Negative Cooperativity in the Molecular Recognition of Excitatory Amino-Acid Derivatives by Synthetic Allosteric 1,1′-Binaphthalene Receptors

The optically active allosteric receptors (−)-(R,R)- 3 and (+)-(R,R)- 4 were synthesized for the molecular recognition of the N-(benzyloxy)carbonyl (N-Cbz)-protected excitatory amino acids aspartic acid (Asp, 1 ) and glutamic acid (Glu, 2 ). These macrocyclic structures consist of two 1,1′-binaphthalene moieties connected by two but-2-yne-1,4-diyl (for (−)-(R,R)- 3 ) or p-xylylene (for (+)-(R,R)- 4 ) bridges between the O-atoms in the minor grooves. Each 1,1′-binaphthalene moiety contains two 2-acetamidopyridin-6-yl (CONH(py)) H-bonding sites in the major groove to bind excitatory amino-acid derivatives via two COOH█bk█⋅⋅⋅█ek█CONH(py) H-bonding arrays and additional secondary electrostatic interactions. The formation of stable complexes with 1 : 2 host-guest stoichiometry was proven by the evaluation of fluorescence binding titrations using a multiple-wavelength nonlinear least-squares curve-fitting procedure, Job plot analysis, and solubilization experiments. Complexation of the first excitatory amino-acid guest at binding site 1 reduces the affinity for the second guest at binding site 2. As measures for the negative cooperativity between the two sites, the ratios of the association constants for the first and second binding events, {K_a(1 : 1)/K_a(1 : 2)}_corr. (corrected for the statistical preference of the 1 : 1 complex formation), were found to adopt values between 1.4 and 2.4, and the Hill coefficients n_H varied between 0.49 and 0.59.     

Solvatochromic studies of interactions between surfactants and poly(N-substituted acrylamide)s

Interactions between poly(N-substituted acrylamide)s and surfactants, such as sodium dodecyl sulfate (SDoS) and sodium decyl sulfate (SDeS), in aqueous solutions were investigated using a solvatochromic probe. The polymers used were poly(N,N-dimethylacrylamide) (PDMA), poly(N-isopropylacrylamide) (PIPA), poly(N-acryloylpyrrolidine) (PAPR), and poly(vinylpyrrolidone) (PVPy) for comparison. They were labeled with pyridinium dicyanomethylide chromophore as a solvatochromic probe, and the changes in the microenvironment polarity of the polymer upon association with surfactant micelles were investigated by monitoring the λ_max in the absorption spectra of the probe molecule. It was found that the Gibbs free energy of micelle stabilization by polymer complexation for SDoS is 7.6, 4.1, and 2.2 kJ mol⁻¹, and for SDeS 5.1, 2.9, and 0.8 kJ mol⁻¹ with PIPA, PAPR, and PDMA, respectively. These results indicate that the complexation between polymer and surfactant is influenced not only by the alkyl-chain length of the surfactant, but also by the polymer side groups.     

Pulsed field gradient NMR investigation of solubilization equilibria in amino acid and dipeptide terminated micellar and polymeric surfactant solutions

Pulsed field gradient NMR spectroscopy was used to investigate the association of toluene, chlorobenzene and benzyl alcohol with amino acid and dipeptide terminated polymerized surfactants (PS). The diffusion coefficient for each probe was measured in the presence and absence of the polymers and the mole fraction of bound probe molecules, f_b, was calculated. For all solutions investigated, the probes associated more strongly with unpolymerized surfactant micelles than with corresponding PS. For example, the toluene f_b values for association with sodium undecanoyl valinate micelles and the PS poly(sodium undecanoyl valinate) were 0.88 and 0.15, respectively. The relatively weak probe–polymer association was attributed to the polarity and fluidity of the polymers' hydrocarbon cores and to the fact that these PS have smaller aggregation numbers than the corresponding unpolymerized surfactant micelles. Copyright © 2002 John Wiley & Sons, Ltd.     

Influence of the Alkyl Tail Length on the Anionic Surfactant-PVP Interaction

The effect of the surfactant tail length on the interaction between sodium alkylsulfates (C _n OS, n = 6, 8, 10, 12) and poly(vinylpyrrolidone) (PVP) in aqueous solution has been investigated by electron paramagnetic resonance (EPR) spectroscopy employing TEMPO-choline (TC) as a spin probe. Experimental evidence show that all of the considered surfactants molecularly interact with PVP. However, the cooperative behavior of the surfactant molecules when self-aggregating onto the polymer strongly increases with the surfactant tail length. In fact, in the case of C₆OS, the TC EPR parameters indicate that surfactant monomers randomly associate with the polymer chain. In the case of C₈OS, formation of surfactant-polymer clusters occurs simultaneously to free micellization. In the case of C₁₂OS and C₁₀OS, the nitrogen isotropic hyperfine coupling constant of TC (< A _N >) shows that formation of surfactant-polymer clusters occurs. The correlation time (τ _C ) of the nitroxide in the same systems shows that electrostatic repulsion among the clusters, formed on the PVP macromolecules, favors a broadening of the polymer coil and a stiffening of its chain. The average number of surfactant molecules participating in each cluster adsorbed onto the polymer, as determined by fluorescence quenching measurements, is much higher for C₁₂OS than for C₁₀OS.     

Magnetic Relaxation Probing of the State of Diheptyl Dithiophosphate Ions in Water and Aqueous Triton X-100 Solutions

The nuclear magnetic relaxation was used to study the state of diheptyl dithiophosphoric acid (D7DTP, L₇) anions in water and aqueous solutions of the nonionic surfactant, Ttiton X-100, at 298 K in the presence of paramagnetic probes, Mn²⁺ions. It was found that increase in the spin-lattice relaxation rate of water protons is caused by formation of simple and mixed (with surfactant) aggregates of D7DTP. Unlike the Mn²⁺–sodium dodecyl sulfate –Triton X-100 system, studied previously an influence of a probe concentration was found at surfactant concentration close to the CMC. It was suggested that two types of mixed species containing diheptyl dithiophosphate ions, Mn(II), and nonionic surfactant can be formed: micellar aggregates, {Mn(L₇)₂(TX)}, and polynuclear associates, [Mn _x (L₇) _y (tx) _z ]. The associates likely contain surfactant in the form of monomers (tx).     

Kinetics of Reactions of α,β-Unsaturated Surfactants with Secondary Amines in a Solution

The kinetics of reactions of acrylamide derivatives (acrylamidotrihydroxymethylmethane (TA), sodium 4-acrylamidobutanoate (AA₃), sodium 6-acrylamidohexanoate (AA₅), and sodium 11-acrylamido-undecanoate (AA10)) with piperidine and morpholine in water (for TA, also in DMF, DMSO, and formamide) is studied at 293 K. These compounds are weak surfactant monomers. The critical concentrations of micelle formation (CCM) for them are determined. The self-association of AA₃, AA₅, and AA₁₀ producing micelles results in a decrease in their reactivity compared to the monomeric forms. The rates of the reactions of surfactant monomers (SM) with morpholine and piperidine are described by the second-order rate law w ₀ = k[SM]₀[Amine]₀. An empirical equation is derived that relates the CCM values to the rate constant for the reaction of a surfactant monomer with a secondary amine with charges on the -carbon and oxygen atoms of the amide group of a surfactant monomer. The rates of the reactions of TA with piperidine and morpholine are determined by the electrophilicity (acidity) of the medium, which is favorable for the Michael reaction.     

Specific interactions in reversed micelles: Oxidation of Fe(bpy)32+ by S2O82− in AOT-oil-water microemulsions

The oxidation of Fe(bpy)₂²⁺ by peroxodisulphate (bpy = 2,2′-bipyridine) has been studied in a variety of sodium bis(2-ethylhexyl)sulfosuccinate(aerosol-OT or AOT)-oil-water microemulsions by changing the nature of the oil phase, the surfactant concentration, and the molar ratio w = [H₂O]/[AOT]. Kinetic results show that the influence of surfactant concentration is due to a dilution effect. On the other hand, the comparison between the reaction rate in conventional aqueous solution with that in AOT w/o microemulsions seems to indicate that the iron(II) species is distributed between the aqueous phase and the interphase. © 1995 John Wiley & Sons, Inc.     

Alterations in phospholipid bilayers caused by sodium dodecyl sulphate/triton X-100 mixed systems

The mechanisms governing the subsolubilizing and solubilizing interaction of sodium dodecyl sulphate (SDS)/Triton X-100 mixtures and phosphatidylcholine liposomes were investigated. Permeability alterations were detected as a change in 5(6)-carboxy-fluorescein (CF) released from the interior of vesicles and bilayer solubilization as a decrease in the static light-scattered by liposome suspensions. Three parameters were described as the effective surfactant/lipid molar ratios (Re) at which the surfactant system a) resulted in 50% of CF release (Re _50%CF); b) saturated the liposomes (Re _SAT;c) led to a complete solubilization of these structures (Re _SOL). From these parameters the corresponding surfactant partition coefficientsK _50%CF,K _SAT andK _SOL were determined. The free surfactant concentrationsS _W were lower than the mixed surfactant CMCs at subsolubilizing level, whereas they remained similar to these values during saturation and solubilization of bilayers in all cases. Although theRe increased as the mole fraction of the SDS rose (X _SDS), theK parameters showed a maximum atX _SDS values of about 0.6, 0.4 and 0.2 forK _50%CF,K _SAT andK _SOL respectively. Thus, the higher the surfactant contribution in surfactant/lipid system, the lower theX _SDS at which a maximum bilayer/water partitioning of mixed surfactant systems added took place and, consequently, the lower the influence of the SDS in this maximum bilayer/water partitioning.Abbreviations PC Phosphatidylcholine - PIPES piperazine-1,4 bis (2-ethanesulphonic acid) - SDS sodium dodecyl sulphate - X _SDS mole fraction of sodium dodecyl sulphate in the mixed system - CF 5(6)-carboxyfluorescein - Re effective surfactant/lipid molar ratio - Re _50%CF effective surfactant/lipid molar ratio for 50% CF release - Re _SAT effective surfactant/lipid molar ratio for bilayer saturation - Re _SOL effective surfactant/lipid molar ratio for bilayer solubilization - S _W surfactant concentration in the aqueous medium - S _{W, 50%CF} surfactant concentration in the aqueous medium for 50% CF release - S _{W, SAT} surfactant concentration in the aqueous medium for bilayer saturation - S _{W, SOL} surfactant concentration in the aqueous medium for bilayer solubilization - S _B surfactant concentration in the bilayers - K bilayer/aqueous phase surfactant partition coefficient - K _50%CF bilayer/aqueous phase surfactant partition coefficient for 50% CF release - K _SAT bilayer/aqueous phase surfactant partition coefficient for bilayer saturation - K _SOL bilayer/aqueous phase surfactant partition coefficient for bilayer solubilization - PL phospholipid TLC-FID, thin-layer chromatography/flame ionization detection system - PI polydispersity index - CMC critical micellar concentration - r ² regression coefficient     

Studies on the Physicochemical Properties of 3-Alkoxyl-2-hydroxypropyl Trimethylammonium Chloride–Sodium Dodecyl Sulfonate Binary-Surfactant Aqueous Systems

Surface tension, micelle formation, surface adsorption, and solubilization of dimethylaminoazobenzene (DMAB) are studied in aqueous solutions of 3-alkoxyl-2-hydroxypropyl trimethylammonium chloride (alkoxyl = C_nH_2n+1O, n = 8, 12, 14, 16), of sodium dodecyl sulfonate, and of mixtures of these cationic surfactants and the anionic surfactant at 40°C. Synergistic effects on micelle formation, surface tension reduction, and solubilization enhancement of DMAB are observed in the cationic–anionic mixed surfactant systems. The experimental results are discussed in the light of the interactions between the two kinds of surfactant ions.     

Ligandless C‐C bond formation via Suzuki–Miyaura reaction in micelles or water–ethanol solution using PdPtZn and PdZn nanoparticle thin films

PdPtZn and PdZn nanoparticle (NP) thin films were synthesized by the reduction of [PdCl₂(cod)], [PtCl₂(cod)] (cod = cis,cis‐1,5‐cyclooctadiene) and [Zn(acac)₂] (acac = acetylacetonate) complexes at an oil–water interface. The structure and morphology of the as‐prepared NPs were characterized with X‐ray diffraction, transmission electron microscopy and energy dispersive analysis of X‐rays. Catalytic activity of the prepared NPs was investigated in the Suzuki–Miyaura cross‐coupling reaction in H₂O–EtOH and various micellar media systems such as cetyltrimethylammonium bromide (cationic surfactant), sodium dodecylsulfate (anionic surfactant) and Pluronic P123 (non‐ionic surfactant). PdPtZn and PdZn thin films exhibited higher catalytic activity compared to Pd thin film in the Suzuki–Miyaura coupling reaction due to the appropriate interaction between palladium, platinum and zinc metals. Copyright © 2015 John Wiley & Sons, Ltd.     

Preparation of substituted ionic carbohydrate polymers and their interactions with ionic surfactants

The formation of micelles of hexadecyltrimethylammonium chloride (CTAC) and sodium dodecylsulfate (SDS) in aqueous solutions containing charged polysaccharides was studied by steady-state and time-resolved fluorescence measurements using pyrene as a photophysical probe. Micropolarity studies using the I₁/I₃ ratio of the vibronic emission bands of pyrene and the behaviour of the I_E/I_M ratio between the excimer and monomer emissions show the formation of hydrophobic domains. The interactions between the polyelectrolytes and surfactants of opposite charge lead to the formation of induced pre-micelles at surfactant concentrations lower than the critical micellar concentration (cmc) of the surfactants. At similar concentrations, the I_E/I_M ratio shows a peak. This aggregation process is assumed to be due to electrostatic attractions. At higher surfactant concentrations, near the critical micellar concentration, micelles with the same properties as those found in pure aqueous solution are formed. On the other hand, systems containing polyelectrolytes and surfactants of the same charge do not show this behaviour at low concentrations. The presence of long alkyl chains bound to the polyelectrolytes also induces the formation of free micelles at concentrations somewhat below the aqueous cmc.     

Interaction of slightly cross-linked gels of poly(diallyldimethylammonium bromide) with sodium dodecyl sulfate. Diffusion of surfactant ions in gel

The dynamics of the changing microenvironment of the fluorescent probe pyrene in slightly cross-linked networks of poly(diallyldimethylmmonium bromide) during diffusion of sodium dodecyl sulfate (SDS) in the gel phase has been investigated by fluorescence spectroscopy. Values of the spectral ratio I₃/I₁ for pyrene monomer included in SDS micelles in the swollen networks fall between the corresponding values for pyrene in water and for pyrene dissolved in SDS micelles in aqueous solution. In the narrow interval of the surfactant concentrations in the gel phase, the formation of pyrene excimers is observed. The values of the critical micelle concentration in the gel phase (ca. 5 × 10^?4 to 8 × 10^?4 mol/L) are tenfold lower than in aqueous solutions of the surfactant. The effective micellar diffusion coeffecient D in the gel phase increases with increasing swelling of the network. © 1993 John Wiley & Sons, Inc.     

Microcalorimetric determination of effect of the antioxidant (Quercetin) on polymer/surfactant interactions

Isothermal titration calorimetry (ITC) and batch calorimetry techniques have been used to evaluate the effect of added antioxidant (Quercetin, QN) on the binding between a polymer/surfactant complex, namely the sodium salt of polystyrene sulfonate (PSS) and typical anionic surfactant sodium dodecylsulfate (SDS). An indirect isotherm approximation method and the Satake–Yang model have been used to evaluate the binding parameter (Ku), adsorption cooperativity (u), and the Gibbs free energy of cooperative and non-cooperative binding (ΔG _C and ΔG _N) from the ITC data. The enthalpy of dissolution of QN into various PSS/water and PSS/SDS/water solutions has been evaluated from batch calorimetry to study the energetics of the polymer/surfactant binding in the presence of QN.     

Phase behavior and solution properties of sodium (3-dodecanoyloxy-2-hydroxy-propyl) succinate in water

Sodium (3-dodecanoyloxy-2-hydroxy-propyl) succinate (SLGMS) is a conjugated anionic surfactant in which a glycerol residue connects with a hydrophilic sodium succinate and dodecanoate. Aqueous micellar phase (W_m), hexagonal (H₁), bicontinuous cubic (V₁), and lamellar (L_α) phases are successively formed with increasing the surfactant concentration in a binary SLGMS-water system. The Krafft point is below 0 °C. The effective cross sectional area per surfactant molecule, a _s, in the H₁ phase is almost constant, 0.5 nm², and the shape of cylindrical micelle is almost unchanged with surfactant concentration. The cmc value of SLGMS measured by means of surface tension, electrical conductivity, and fluorescence probe methods is in the range of 4∼9 × 10⁻⁵ mol/l that is much lower than that of sodium dodecanoate, 2 × 10⁻² mol/l, or SDS, 8 × 10⁻³ mol/l. Hence, it is considered that the polar glycerol part in the SLGMS acts as a hydrophobic part. The solubilization of oil in the SLGMS solution is much higher than that in the SDS solution and this also suggests that the glycerol and succinic units act as lipophilic moieties. Received: 15 June 2000/Accepted: 27 July 2000     

Molecular dynamics of electrosprayed water nanodroplets containing sodium bis(2‐ethylhexyl)sulfosuccinate

The behavior of aqueous solutions of sodium bis(2‐ethylhexyl)sulfosuccinate (AOTNa) subject to electrospray ionization (ESI) has been investigated by molecular dynamics (MD) simulations at three temperatures (350, 500 and 800 K). We consider several types of water nanodroplets containing AOTNa molecules and composed of a fixed number of water molecules (1000), N⁰_AOT AOT^? anions (N⁰_AOT = 0, 5, 10) and N⁰_Na sodium ions (N⁰_Na = 0, 5, 10, 15, 20): in a short time scale (less than 1 ns), the AOTNa molecules, initially forming direct micelles in the interior of the water nanodroplets, are observed in all cases to diffuse nearby the nanodroplet surface, so that the hydrophilic heads and sodium ions become surrounded by water molecules, whereas the alkyl chains lay at the droplet surface. Meanwhile, evaporation of water molecules and of solvated sodium ions occurs, leading to a decrease of the droplet size and charge. At 350 K, no ejection of neutral or charged surfactant molecules is observed, whereas at 500 K, some fragmentation occurs, and at 800 K, this event becomes more frequent. The interplay of all these processes, which depend on the values of temperature, N⁰_AOT and N⁰_Na eventually leads to anhydrous charged surfactant aggregates with prevalence of monocharged ones, in agreement with experimental results of ESI mass spectrometry. The quantitative analysis of the MD trajectories allows to evidence molecular details potentially useful in designing future ESI experimental conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

A UV-Visible Study of Partitioning of Pyrene in an Anionic Surfactant Sodium Dodecyl Sulfate

The interaction of hydrophobic dye pyrene with sodium dodecyl sulphate (SDS), an anionic surfactant, was studied in the process of solubilization. Difference UV-Visible spectroscopy was used to carry out the study. The partition coefficient (K_x), and number of dye molecules incorporated per micelle (n) was calculated. High K_x value shows that pyrene is partitioned strongly from polar to nonpolar environment. Steady-state fluorescence spectroscopy is used to check the environment of the pyrene as it is a well-known fluorescent probe. Onset of slope in curves is used to determine the critical micelle concentration (CMC).     

Spectroscopic analysis of naphtholazobenzimidazole in organised solution

Micelle–water partition coefficient (K_x ) of naphtholazobenzimidazole dye (NAB) in aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulphate (SDS) has been determined spectrophotometerically. Changes in absorption patterns of dye caused by surfactants and solvents were quantified in terms of dye–surfactant ratio (n _D/n _S) and solvent water partition coefficients (P), respectively. Multiple residence sites have been suggested for dye molecules within micelles, based on shifts in azo-hydrazone tautomeric equilibrium. Micelle–water partition coefficients were used to evaluate the influence of dye on critical micelle concentration of CTAB and SDS. At same micelle concentration, M, the relative solubility of NAB was greater in cationic surfactant CTAB than in anionic surfactant SDS.     

Effect of Alcohols on the CMC and Thermodynamic Functions of Anionic Surfactants in DMF/Long‐Chain Alcohol Using Microcalorimetric Method

The critical micelle concentration (CMC) of two kinds of anionic surfactant (including sodium laurate (SLA) and sodium dodecyl sulfate (SDS)) in mixed alcohol and N, N‐dimethyl formamide solvent (DMF) were investigated through measuring power‐time curves by titration microcalorimetry. From data of the lowest point and the area of the power‐time curves, their CMC and ΔH _m ⁰ can be obtained. According to standard thermodynamic equations, ΔG _m ⁰ and ΔS _m ⁰ also can be calculated. For different surfactant, the influences of the carbon number and the concentration of alcohol on the CMC and standard thermodynamic functions are different in DMF polar medium. These thermodynamic functions for micelle formation can be further interpreted.     

FLUORESCENCE PROBE STUDIES ON THE SECOND CRITICAL MICELLE CONCENTRATION OF SEVERAL KINDS OF SURFACTANTS

The fluorescence intenstiy ratios (F₂/F₁) of excimer (F₂) to monomer (F₁) of probe (pyrene) were measured as a function of the concentration of the surfactant in several systems, which consist of sodium hexadecyl sulfonate (C₁₆As)-anionic surfactant, cetyltrimethylammonium bromide (CTAB)-cationic surfactant and pentaoxyethylene decyl ether (C₁₀E₅)-nonionic surfactant. The second CMC values were obtained according to F₂/F₁ ~ surfactant concentration curve break. In addition, we also studied the variation of second CMC vs NaCl concentration. It was found that the second CMC values of C₁₆A₅ and CTAB decreased with the increase of NaCl concentration. But the present of NaCl had no influence on the second CMC value of C₁₀E₅. Results showed that sphere-shaped micelles turn into rod-like micelle at the second CMC. The results were interpreted in terms of diffusion of pyrene molecules and the distribution of pyrene among micelles.     

Phase Diagrams and Solubilization of Chlorocarbons in Chlorocarbon/Water/Anionic Surfactant/Alcohol Microemulsion Systems

The solubilization abilities of various chlorocarbons were investigated in a middle phase microemulsion system anionic surfactant sodium dodecyl sulfate (SDS) or sodium dodecyl sulfonate (AS)/n-butanol/chlorocarbon/brine with a ε-β fishlike phase diagram. The composition of the balanced interfacial layer of the microemulsion and some other parameters are calculated. The result shows that surfactant little dissolves in water and chlorocarbon phases, while alcohol mainly dissolves in water and oil phases besides in the interfacial layer. The order of the solubilization ability is dichloromethane (CH₂Cl₂) ～ carbon tetrachloride (CCl₄) > tetrachloroethylene (PCE) > o-dichloro-benzene. The solubility of the alcohol decreases with the increase in NaCl concentrations, which should be compensated by the increase in the amount of alcohol as cosolvent (Cs), so as to maintain the balanced interfacial layer. Salinity has little effect on the partition of surfactant between phases.