 Oct 11.
Artículo en Inglés | MEDLINE | ID: mdl-28949513

RESUMEN

How are water's material properties encoded within the structure of the water molecule? This is pertinent to understanding Earth's living systems, its materials, its geochemistry and geophysics, and a broad spectrum of its industrial chemistry. Water has distinctive liquid and solid properties: It is highly cohesive. It has volumetric anomalies-water's solid (ice) floats on its liquid; pressure can melt the solid rather than freezing the liquid; heating can shrink the liquid. It has more solid phases than other materials. Its supercooled liquid has divergent thermodynamic response functions. Its glassy state is neither fragile nor strong. Its component ions-hydroxide and protons-diffuse much faster than other ions. Aqueous solvation of ions or oils entails large entropies and heat capacities. We review how these properties are encoded within water's molecular structure and energies, as understood from theories, simulations, and experiments. Like simpler liquids, water molecules are nearly spherical and interact with each other through van der Waals forces. Unlike simpler liquids, water's orientation-dependent hydrogen bonding leads to open tetrahedral cage-like structuring that contributes to its remarkable volumetric and thermal properties.

11.
Phys Chem Chem Phys ; 20(48): 30340-30350, 2018 Dec 12.
Artículo en Inglés | MEDLINE | ID: mdl-30488933

RESUMEN

Ion-specific effects at the protein surface are investigated here in light of the changes they infer to surface water dynamics, as observed by 1H NMR relaxation (at 20 MHz). Two well-known proteins, hen egg-white lysozyme (LZM) and bovine serum albumin (BSA), show qualitatively opposite trends in the transverse relaxation rate, R2(1H), along a series of different monovalent salt anions in the solution. Presence of salt ions increases R2(1H) in the case of lysozyme and diminishes it in the case of BSA. The effect magnifies for larger and more polarizable ions. The same contrasting effect between the two proteins is observed for protein-solvent proton exchange. This hints at subtle effects ion-binding might have on the accessibility of water surface sites on the protein. We suggest that the combination of the density of surface charge residues and surface roughness, at the atomic scale, dictates the response to the presence of salt ions and is proper to each protein. Further, a dramatic increase in R2(1H) is found to correlate closely with the formation of protein aggregates. The same ordering of salts in their ability to aggregate lysozyme, as seen previously by cloud point measurements, is reproduced here by R2(1H). 1H NMR relaxation data is supplemented by 35Cl and 14N NMR relaxation for selected salt ions to probe the ion-binding itself.


Asunto(s)
Muramidasa/química , Albúmina Sérica Bovina/química , Soluciones/química , Agua/química , Animales , Aniones , Bovinos , Pollos , Difusión , Multimerización de Proteína , Espectroscopía de Protones por Resonancia Magnética , Protones
12.
J Mol Liq ; 270: 74-80, 2018 Nov 15.
Artículo en Inglés | MEDLINE | ID: mdl-30872874

RESUMEN

Isothermal titration calorimetry was used to determine the temperature and salt concentration dependence of the enthalpy of mixing, Δmix H, of bovine serum albumin (BSA) in aqueous buffer solutions with several low molecular weight salts. Three buffers were used: acetate (pH = 4.0), MOPS (7.2), and borate (9.2). Since the isoionic point of BSA is at pI ≈ 4.7, the net charge of BSA in acetate buffer was positive (≈ +20), while in the other two buffer solutions it was negative (≈ -15 in MOPS and ≈ -25 in borate). The majority of the recorded heat effects were exothermic, while only at pH = 9.2 a weak endothermic effect upon mixing BSA with LiCl, NaCl, and KCl was observed. For all buffer solutions the absolute values of Δmix H of sodium salts followed the order: NaCl < NaBr < NaNO3 < NaI < NaSCN, which is the reverse Hofmeister series for anions. The magnitude of the effects was the largest in acetate buffer and decreased with an increasing pH value of the solution. While the effect of varying the anion of the added salts was strongly pronounced at all pH values, the effect of the cation (LiCl, NaCl, KCl, RbCl and CsCl salts) was weak. The most interesting feature of the results obtained for pH > pI was the fact that Δmix H were considerably more sensitive to the anion (co-ion to the net BSA charge) than to the cation species. This indicated that anions interacted quite strongly with the BSA even at pH values where the net charge of the protein was negative. We showed that Δmix H at high addition of salts correlated well with the enthalpy of hydration of the corresponding salt anion. This finding suggested, consistently with some previous studies, that a part of the exothermic contribution to Δmix H originated from the hydration changes upon the protein-salt interaction. Theoretical analysis, based on the primitive model of highly asymmetric electrolyte solutions solved within the mean spherical approximation, was used to estimate Coulomb effects upon mixing.

13.
J Mol Liq ; 228: 126-132, 2017 Feb.
Artículo en Inglés | MEDLINE | ID: mdl-28503012

RESUMEN

The volumetric (partial and apparent molar volumes) and calorimetric properties (apparent heat capacities) of aqueous cationic polyelectrolyte solutions - ionenes - were studied using the oscillating tube densitometer and differential scanning calorimeter. The polyion's charge density and the counterion properties were considered as variables. The special attention was put to evaluate the contribution of electrostatic and hydrophobic effects to the properties studied. The contribution of the CH2 group of the polyion's backbone to molar volumes and heat capacities was estimated. Synergistic effect between polyion and counterions was found.

14.
J Mol Liq ; 228: 4-10, 2017 Feb.
Artículo en Inglés | MEDLINE | ID: mdl-28450755

RESUMEN

The phase behavior of the continuous shouldered well model fluid proposed by Franzese [J. Mol. Liq. 136 (2007) 267] was examined using the Monte Carlo computer simulations in the grand canonical ensemble. The essential parts of the vapour-liquid and liquid-liquid coexistence envelopes were obtained. The Widom lines departing from coexistence envelopes were calculated using maxima of the fluctuations of the number of particles as a function of chemical potential along various isotherms. The region embracing anomalies in the properties of the model was located using the approximate criterion that involves the excess pair entropy.. The temperature of maximum density line was built by performing canonical Monte Carlo simulations. Our results are consistent with previous results from molecular dynamics constant pressure-constant temperature simulations and provide wider insight into the phase behavior of the model by using the chemical potential as the external parameter.

15.
Acta Chim Slov ; 62(3): 555-64, 2015.
Artículo en Inglés | MEDLINE | ID: mdl-26454589

RESUMEN

The Zaire Ebola viral protein VP24 selectively inhibits nuclear import of STAT1 and as such blocks interferon-induced antiviral responses vital for cell's emergency. Inhibition of VP24 with small molecule inhibitor may neutralize the threatening action of Ebola virus. We performed molecular docking of compounds from a selected small library of plant polyphenols on to VP24. Our research shows that 1,2,3,6-tetragalloyl glucose, epigallocatechin gallate, chlorogenic acic, oleuropein and miquelianin represent promising leads for further studies.


Asunto(s)
Membrana Celular/virología , Ebolavirus , Simulación del Acoplamiento Molecular , Plantas/química , Polifenoles/metabolismo , Proteínas Virales/metabolismo , Sitios de Unión , Humanos , Ligandos , Polifenoles/química , Unión Proteica , Conformación Proteica , Termodinámica , Proteínas Virales/química , alfa Carioferinas/química , alfa Carioferinas/metabolismo
16.
Cell Mol Life Sci ; 70(8): 1483-92, 2013 Apr.
Artículo en Inglés | MEDLINE | ID: mdl-23224430

RESUMEN

Astrocytes interact with neurons and endothelial cells and may mediate exchange of metabolites between capillaries and nerve terminals. In the present study, we investigated intracellular glucose diffusion in purified astrocytes after local glucose uptake. We used a fluorescence resonance energy transfer (FRET)-based nano sensor to monitor the time dependence of the intracellular glucose concentration at specific positions within the cell. We observed a delay in onset and kinetics in regions away from the glucose uptake compared with the region where we locally super-fused astrocytes with the D-glucose-rich solution. We propose a mathematical model of glucose diffusion in astrocytes. The analysis showed that after gradual uptake of glucose, the locally increased intracellular glucose concentration is rapidly spread throughout the cytosol with an apparent diffusion coefficient (D app) of (2.38 ± 0.41) × 10(-10) m(2) s(-1) (at 22-24 °C). Considering that the diffusion coefficient of D-glucose in water is D = 6.7 × 10(-10) m(2) s(-1) (at 24 °C), D app determined in astrocytes indicates that the cytosolic tortuosity, which hinders glucose molecules, is approximately three times higher than in aqueous solution. We conclude that the value of D app for glucose measured in purified rat astrocytes is consistent with the view that cytosolic diffusion may allow glucose and glucose metabolites to traverse from the endothelial cells at the blood-brain barrier to neurons and neighboring astrocytes.


Asunto(s)
Astrocitos/metabolismo , Citosol/metabolismo , Glucosa/metabolismo , Animales , Transporte Biológico , Células Cultivadas , Difusión , Transferencia Resonante de Energía de Fluorescencia , Cinética , Modelos Biológicos , Ratas , Análisis de la Célula Individual
17.
Annu Rev Biophys ; 2023 Oct 31.
Artículo en Inglés | MEDLINE | ID: mdl-37906740

RESUMEN

Protein-protein association and aggregation are fundamental processes that play critical roles in various biological phenomena, from cellular signaling to disease progression. Understanding the underlying biophysical principles governing these processes is crucial for elucidating their mechanisms and developing strategies for therapeutic intervention. In this review, we provide an overview of recent experimental studies focused on protein-protein association and aggregation. We explore the key biophysical factors that influence these processes, including protein structure, conformational dynamics, and intermolecular interactions. We discuss the effects of environmental conditions such as temperature, pH and related buffer-specific effects, and ionic strength and related ion-specific effects on protein aggregation. The effects of polymer crowders and sugars are also addressed. We list the techniques used to study aggregation. We analyze emerging trends and challenges in the field, including the development of computational models and the integration of multidisciplinary approaches for a comprehensive understanding of protein-protein association and aggregation. Expected final online publication date for the Annual Review of Biophysics, Volume 53 is May 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

18.
J Chromatogr A ; 1706: 464245, 2023 Sep 13.
Artículo en Inglés | MEDLINE | ID: mdl-37527569

RESUMEN

Prediction of analyte retention times requires prior knowledge of the column void volume, the measurement of which is still highly contested within the literature and therefore experimental based prediction is often used. In this study, we investigated deuterated acetonitrile as an isotopically labelled mobile phase component to observe its elution behaviour in a binary mixture with water at 25 different mobile phase compositions (from 5 to 95 vol.% of acetonitrile), on two stationary phases (C8 and C18), and at two temperatures (30 and 40 °C) using LC-MS. The same experimental design was additionally used for three commonly used neutral void volume markers: uracil, phloroglucinol and N,N-dimethylformamide. Temperature was observed to influence the elution of acetonitrile in an inversely proportional manner with higher temperatures coinciding with lower elution times. By utilizing a three-way ANOVA, the composition of the mobile phase has been shown to have a significant effect on deuterated acetonitrile and other investigated void volume markers, demonstrating the fact that both void volume markers and acetonitrile itself exhibit retention-like behaviour. Excess adsorption isotherms for acetonitrile were calculated using deuterated acetonitrile elution data. The comparison of void volumes, obtained with conventional neutral void volume markers, revealed the former to be 24-36% lower than the void volume obtained using deuterated acetonitrile, as an isotopically labelled mobile phase component. For a water:acetonitrile mobile phase, the minor disturbance method using deuterated acetonitrile to obtain an integral average void volume (2.08 and 2.05 mL for C18 at 30 and 40 °C, respectively and 2.16 and 2.13 mL for C8 at 30 and 40 °C, respectively) was found to be the most appropriate method for determining the elusive column void volume.


Asunto(s)
Agua , Cromatografía Liquida/métodos , Agua/química , Temperatura , Espectrometría de Masas , Indicadores y Reactivos , Acetonitrilos/química , Cromatografía Líquida de Alta Presión
19.
Phys Chem Chem Phys ; 14(6): 2024-31, 2012 Feb 14.
Artículo en Inglés | MEDLINE | ID: mdl-22231588

RESUMEN

Aliphatic x,y-ionenes are polyelectrolytes in which x and y denote the numbers of methylene groups separating quaternary ammonium ions. They represent useful model substances for studying hydrophobic and charge effects in aqueous solutions. We used isothermal titration calorimetry to measure the enthalpies of mixing, ΔH(mix), of 3,3- and 6,6-ionene fluorides and bromides with low molecular weight salts (NaF, NaCl, NaBr, and NaI) at 298 K in water. The signs and magnitudes of the measured enthalpies depend on the hydrophobicity of the ionene and on the nature of the added salt. For example, addition of sodium fluoride to solutions of 3,3- and 6,6-ionene fluorides produced endothermic effects, while addition of sodium bromide to 3,3-ionene bromide resulted in a strong exothermic effect. Interestingly, mixing of 6,6-ionene bromide and NaBr solutions in water gave a small exothermic heat effect. Polyelectrolyte theories, based on continuum-solvent models, predict enthalpies of mixing to be positive (endothermic) for all the solutions examined in this work. The ion-specific effect is more strongly expressed in ionene solutions with higher charge density (3,3-ionene). The most important result of this work is the finding that the enthalpy of mixing of 3,3- (and of 6,6-ionene) fluorides with sodium halides can be expressed as a linear function of the enthalpy of hydration of the halide counterions. The experimental results were complemented with an explicit water molecular dynamics simulation of solutions of oligoions modelling 3,3- and 6,6-ionenes. The computer simulation results for various nitrogen-counterion pair distribution functions were in most cases consistent with the enthalpy measurements.

20.
Phys Chem Chem Phys ; 14(19): 6805-11, 2012 May 21.
Artículo en Inglés | MEDLINE | ID: mdl-22491212

RESUMEN

Enthalpies of mixing of aliphatic 3,3 and 6,6-ionene fluorides with low molecular weight salts (sodium formate, acetate, nitrate, chlorate(v), and thiocyanate), all dissolved in water, were determined. In addition, to complement our previous study (Luksicet al., Phys. Chem. Chem. Phys., 2012, 14, 2024), new measurements were performed where aqueous solutions of 3,3 and 6,6-ionene bromides were mixed with solutions of sodium fluoride, chloride, bromide, and iodide. Electrostatic theory, based on Manning's limiting law or the Poisson-Boltzmann equation, predicted the enthalpy of mixing to be endothermic in all the cases, while experiments showed that this is not always true. When an aqueous solution of 3,3-ionene fluoride was mixed with a solution of sodium fluoride (or formate and acetate) in water, the effect was indeed endothermic. For all other salts, i.e. sodium chlorate, nitrate, and thiocyanate, heat was released upon mixing. The situation was similar for 6,6-ionene fluoride solutions with an exception of mixing with sodium chlorate, where the effect was endothermic. The enthalpy of mixing was strongly correlated with the enthalpy of hydration of the counterion of the low molecular weight salt. A lyotropic series, similar to that of Hofmeister, was obtained. To examine also the effect of co-ions, ionene bromides were titrated with tetramethyl-, tetraethyl-, or tetrapropylammonium bromides. The enthalpy was exothermic for all mixtures while, somewhat unexpectedly, the co-ion specific effect was quite strong.

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