Thermodynamics of monolayers formed by mixtures of phosphatidylcholine/phosphatidylserine

In this work we obtain the thermodynamic properties of mixed (1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine) PC and (1-stearoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (sodium salt)) PS monolayers. Measurements of compressibility (isotherms, bulk modulus, and excess area per molecule) and surface potential show that the properties of monolayers at the air-water interface depend on the concentration of ions (Na(+) and K(+)) and the proportion of PS in the mixture. The dependence on PS arises because the molecule is originally bound to a Na(+) counterion; by increasing the concentration of ions the entropy changes, creating a favorable system for the bound counterions of PS to join the bulk, leaving a negatively charged molecule. This change leads to an increase in electrostatic repulsions which is reflected by the increase in area per molecule versus surface pressure and a higher surface potential. The results lead to the conclusion that this mixture of phospholipids follows a non ideal behavior and can help to understand the thermodynamic behavior of membranes made of binary mixtures of a zwitterionic and an anionic phospholipid with a bound counterion.     

Effects of the nature of counterions in solution on interactions of phenoxazones, xanthones, and carbazoles containing benzo-crown groups with DNA

The interactions of DNA with phenoxazones, xanthones, and carbazoles containing the (benzo-18-crown-6)-4′-yl and (benzo-15-crown-5)-4′-yl radicals bonded to the chromophore via spacers of different lengths in the presence of Na⁺ and K⁺ ions were studied by spectrophotometry, circular dichroism, and dynamic birefringence. The thermodynamic parameters of the binding of the compounds with DNA and changes in the macromolecular parameters of the DNA molecule during complexation were determined. Based on the results of these studies, we suggested the models of bonding of these compounds to the double helix of DNA. It is shown that the mode of DNA binding with a phenoxazone derivative containing two (benzo-15-crown-5)-4′-yl radicals bonded via a fragment of glycine to chromophore depends on the type of the counterion in solution. In the presence of Na⁺, the chromophore is intercalated into the double helix of DNA; in the presence of K⁺, it is bound to DNA in the form of a dimer outside the double helix. The type of the counterion does not affect the mode of binding of other crown-containing compounds of actinocin with DNA. For compounds containing the (benzo-18-crown-6)-4′-yl radical, the mode of binding to DNA adepends only on the spacer length.     

Polymerization and molecular recognition in the monolayers of the surface-active derivatives of crown-ethers

The four surface-active derivatives of crown-ethers with a variety of fatty alkyl chains were synthesized and studied in monolayers at various conditions. The areas per crown-ether molecule in monolayers are increasing significantly in the presence of various alkali metal cations in the aqueous subphase. These effects can be explained as complex formation between crown-ethers in the monolayers and cations from aqueous subphase, causing a change in the conformation of the polyether ring of the molecule at the interface. This is accompanied with the change in the cation selectivity (Na⁺ > Li⁺ > K⁺ > Cs⁺) as compared with the series in volume (K⁺ > Cs⁺ > Na⁺ > Li⁺). Polymerization of the crown-ether monolayer allows to stabilize the conformation of the molecule at the interface as if tuned to the definite cation.     

Temperature Dependant Micellization of AOT in Aqueous Medium: Effect of the Nature of Counterions

The lithium, potassium, and ammonium salts of bis (2‐ethylhexyl) sulphosuccinic acid have been prepared from the sodium salt (AOT) by applying ion‐exchange technique. The critical micellization concentrations (cmc) of the surfactants with four different counterions have been determined at a temperature range of 10°C to 40°C using surface tension as well as electrical conductivity measurements. Observed data have been utilized to evaluate the ionization degree (counter ion association constant),α, and various thermodynamic parameters of micellization viz, free energy, enthalpy, entropy changes of micelle formation, and also the surface parameters (Γ_max, A_min) in aqueous media. The value of cmc decreases with hydrated ionic size of the counter ions (except K⁺) and follows the order NH₄ ⁺>Na⁺>Li⁺>K⁺. While large negative free energy change (ΔG⁰ _m) and the positive entropy change (ΔS⁰ _m) favor the micellization process thermodynamically, nature of their variation with counterion supports the involvement of counterion size factor in micellization process via a change in the hydrophilicity of surfactant head group.     

Effects of Cs and Na ions on the interfacial properties of dodecyl sulfate solutions

The competitive binding of counterions to anionic dodecyl sulfate ions in aqueous solutions of cesium dodecyl sulfate (CsDS) and sodium dodecyl sulfate (SDS) mixtures, which significantly influences the critical micelle concentration (cmc) and surface (or interfacial) tension of surfactant solutions, was investigated. The cmc and degree of counterion binding were obtained through electrical conductivity measurements. The curve of cmc versus the mole fraction of CsDS in the surfactant mixture was simulated by Rubingh's equations, which enabled us to estimate the interaction parameter in micelles (W _R) based on the regular solution approximation. The curve-fitting exhibited a slightly negative value (W _R=−0.1), indicating that the mixing (SDS+CsDS) enhances micelle formation owing to a greater interaction between surfactant molecules and counterions than in pure systems (SDS). On going from SDS, SDS:CsDS(75:25), SDS:CsDS(50:50), SDS:CsDS(25:75) to CsDS, interfacial tension at the hexadecane/surfactant-solution interface showed a negative deviation from the mixing rule (interaction parameter in adsorbed film W _A=−0.38), indicating the replacement of Na⁺ bound to anionic dodecyl sulfate by Cs⁺ ions owing to the stronger interaction between the Cs⁺ and the dodecyl sulfate ions. Droplet sizes of emulsion formed with hexadecane and aqueous dodecyl sulfate solutions were investigated using the light scattering spectrophotometer. The higher binding capacity of Cs⁺, having a smaller hydrated ionic size than Na⁺, also resulted in a negative deviation in emulsion droplet size in mixed systems. Received: 10 May 2000/Accepted: 11 August 2000     

Contribution of Na counterions to H-AOT&Na-AOT-based W/O microemulsion formation using aqueous NaOH solutions as estimated by pyranine absorbance

The objective of this study is to estimate the contribution of Na⁺ as a counterion in the formation of H-AOT&Na-AOT-based W/O microemulsions using aqueous NaOH solution by pyranine absorbance measurements. A mixture of an aqueous NaOH solution containing pyranine/H-AOT&Na-AOT/isooctane was emulsified by changing the mixing ratio of Na-AOT (X_Na-AOT = 0–1) and the mole fraction of NaOH (X_NaOH = [NaOH]/the AOT⁻ concentration in the water pool = 0–1). The phase behavior of the emulsified mixture was evaluated from the absorbance of pyranine at the isosbestic point and by visual observations. W/O microelumsions are formed at the mid-range of X_Na-AOT, whereas the emulsified mixture separates into two phases at lower X_Na-AOT and higher X_Na-AOT. The two phase boundaries shift toward lower X_Na-AOT as with increasing X_NaOH. The phase behavior depends on the degree of screening of electrostatic repulsions between the polar headgroups of AOT⁻ by the Na⁺ counterion. Interestingly, nano-sized W/O microemulsions are formed without phase separation using a highly concentrated NaOH aqueous solution when the Na-AOT mixing ratio is appropriately adjusted. The phase behavior was plotted as X_NaOH versus X_Na-AOT, and the correlation equations for the two phase boundaries were obtained by fitting the points. The contribution of the Na⁺ counterion from NaOH to W/O microemulsion formation was estimated by the correlation equations. The absorbance of pyranine and the size of W/O microemulsions, as measured by DLS, were plotted as a function of X_Na⁺=(x[Na⁺   from   NaOH]+[Na⁺   from   Na-AOT])/[AOT⁻], in which x is the ratio contributed by NaOH. The absorbance and size correlates well with X_Na⁺, indicating that X_Na⁺ is a meaningful parameter for quantitatively estimating phase behavior and size variation.     

Polymérisation anionique des diènes. III. Etude thermodynamique de la propagation du butadiène et de i'isoprène par les organo-alcalins

The present work is concerned with the influence of the polymerization temperature on the propagation mechanisms of polyisoprenyl- and polybutadienyl-alkali metals. The thermodynamic parameters of the contact ion pairs and free ions propagations have been calculated. With Li⁺ in dioxane solvent, the vinyl propagation is stereospecific (for isoprene, the propagation is mainly 4–3). In comparison with benzene, the vinyl propagation of the polydienyl-Li contact ions pairs should be due to complexation of Li⁺ by dioxane, an electron donor having a weak dielectric constant. In general, the stereospecificity of the propagation of contact ion pairs decreases with increasing counterion size; little difference has been observed with K⁺ and Cs⁺ ion pairs in dioxane and benzene media. For isoprene, the methyl substituant should have chiefly a steric effect in the propagation of free ions, whereas it should confer a polar character to the isoprene molecule in the presence of ions pairs.     

Enrichment and distribution of counterions in spherical polyelectrolyte brushes probed by SAXS

The spatial correlation of counterions [Li⁺, Na⁺, Rb⁺, Cs⁺, NH₄⁺, (CH₃)₄N⁺] with spherical polyelectrolyte brushes (SPBs), which consist of a PS core and chemically grafted PSS chains, was comprehensively studied through a combination of SAXS, DLS, and Zeta potential. Results show that the SAXS intensity profiles of the brush appears to be “insensitive” to the concentration of Na⁺. By contrast, introducing salt ions with a density lower than sodium [NH₄⁺, (CH₃)₄N⁺ and Li⁺] into the brush layer will cause a decrease in the scattering intensity while introducing those with a higher density than sodium (Rb⁺ and Cs⁺) will cause the opposite result. As verified by the combined results of SAXS, DLS, and Zeta potential, the collapse of the brush and the enrichment of the counterions in the brush layer occur simultaneously. It was further demonstrated that the concentration of counterions enriched in the innermost layer of the brush shell can be enhanced up to hundreds of times compared with the bulk concentration, and the counterion distribution in SPB shell follows a radial attenuation distribution. SAXS is confirmed to be powerful in probing the enrichment and distribution of counterions within SPB. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 738–747     

Measurement of Local Sodium Ion Levels near Micelle Surfaces with Fluorescent Photoinduced‐Electron‐Transfer Sensors

The Na⁺ concentration near membranes controls our nerve signals aside from several other crucial bioprocesses. Fluorescent photoinduced electron transfer (PET) sensor molecules target Na⁺ ions in nanospaces near micellar membranes with excellent selectivity against H⁺. The Na⁺ concentration near anionic micelles was found to be higher than that in bulk water by factors of up to 160. Sensor molecules that are not held tightly to the micelle surface only detected a Na⁺ amplification factor of 8. These results were strengthened by the employment of control compounds whose PET processes are permanently “on” or “off”.     

Counterion binding by poly(styrenesulfonate)

Binding isotherms of poly(styrenesulfonate) with hydrogen and selected alkali ions have been determined by dialysis equilibrium and dye spectroscopy, employing Pb²⁺, Mg²⁺, and Ca²⁺ as the competing divalent ions. The observed degrees of counterion binding, Θ_z, defined as the number of bound counterfoils of valence z per poly ion site-group, agree quite well with those predicted by Manning's two variable theory. The binding preference follows the order Cs⁺ > Rb⁺ > K⁺ > Na⁺ > H⁺ >Li⁺, indicating that the binding process is of a territorial nature. Independently performed light scattering measurements show that a plot of the radius of gyration, >S _z>, against the actual polyion charge gives a sigmoidal curve. This result is taken to indicate that (1) a polyion is, in general, not completely neutralized in the Θ-state and that (2) a polyion may be not fully stretched when the polyion charge density is largest.     

Influence of Molecular Organization of Photo-active Azo-phanes on the Reactivity in Monolayers at the Air–Water Interface

The molecular organization of monolayers at the air–water interface of an amphiphilic azo-phane with unbranched n-dodecyl substituents differs from that of the analog with bulky ‘tert’-octyl substituents as seen in the area per molecule. Complexation with sodium ions from the aqueous subphase, as deduced from measurement of the surface potential, is facilitated by closer approach of the macrocycles for the n-dodecyl-substituted azo-phane, since two macrocycles form the complex with Na⁺. The compensation of the positive charge after complexation in the case of a two-component monolayer of the n-dodecyl-substitued azo-phane and octadecanoic acid, molar ratio azo-phane to acid = 2:1, enhances even more the complexation. The complex equilibrium constants and the contributions of the hydrophilic head group region to the surface potential are evaluated from Langmuir isotherm fits to the dependencies of the surface potential on the NaCl concentration in the aqueous subphase for the three monolayer systems investigated, i.e., the two different pure azo-phane monolayers and the two-component monolayer.     

Competitive binding exchange between alkali metal ions (K+, Rb+, and Cs+) and Na+ ions bound to the dimeric quadruplex [d(G4T4G4)]2: a 23Na and 1H NMR study

A comparative study of the competitive cation exchange between the alkali metal ions K⁺, Rb⁺, and Cs⁺ and the Na⁺ ions bound to the dimeric quadruplex [d(G₄T₄G₄)]₂ was performed in aqueous solution by a combined use of the ²³Na and ¹H NMR spectroscopy. The titration data confirm the different binding affinities of these ions for the G‐quadruplex and, in particular, major differences in the behavior of Cs⁺ as compared to the other ions were found. Accordingly, Cs⁺ competes with Na⁺ only for the binding sites at the quadruplex surface (primarily phosphate groups), while K⁺ and Rb⁺ are also able to replace sodium ions located inside the quadruplex. Furthermore, the ¹H NMR results relative to the CsCl titration evidence a close approach of Cs⁺ ions to the phosphate groups in the narrow groove of [d(G₄T₄G₄)]₂. Based on a three‐site exchange model, the ²³Na NMR relaxation data lead to an estimate of the relative binding affinity of Cs⁺ versus Na⁺ for the quadruplex surface of 0.5 at 298 K. Comparing this value to those reported in the literature for the surface of the G‐quadruplex formed by 5′‐guanosinemonophosphate and for the surface of double‐helical DNA suggests that topology factors may have an important influence on the cation affinity for the phosphate groups on DNA. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of carbonyl carbon atom replacement in acetone molecule (ACN) by sulfur atom (DMSO): Part II. Thermodynamic functions of complex formation of crown ethers with Na<Superscript>+</Superscript> in mixed solvents

The thermodynamic functions of complex formation of benzo-15-crown-5 ether (B15C5) and sodium cation (Na⁺) in acetone–water mixtures at 298.15 K have been calculated. The equilibrium constants of B15C5/Na⁺ complex formation have been determined by conductivity measurements. The enthalpic effect of complex formation has been measured by the calorimetric method. The complexes are enthalpy-stabilized but entropy-destabilized in acetone–water mixtures. The effects of hydrophobic hydration, preferential solvation of B15C5 by a molecule of water and acetone, respectively and the solvation of Na⁺ on the complex formation processes have been discussed. The calculated thermodynamic functions of B15C5/Na⁺ complex formation and the effect of benzene ring on the complex formation have been compared with analogous data obtained in dimethylsulfoxide–water mixtures. The effect of carbonyl atom replacement in acetone molecule by sulphur atom (DMSO molecule) on the thermodynamic functions of complex formation has been analysed.     

Formation of the aggregates of actinocin derivatives containing 4′-benzo-15-crown-5 radicals in amide groups and their interaction with a DNA molecule

Methods of spectrophotometry, spectropolarimetry, and viscometry are used to study the self-organization in the solution of crown-containing actinocin derivative (I) exhibiting antitumor activity and the interaction of the formed aggregates with a DNA molecule. The presence of the 4′-benzo-15-crown-5 radical in the structure of the studied compound determines the observed differences in its complexation with Na⁺ and K⁺ ions. The process of aggregation in the presence of K⁺ ions is accompanied by a shift of the long-wave band in the absorption spectrum to short-wave (the formation of H type aggregates) or long-wave (the formation of J type aggregates) regions depending on the K⁺ ion concentration in the solution. In the presence of Na⁺ ions, regardless of their concentration in the solution, J type aggregates form. A scheme of complex formation and their mutual transformations with changes in the ionic composition of the medium is proposed. A study of the interaction of this compound with DNA shows that in the presence of K⁺ ions it binds to the DNA molecule in the form of monomers and/or dimers without producing large supramolecular aggregates. The H and J structures formed in K⁺-containing solutions of compound I are broken in the interaction with DNA. If a solution of compound I is added to a DNA solution containing Na⁺ ions, the J type aggregates are formed directly on the surface of the DNA molecule. At the same type, the J structures originally formed in the Na⁺-containing solution of compound I practically do not interact with DNA. A study of this system shows that the introduction of the crown group in the compound molecule with a heterocyclic chromophore provides the opportunity to affect its affinity and binding to the DNA molecule by means of the ionic composition of the medium.     

The interactions between cholesterol and phospholipids located in the inner leaflet of humane erythrocytes membrane (DPPE and DPPS) in binary and ternary films—The effect of sodium and calcium ions

The studies on the influence of cholesterol on phospholipids accumulated in inner leaflet of membrane are performed rather rarely, especially in the presence of electrolytes, which are present in membrane environment. Therefore, in this work the interactions between cholesterol and saturated phosphatidylethanolamine (PE) and phosphatidylserine (PS) were studied in binary (phospholipid/cholesterol) and ternary (PS/PE/cholesterol) monolayers in the presence and absence of sodium and calcium ions. The composition of ternary films was estimated to reflect the proportion of PSs to PEs in inner layer of human erythrocyte membrane. The influence of electrolytes on pure PS and PE films was also analyzed. It was found that both sodium and calcium ions affect the condensation of DPPS films, and influence the interactions in DPPS/cholesterol monolayers. On the other hand, no effect of these ions on DPPE films as well as on DPPE/cholesterol interactions in the mixed systems was observed. The results obtained for ternary mixtures prove that in the presence of Na⁺ the interactions between the lipids are more favorable than in the absence of these ions. This is in contrast to the effect of Ca²⁺. All the results were thoroughly analyzed in the context of the structure of polar heads of the investigated phospholipids.     

Photoelectron intensity spectroscopy  analytical and structural information from peis applied to zeolites

X-ray photoelectron spectroscopy (XPS or ESCA) based on calculated photoionization cross sections was used to investigate the surface Si/Al ratio of different zeolites and the cation distribution in Ag- and Ca-exchanged NaA-zeolites. We find the same module at the surface as in the bulk, but a strong de-alumination of the surface after HCl treatment. The Ag⁺ ions are enriched and the Na⁺ ions depleted in the outer cavities of the zeolite crystals, both in strong dependence on the Ca content.     

Thermodynamic and spectroscopic properties of mixtures of β-lactoglobulin and dioleylphosphatidylcholine

 The behavior of the single spreading monolayers of β-lacto-globulin (β-LG) and dioleyl-phosphatidylcholine (DOPC), as well as their mixtures, has been studied on subphases containing Na⁺ or Ca⁺⁺ ions. The results show an influence of temperature and subphase on the studied systems. The behavior of the areas as a function of the weight fraction of the two components shows significative and prevalently positive deviations from the additivity and their bidimensional miscibility. The variation of ΔG ^ex, ΔH ^ex and ΔS ^ex calculated for the DOPC–β-LG mixture having maximum deviation on two different supports allows to deduce that the interactions are prevalently repulsive. FTIR–ATR spectra of transferred plurilayers show that DOPC has a surface orientation which can originate the miscibility between the protein and DOPC. Received: 3 February 1997 Accepted: 3 June 1997     

Determination of the Structure and Effective Dielectric Constant of Hydrated Ions

Abstract

Debye's equation for the salting in or out of nonpolar compounds, such as benzene, in aqueous salt solutions was expanded so as to determine the effective dielectric decrement and constant of the hydrated domain of an ion. For ions having an electrostatic charge per surface area less than or equal to that of the K⁺ or Cl^? ions, this domain consists of a single layer of water molecules loosely or negatively hydrated to the ion; i.e., the domain consists of a mono-molecular B region. For ions having an electrostatic charge per unit surface area approximately equal to that of the Na⁺ and F^? ions, there exists no B region and only one layer of tightly bound or positively hydrated water (a monomolecular A region). Since the electrostatic field does not appreciably influence water molecules beyond this A region, such ions have an effective dielectric constant that is near zero, as in relatively inert molecules such as hydrocarbons. For all other ions, such as H⁺, Li⁺, Mg²⁺⁰,Cr²⁺, Sr²⁺, Ba²⁺, and other multivalent ions, there exists only one monomolecular A region followed by one monomolecular B region. The effective value of the dielectric constant of such an ion is obtained from its B region, since its A region cannot be penetrated. The effective dielectric decrement or constant of any B region as measured by benzene solubility goes through a maximum as the electrostatic charge per unit surface area (C/A) is decreased because a large C/A restricts the orientation of the hydrated water molecules and a low value of C/A allows competitive interaction between surrounding water molecules. Thus both small and large values of C/A decrease the solubility of benzene, i.e., decrease i t s ability to penetrate into the medium. A decrease in the macroscopic dielectric constant of water upon the addition of salt is due to the destruction of the clusters of water by the ions, or to the addition of ions which have effective dielectric constants less than that of water, or both. All hydrated ions o r molecules which salt-in or salt -out benzene have, respectfully, effective dielectric constants greater or less than that of water.     

Effect of Na+ and Ca2+ Ions on a Lipid Langmuir Monolayer: An Atomistic Description by Molecular Dynamics Simulations

Studying the effect of alkali and alkaline‐earth metal cations on Langmuir monolayers is relevant from biophysical and nanotechnological points of view. In this work, the effect of Na⁺ and Ca²⁺ on a model of an anionic Langmuir lipid monolayer of dimyristoylphosphatidate (DMPA^?) is studied by molecular dynamics simulations. The influence of the type of cation on lipid structure, lipid–lipid interactions, and lipid ordering is analyzed in terms of electrostatic interactions. It is found that for a lipid monolayer in its solid phase, the effect of the cations on the properties of the lipid monolayer can be neglected. The influence of the cations is enhanced for the lipid monolayer in its gas phase, where sodium ions show a high degree of dehydration compared with calcium ions. This loss of hydration shell is partly compensated by the formation of lipid–ion–lipid bridges. This difference is ascribed to the higher charge‐to‐radius ratio q/r for Ca²⁺, which makes ion dehydration less favorable compared to Na⁺. Owing to the different dehydration behavior of sodium and calcium ions, diminished lipid–lipid coordination, lipid–ion coordination, and lipid ordering are observed for Ca²⁺ compared to Na⁺. Furthermore, for both gas and solid phases of the lipid Langmuir monolayers, lipid conformation and ion dehydration across the lipid/water interface are studied.     

Miscibility of phospholipids and cholesteryl acetate in mixed monolayers on aqueous surfaces

Summary The surface pressure — area per molecule curves (F A curves) of mixed monolayers of phosphatidyl serine (PS) and cholesteryl acetate (CA), and those of dimyristoyl phosphatidyl choline (DMPC) and CA were measured on aqueous surfaces. ThoseF — A curves showed kink points, which were considered to be the collapse point of the monolayers. Then, the collapse pressure was determined as the surface pressure at the collapse points. On the basis of the phase diagrams, drawn by plotting the measured values of the collapse pressure as a function of the composition, the miscibility of the lipids in the mixed monolayers was discussed. Thus, it has been concluded that PS and CA, and also DMPC and CA are completely miscible in the monolayers on water. On the other hand, it has been also found that, on aqueous solutions of 100 mM CaCl₂, PS and CA are immiscible in the monolayers because of the aggregation of PS molecules induced by Cat²⁺.