Isothermal titration calorimetry and dynamic light scattering studies of interactions between gemini surfactants of different structure and Pluronic block copolymers

The interactions between triblock copolymers of poly(ethylene oxide) and poly(propylene oxide), P103 and F108, EO(n)PO(m)EO(n), m=56 and n=17 and 132, respectively, and m-s-m type gemini surfactants, m=8, 10, 12, and 18, and s = 3, 6, 12, and 16, have been studied in aqueous solution using isothermal titration calorimetry and dynamic light scattering techniques. The enthalpograms of F108 as a function of surfactant concentration show one broad peak at polymer concentrations C(p) < or = 0.50 wt%, below the cmc of the copolymer at 25 degrees C. It is attributed to interactions between the surfactant and the triblock copolymer monomer. DLS results show hydrodynamic radii (R(h)) initially consistent with copolymer monomers that change to values consistent with gemini surfactant micelles as the surfactant concentration is increased. In P103 solutions at C(p) > or = 0.05 wt%, two peaks appear in the enthalpograms, and they are attributed to the interactions between the gemini surfactant and the micelle or monomer forms of the copolymer. An origin-based nonlinear fitting program was employed to deconvolute the two peaks and to obtain estimates of peak properties. An estimate of the fraction of copolymer in aggregated form was also obtained. The enthalpy change due to interactions between the surfactants and P103 aggregates is very large compared to values obtained for traditional surfactants. This suggests that extensive reorganization of copolymer aggregates and surrounding solvent occurs during the interaction. DLS results for the P103 systems containing C(p) > or = 0.05% show evidence of very large aggregates in solution, likely P103 micelle clusters. The transitions observed in the hydrodynamic radii are consistent with a breakdown of micelle clusters with addition of gemini surfactant, followed by mixed micelle formation and/or deaggregation into monomer P103. This is followed by interactions similar to those typically observed in surfactant-nonionic polymer systems. Mechanisms for the interaction and the observed structural changes are discussed.     

Interaction of ethylene oxide-propylene oxide copolymers with ionic surfactants studied by calorimetry: random versus block copolymers

The present study used calorimetric techniques to follow the interaction of random and block ethylene oxide (EO)-propylene oxide (PO) copolymers with ionic surfactants. Features such as the intensity of the interaction (evaluated through their critical aggregation concentrations) and the profile of the isothermal titration calorimetry (ITC) curves were comparatively analyzed for random and block copolymers with similar composition (number of EO and PO units). Random copolymers displayed an interaction similar to that observed with other hydrophilic homopolymers with the additional characteristic that the intensity of the interaction increased with the increase in the copolymer hydrophobicity (as determined by its PO content), revealing that these copolymers display an intermediate behavior between PEO and PPO. For nonaggregated block copolymers (unimers) with large enough EO blocks (molar mass above 2000 g mol-1), ITC curves revealed that the anionic surfactant sodium dodecylsulfate (SDS) interacts with the PO and EO blocks almost independently, being more favorable with the PO block, which controls the critical aggregation concentration (cac) value. Effects of temperature and of the nature of the ionic surfactants on their interaction with these copolymers were found to agree with the previously reported trends.     

A study of α-cyclodextrin with a group of cationic gemini surfactants utilizing isothermal titration calorimetry and NMR

Isothermal titration calorimetry has been applied to determine the stability constants, stoichiometry, formation enthalpies, entropies, and Gibbs free energies for the complexes of α-cyclodextrin (α-CD) with a series of bis-quarternary ammonium surfactants, (C_nN)₂Cl₂ (n = 12, 14, 16), in aqueous solutions at 293.15 K. The observed stability constants of the complexes are very large. For these quite stable inclusion complexes, the stoichiometry of most stable complexes changes from 2:1 to 6:1 as the number of methylenes (–CH₂–) in each of the hydrophobic tail is increased from 12 to 16. According to the same change of the hydrophobic chain, both formation enthalpy and formation entropy evidently decrease. The results also indicate that the association processes are characterized by both favorable enthalpy changes and unfavorable entropy changes. Chemical shift data of all protons in the CD molecule, induced by the formation of the (α-CD + (C₁₂N)₂Cl₂) complexes have been determined by Proton NMR spectroscopy.     

Glass transition temperatures of ABA poly(styrene-b-isoprene) block copolymers by differential scanning calorimetry

The glass transition behavior of two sets of ABA poly(styrene-b-isoprene) block copolymers was examined by differential scanning calorimetry. In one series, the triblock copolymers had different total molecular weights and the same (30 wt %) polyisoprene content, in the other, the molecular weight was constant (30,000 g/mol) and the elastomer content was the variable. For all triblock copolymers studied, the data show an inward shift for the glass transition temperatures T_g of the corresponding homopolymers. This shift increases for the rigid-phase T_g as the polystyrene block length decreases. Depending on their molecular characteristics, two, three, or only one T_g were found. The third T_g was interpreted in terms of the existence of an interphase. Some of these conclusions could be confirmed by transmission electron microscopy.     

Interactions of Humicola insolens cutinase with an anionic surfactant studied by small-angle neutron scattering and isothermal titration calorimetry

The interaction of cutinase from Humicula insolens (HiC) and sodium dodecyl sulfate (SDS) has been investigated by small-angle neutron scattering (SANS) and isothermal titration calorimetry (ITC). The concerted interpretation of structural and thermodynamic information for identical systems proved valuable in attempts to elucidate the complex modes of protein-detergent interaction. Particularly so at the experimental temperature 22 degrees C, where the formation of SDS micelles is athermal (deltaH = 0), and the effects of protein-detergent interactions stand out clearly in the thermograms. It was found that the effect of SDS on cutinase depended strongly on the sample composition. Thus, addition of SDS corresponding to a molar ratio, n(s) = n(SDS)/n(HiC) of about 10, was associated with the formation of HiC/SDS aggregates, which include more than one protein molecule. The SANS results suggested that on the average such adducts contained two HiC, and the ITC traces showed that they form and break down slowly. At slightly higher SDS concentrations (n(s) = 10-25) these "dimers" dissociated, and the protein denatured. The denaturation showed the characteristic positive enthalpy change, but the SDS denatured state of HiC was unusually compact with a radius of gyration close to that of the native conformation. Further titration with SDS was associated with exothermic binding to the denatured protein until the saturation point at about n(s) = 90. At this point, the free monomer concentration was 2.2 mM and the binding number was approximately 40 SDS/HiC. Interestingly, this degree of SDS binding (approximately 0.5 g of SDS/g of HiC) is less than half the amount bound to typical water-soluble proteins.     

Dicynthaurin (ala) monomer interaction with phospholipid bilayers studied by fluorescence leakage and isothermal titration calorimetry

The interaction of the antimicrobial peptide dicynthaurin (ala) monomer with model membranes of zwitterionic and negatively charged lipids and mixtures thereof was studied by means of isothermal titration calorimetry (ITC), fluorescent leakage, and dynamic light scattering (DLS) measurements. For the ITC analysis, we have applied the surface partitioning equilibrium model which shows that the interaction is predominately driven by hydrophobic effects (Kb between 2 x 10(4) and 1 x 10(5) M(-1)). Under low salt conditions, the enhanced electrostatic interaction leads to larger peptide concentrations immediately above the vesicle surface, which initiates the insertion of the peptide into the bilayer more effectively. Fluorescent leakage measurements have shown a fast leakage of the fluorescent dye within seconds after peptide addition. The analysis of the leakage kinetics was performed in terms of an initial pore formation model (up to t = 1000 s) that takes the reversible surface aggregation of bound peptide monomers into account. From this analysis, a minimum aggregation number of n = 7 +/- 2 per pore is obtained.     

Interactions of cationic gemini surfactants with hydrophobically modified poly(acrylamides) studied by fluorescence and microcalorimetry

Steady-state fluorescence, time-resolved fluorescence quenching, and isothermal titration microcalorimetry have been used to study the interactions of cationic gemini surfactants alkanediyl-alpha,omega-bis(dodecyldimethylammonium bromide) (C(12)C(S)C(12)Br(2), S = 3, 6, and 12) with hydrophobically modified poly(acrylamide) (HMPAM) and unmodified poly(acrylamide) (PAM). Without addition of gemini surfactant, 0.2 wt % HMPAMs except PAM have already self-aggregated into hydrophobic aggregates. Different from single-chain surfactants, C(12)C(S)C(12)Br(2) have stronger interactions with HMPAMs to form surfactant/polymer aggregates, even with PAM. Addition of C(12)C(S)C(12)Br(2) may cause the disruption of HMPAM hydrophobic aggregates and the formation of mixed micelles. It is found that HMPAMs generate lower micropolarity of mixed micelles, larger values of enthalpy of interaction (DeltaH(ps)), and nearly constant values of Gibbs free energy of interaction (DeltaG(ps)). On the other hand, C(12)C(S)C(12)Br(2) with longer spacer brings out slightly lower micropolarity of mixed micelles, owing to the lower electrostatic repulsion between surfactant headgroups. Especially for C(12)C(12)C(12)Br(2), the values of DeltaH(ps) are much more endothermic and the values of DeltaG(ps) are much less negative. The weaker interactions of C(12)C(12)C(12)Br(2) with HMPAMs arise from the marked reduction of attraction between surfactant headgroups and polymer hydrophilic groups induced by its longer spacer.     

Interactions between gemini surfactants, 12-s-12, and beta-cyclodextrin as investigated by NMR diffusometry and electric conductometry

The interaction between beta-cyclodextrin (CD) and gemini surfactant of the type alkyl-alpha,omega-bis(dodecyldimethylammonium bromide) with different spacer lengths of 2, 8, and 10 carbons has been investigated by means of electric conductivity (EC) and proton self-diffusion NMR at 298 K. The formation of a 2:1 (CD:gemini) complex in a two-step mechanism is observed with the first association constant (K(11)) higher than the second one (K(21)), but both relatively small in comparison with single C(12)-tailed surfactant. The value of the association constants increased with spacer length both for the first and second associated CD, which indicates that the available space on the gemini molecule is important. The magnitudes of the association constant both for the first and second complexation are discussed. The first association constant is small (when compared with the homologous single-chain surfactant) due to hydrophobic interaction between the hydrocarbon tails within the gemini molecule, while the second association constant shows no cooperativity and its magnitude is discussed in terms of steric constrains.     

Investigation of copper(II) complexation by glycylglycine using isothermal titration calorimetry

Isothermal titration calorimetry (ITC) and potentiometric titration methods have been used to study the process of proton transfer in the copper(II) ion-glycylglycine reaction. The stoichiometry, conditional stability constants, and thermodynamic parameters (ΔG, ΔH, and ΔS) for the complexation reaction were determined using the ITC method. The measurements were carried out at 298.15 K in solutions with a pH of 6 and the ionic strength maintained with 100 mM NaClO₄. Carrying out the measurements in buffer solutions of equal pH but different enthalpies of ionization of its components (Mes, Pipes, Cacodylate) enabled determination of the enthalpy of complex formation, independent of the enthalpy of buffer ionization. The number of protons released by glycylglycine on account of complexation of the copper(II) ions was determined from calorimetric and potentiometric measurements.     

Interaction between Pluronic F127 and dioctadecyldimethylammonium bromide (DODAB) vesicles studied by differential scanning calorimetry

A number of fundamental studies on the interactions between lipid bilayers and (ethylene oxide)-b-(propylene oxide)-b-(ethylene oxide) copolymers (PEO-PPO-PEO, Pluronics) have been carried out recently as model systems for the complex behavior of cell membranes with this class of polymers often employed in pharmaceutical formulations. We report here a study by differential scanning calorimetry (DSC) of the interactions in water between Pluronic F127 (F127), and the cationic vesicles of di-n-octadecyldimethylammonium bromide (DODAB), as a function of concentration of the two components (DODAB 0.1 and 1.0 mM; F127 0.1 to 5.0 mM) and of the sample preparation protocol. The DSC studies follow the critical micellization temperature (cmt ≈ 27 °C at 1.0 mM) of F127 and the gel-liquid crystal transition (T(m) ≈ 45 °C) of the DODAB bilayer and of F127/DODAB mixtures. Upon heating past T(m), vesicle/polymer mixtures undergo an irreversible conversion into mixed DODAB/F127 micelles and/or F127-bearing vesicles, depending on the relative amount of each component, together with, in some cases, residual intact F127 micelles or DODAB vesicles. Sample preparation protocol is shown to have little impact on the composition of mixed systems once they are heated above T(m).     

聚(丙烯酰胺/丙烯酸钠/N,N-二辛基丙烯酰胺)与表面活性剂的相互作用

利用粘度法研究了孪尾疏水缔合水溶性聚(丙烯酰胺/丙烯酸钠/N,N-二辛基丙烯酰胺)[P(AM/NaAA/D iC8AM)]与表面活性剂十二烷基硫酸钠(SDS)、十六烷基三甲基溴化铵(CTAB)、壬基酚聚氧乙烯醚(OP-10)的相互作用。研究表明SDS、CTAB的加入,能显著地降低P(AM/NaAA/D iC8AM)水溶液的临界缔合浓度。表明P(AM/NaAA/D iC8AM)与SDS、CTAB的疏水缔合作用较强,而与OP-10的疏水缔合作用较弱。     

Interactions of poly(amidoamine) dendrimers with the surfactants SDS, DTAB, and C12EO6: an equilibrium and structural study using a SDS selective electrode, isothermal titration calorimetry, and small angle neutron scattering

Interactions in aqueous solutions of different generations of poly(amidoamine) (PAMAM) dendrimers containing amine, hydroxyl, or delta-glucolactone functional groups at the periphery with the anionic surfactant sodium dodecyl sulfate (SDS) were investigated. We used a SDS-specific electrode (EMF) for SDS monomer concentration monitoring, isothermal titration calorimetry (ITC) for binding information, and small angle neutron scattering (SANS) for structural studies. ITC experiments monitoring the interaction of the dendrimers with cationic dodecyltrimethylammonium bromide (DTAB) and nonionic hexaethylene glycol mono-n-dodecyl ether (C12EO6) showed no significant binding effects. In contrast, SDS binds to all of the above dendrimers. EMF and ITC data demonstrated a regular trend for both the onset of binding and binding saturation as the generation in each family of dendrimers increased. In addition, generation G6 exhibited a noncooperative binding process at very low SDS concentrations. Furthermore, the onset of cooperative binding in the EMF experiments started at lower concentrations as the weight % (w/v), the size, and the numbers of the internal or surface groups increased. On the other hand, the binding capacity of the dendrimers showed only a small dependence on the above parameters. At SDS concentrations approaching the binding limit and also at selective concentrations within the binding range, SANS measurements indicated that in all cases the bound surfactant is in the micellar form. From the electromotive force (EMF) measurements, ITC data, and SANS data, the stoichiometry of the supramolecular complexes was determined.     

Interactions between poly(acrylic acid) and sodium dodecyl sulfate: isothermal titration calorimetric and surfactant ion-selective electrode studies

Interaction between a monodispersed poly(acrylic acid) (PAA) (M(W) = 5670 g/mol, M(w)/M(n) = 1.02) with sodium dodecyl sulfate (SDS) was investigated using isothermal titration calorimetry (ITC), ion-selective electrode (ISE), and dynamic light scattering measurements. Contrary to previous studies, we report for the first time evidence of interaction between SDS and PAA when the degree of neutralization (alpha) of PAA is lower than 0.2. Hydrocarbon chains of SDS cooperatively bind to apolar segments of PAA driven by hydrophobic interaction. The interaction is both enthalpy and entropy favored (deltaH is negative but deltaS is positive). In 0.05 wt % PAA solution, the SDS concentration corresponding to the onset of binding (i.e., CAC) is approximately 2.4 mM and the saturation concentration (i.e., C(S)) is approximately 13.3 mM when alpha = 0. When PAA was neutralized and ionized, the binding was hindered by the enhanced electrostatic repulsion between negatively charged SDS and PAA chains and improved solubility of the polymer. With increasing alpha to 0.2, CAC increases to approximately 6.2 mM, C(S) drops to 8.6 mM, and the interaction is significantly weakened where the amount of bound SDS on PAA is reduced considerably. The values of CAC and C(S) derived from different techniques are in good agreement. The binding results in the formation of mixed micelles on apolar PAA coils, which then expands and dissociates into single PAA chains. The majority of unneutralized PAA molecules exist as single polymer chains stabilized by bound SDS micelles in solution after the saturation concentration.     

Interaction between poly(ethylene glycol) and water as studied by differential scanning calorimetry

The interaction between poly(ethylene glycol) (PEG) and water was studied by differential scanning calorimetry (DSC). The DSC curves of PEG–water systems were classified into three groups according to the difference in molecular weight. The melting peaks of eutectic mixture appeared for PEG with molecular weight higher than 1000. The eutectic point temperature shifted to higher temperatures and the eutectic point composition shifted to lower concentrations of PEG with increasing molecular weight. The maximum hydration number per ethylene glycol (EG) unit was estimated as 1.6, 2.4, and 3.3 for samples with molecular weights 400, 1540, and 70,000, respectively. No thermal change was found in PEG1540‐water system for a narrow weight fraction range of 0.585–0.605 for overall measuring temperatures due to perfect supercooling. The glass transition temperature shifted to higher temperatures with increasing molecular weight of PEG. A modified Flory–Huggins equation was used to fit curves for experimental liquidus data in phase diagrams. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 496–506, 2001     

Studies of the binding mechanism between aptamers and thrombin by circular dichroism, surface plasmon resonance and isothermal titration calorimetry

Thrombin, a multifunctional serine protease, has both procoagulant and anticoagulant functions in human blood. Thrombin has two electropositive exosites. One is the fibrinogen-binding site and the other is the heparin-binding site. Over the past decade, two thrombin-binding aptamers (15-mer and 29-mer) were reported by SELEX technique. Recently, many studies examined the interactions between the 15-mer aptamer and thrombin extensively, but the data on the difference of these two aptamers binding to thrombin are still lacking and worth investigating for fundamental understanding. In the present study, we combined conformational data from circular dichroism (CD), kinetics and thermodynamics information from surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC) to compare the binding mechanism between the two aptamers with thrombin. Special attentions were paid to the formation of G-quadruplex and the effects of ions on the aptamer conformation on the binding and the kinetics discrimination between specific and nonspecific interactions of the binding. The results indicated reasonably that the 15-mer aptamer bound to fibrinogen-binding site of thrombin using a G-quadruplex structure and was dominated by electrostatic interactions, while the 29-mer aptamer bound to heparin-binding site thrombin using a duplex structure and was driven mainly by hydrophobic effects.     

Binding of 8-anilinonaphthalene sulfonate to dimeric and tetrameric concanavalin A: energetics and its implications on saccharide binding studied by isothermal titration calorimetry and spectroscopy

The binding of 8-anilinonaphthalene sulfonate to concanavalin A has been investigated. Isothermal titration calorimetry (ITC) and circular dichroism studies have been performed under different experimental conditions to understand the binding quantitatively and evaluate contributions of different forces responsible for it. Isothermal titration calorimetric results of concanavalin A with ANS at pH 5.2 and 2.5, where it exists as a dimer, indicated binding heterogeneity and two classes of noninteracting sites. Enhancement of the binding constants from native to pH 2.5 suggests that the ANS binding is strongly influenced by the protein charge and the favorable alteration in the structure of concanavalin A as suggested by near-UV CD results. No binding was observed with the tetrameric form of concanavalin A, indicating shielding of sites due to dimerization of canonical dimers. The results have also demonstrated existence of a hydrophobic binding site that is distinct from the saccharide binding site.     

Interactions between ethyl(hydroxyethyl) cellulose and lysine-based surfactants in aqueous media

In this work, the interaction between ethyl(hydroxyethyl) cellulose (EHEC) and three dimeric lysine-based surfactants of distinct chain length (C₆, C₈ and C₁₀) have been assessed and the system was evaluated in terms of its temperature-dependent gelling capacity. The viscosity profile depends on the specific surfactant, its concentration and temperature. The observed profiles reflected polymer–polymer associations at elevated temperatures and polymer-surfactant interactions, implying the formation of micellar-type associations. The systems induce gelation at higher temperatures. Longer chain-length surfactants induce gelation at lower concentrations due to their stronger tendency to self-assemble. The thermo-responsive gels showed gel strength generally lower than 20 Pa.sⁿ and a fractal dimension of 2.3–2.4, respectively, indicating the formation of soft gels comprising a tight and homogeneous network. The weakest gel was produced in the presence of the C₆ surfactant. 2D Small-Angle Light Scattering patterns showed a pronounced effect of temperature in terms of the evolution of large hydrophobic clusters, an event precluded when high concentrations of the longer chain surfactants were used.