The focus of this study is to understand the origin of the chiral recognition for a host–guest system containing complexes with different stoichiometries. Each enantiomer of 2-naphthyl-1-ethanol forms two different 1:1 complexes with β-cyclodextrin, leading to the formation of three different 2:2 complexes. One of these 2:2 complexes leads to excimer emission of the guest. Fluorescence studies were employed to determine the binding isotherms for the 1:1 and 2:2 complexes. No chiral discrimination was directly observed for the formation of the 1:1 complexes, while higher equilibrium constants (29% from binding isotherms and 40% from kinetic studies) were observed for the formation of the 2:2 complexes with ( R )-2-naphthyl-1-ethanol when compared to the formation of the 2:2 complexes formed from ( S )-2-naphthyl-1-ethanol. The relaxation kinetics was studied using stopped-flow experiments. The formation of the 2:2 complexes was followed by detecting the excimer emission from one of the 2:2 complexes. The relaxation kinetics was faster for ( S )-2-naphthyl-1-ethanol, where a higher dissociation rate constant, by 47%, was observed, suggesting that the chiral discrimination occurs because the interaction between two cyclodextrins is more favorable for the complexes containing ( R )-2-naphthyl-1-ethanol when compared to ( S )-2-naphthyl-1-ethanol. The same overall equilibrium constants were observed for the 1:1 complexes with both enantiomers showing that at a given cyclodextrin concentration the sum of the two types of 1:1 complexes is the same for both enantiomers. However, analysis of the binding isotherms indicates that the ratio between the two different 1:1 complexes for each enantiomer was different for ( R )- and ( S )-2-naphthyl-1-ethanol.

Vesicles of l-α-dimyristoyl-phosphatidylcholine (DMPC) are known to disintegrate upon treatment with surfactin, a lipoheptapeptide biosurfactant from Bacillus subtilis OKB 105, as was observed by static light scattering (SLS) and cryo-transmission electron microscopy (cryo-TEM) recently. The lysis of DMPC bilayers occurs strongly dependent on the surfactin concentration according to a three-stage model. Unilamellar DMPC vesicles are disrupted to form sheet-like lamellar intermediates at a moderate surfactant concentration, but undergo a transition towards smaller particles of unknown structure at a higher surfactant concentration according to earlier neutron scattering experiments. Here we present direct structural evidence from cryo-electron tomography data that thread-like micelles with a uniform diameter of 6.5nm are organized into loops of different sizes at a surfactin concentration of >15mol%.

To understand the biological action of surfactin from Bacillus subtilis we investigated its effects on the phase transition of L-alpha-dimyristoyl phosphatidylcholine (DMPC)-vesicles from the crystalline to the fluid state using differential scanning calorimetry; light scattering; small angle neutron scattering and cryo-clectron microscopy. DSC-thermograms revealed two phase transition peaks. Light scattering profiles showed two branches with characteristic hysteresis phenomena. With both techniques the same values of the phase transition temperatures T-m1 and T-m2 of 23.5 and 23 degrees C were obtained indicating two forms of DMPC-surfactin aggregates which could be visualized by cryo-electron microscopy. Until 4 mol % surfactin the vesicular form predominated, but was accompanied by bilayered membrane fragments by increasing the biosurfactant concentrations. At surfactin concentrations higher than 15 mol% smaller DMPC-surfactin micelles of ellipsoidal conformation were formed, as demonstrated by small angle neutron scattering. In addition, by "Poor Man's" temperature-jump-relaxation spectroscopy slow transients in the phase transition of vesicular DMPC-surfactin aggregates with relaxation times of 20-30 s were detected which presumably indicate the slow dissipation of intermediate lipid-and surfactin domains formed after the main phase transition on the way to the fluid state. This process is accelerated by surfactin. (c) 2007 Elsevier B.V. All rights reserved.

The interactions of sodium dodecyl sulfate (SDS) with the triblock copolymer L64 (EO13-PO30-EO13) and hexaethylene glycol mono-n-dodecyl ether (C12EO6) were studied using electromotive force, isothermal titration microcalorimetry, differential scanning micocalorimetry, and surface tension measurements. In certain regions of binding, mixed micelles are formed, and here we could evaluate an interaction parameter using regular solution theory. The mixed micelles of L64 with both SDS and C12EO6 exhibit synergy. When L64 is present in its nonassociated state, it forms polymer/micellar SDS complexes at SDS concentrations above the critical aggregation concentration (cac). The cac is well below the critical micellar concentration (cmc) of pure SDS, and a model suggesting how bound micelles are formed at the cac in the presence of a polymer is described. The interaction of nonassociated L64 with C12EO6 is a very rare example of strong binding between a nonionic surfactant and a nonionic polymer, and C12EO6/L64 mixed micelles are formed. We also carried out small angle neutron scattering measurement to determine the structure of the monomeric polymer/micellar SDS complex, as well as the mixed L64/C12EO6 aggregates. In these experiments, contrast matching was achieved by using the h and d forms of SDS, as well as C12EO6. During the early stages of the formation of polymer-bound SDS micelles, SDS aggregates with aggregation numbers of approximately 20 were found and such complexes contain 4-6 bound L64 monomers. The L64/C12EO6 data confirmed the existence of mixed micelles, and structural information involving the composition of the mixed micelle and the aggregation numbers were evaluated.

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.

We investigated the binding of sodium dodecyl sulfate (SDS) to various linear and star polymers of the nonionic methoxyhexa(ethylene glycol) methacrylate (PMHEGMA) and the ionic 2-(dimethylamino)ethyl methacrylate (PDMAEMA), the latter being a polycation at low pH. The dodecyl sulfate ion selective electrode (EMF), isothermal titration calorimetry (ITC), and surface tension (ST) were applied to gain detailed information about interactions. In all cases there is evidence of significant binding of SDS over an extensive SDS concentration range spanning from ca. 10(-6) to 0.1 mol dm(-3). At pH 3, the polymer PDMAEMA is a strong polycation and here the binding is dominated by electrostatic 1:1 charge neutralization with the anionic surfactant. At their natural pH of 8.6, PMHEGMA and PDMAEMA polymers are essentially nonionic and bind SDS in the form of polymer-bound aggregates in the concentration range of ca. 1 x 10(-3) to 3 x 10(-2) mol dm(-3). All the polymers also bind SDS to a lesser extent at concentrations below 1 x 10(-3) mol dm(-3) reaching as low as 10(-7) mol dm(-3). This low concentration binding process involves the polymer and nonassociated SDS monomers. As far as we are aware, this is the first example that such a low concentration noncooperative binding process could be observed in SDS/neutral polymer systems by EMF and ST. We also showed that the nonionic surfactant hexa(ethylene glycol) mono-n-dodecyl ether (C12EO6) and the cationic cetyltrimethylammonium bromide (C16TAB) interact with star PDMAEMA. We believe that the interaction of C12EO6 and CTAB is of similar noncooperative type as the first SDS binding process in the range from ca. 10(-5) to 0.3 x 10(-3) mol dm(-3). At the high concentration binding limit Csat of SDS, the above polymers become fully saturated with bound SDS micelles. We applied small angle neutron scattering (SANS) to determine the structure and aggregation numbers of the star polymer/bound SDS micelles and calculated the stoichiometry of such supramolecular complexes. The SANS data on PDMAEMA star polymers in the presence of C12EO6 showed only a limited monomer binding in contrast to linear PDMAEMA, which showed monomer C12EO6 binding at low concentrations but micellar aggregates at 6 x 10(-3) mol dm(-3).

We investigated the thermodynamic and dynamic behaviour of triblock copolymers of general structure poly-ethylene oxide - poly-propylene oxide - polyethylene oxide (PEO-PPO-PEO) in water. Two representative examples were chosen, L64, (EO)(13)-(PO)(30)-(EO)(13) and F-127, (EO)(97)-(PO)(69)-(EO)(97). The critical micellization temperature (CMT), the enthalpy of micellization (DeltaH(mic)), and aggregation numbers could be determined. The kinetics were characterized by three well-defined relaxation processes from iodine-laser temperature-.jump and stopped flow studies. Monomer insertion in the microsecond time range was followed by a size readjustment and finally micellar growth was observed when macroscopic phase separation was approached. Small angle neutron scattering showed aggregates with wet cores of PPO and coronas of fully hydrated PEO. The influence of added surfactants could also be monitored and interactions below and above the CMT of the triblock copolymers were observed, showing synergistic effects. An outlook into future experiments closes the contribution.

During the last 6 years, chemical relaxation studies of the kinetics of micelle formation in triblock copolymers such as L64 (EO13PO30EO13) and B40 (EO13BO10EO13) Which behave as nonionic surfactants have been studied. The micellar system is characterized by two well-defined relaxation times as predicted by Aniansson and Wall for the isodesmic model A(1) + A(n-1) reversible arrow A(n) in which monomeric A(1) and micellar, A(n), are the principal species. A closer examination of the published work, however, reveals that some issues have not been satisfactorily resolved. For example it is claimed that the fast relaxation time associated with the single step monomer/micelle exchange exists in two time domains, namely, 10(-4)-10(-5) s (T-jump) and 10(-7) s (ultrasonic) for the same triblock copolymers. In an attempt to resolve this predicament, we report here some ultrasonic relaxation measurements on a number of triblock copolymers as micellar systems and also as mixed micellar systems in the presence of hexadecyltrimethylammonium bromide using both H2O and D2O as solvents. We carried out a careful and systematic experimental study including light scattering and surface tension. Following the analysis of the ultrasonic absorption data, the resulting relaxation parameters were incorporated into well-documented relaxation equations to evaluate rate data and test thermodynamic predictions for the monomer/micelle exchange. After careful considerations we concluded that the ultrasonic relaxation observed in micellar solutions of water soluble triblock copolymers is not associated with the monomer/micelle exchange as defined in the Aniansson and Wall treatment. This conclusion is based on (a) the unreasonable values above the diffusion controlled limit of the resulting rate constants and (b) the amplitude of the relaxation process being incompatible with the known thermodynamic parameters for the micellization process. The most likely explanation for the origin of the ultrasonic relaxation in Pluronics is associated with critical phenomena involving concentration fluctuations in the micellar aggregates. We believe that this process is active in ultrasonic relaxation through a coupling with monomer concentration changes which accompany the fluctuations. The fast relaxation times measured in triblock copolymer systems with the iodine laser temperature jump by Holzwarth et al.(12-15) therefore provide the correct rate constants for a monomer micelle/exchange process.

The binding of sodium dodecyl sulfate (SDS) to six random nonionic copolymers with the general chemical name polyvinyl(methyl imidazole-co-pyrrolidone-co-acrylate) were studied using electromotive force measurements (EMF) and isothermal titration calorimetry (ITC). In terms of their composition expressed in mole percent, each polymer contains 45 mol % methyl vinyl imidazole (MVI), 45 mol % vinyl pyrrolidone (VP)1 and 10 mol % of each of six different substituted acrylates. The purpose of the work was to investigate how subtle structural changes in the acrylate monomer affect the binding properties of SDS. The results showed significant differences in the binding behavior of the polymers, which are reflected in the determination of critical constants associated with SDS binding, like binding isotherms, the degree of sodium ion association to the bound SDS micelles, and the binding enthalpies as measured by ITC, especially in the early stages of binding. This opens the possibility of using ITC and EMF experiments which effectively measure the binding process as a mean of monitoring and characterizing subtle differences in structurally related macromolecules.

The iodine laser temperature jump method is used to study adsorption/desorption kinetics at a methylated silica/solution interface. A suspension of C1-derivatized fumed silica is used for the kinetic measurements. The colloidal silica does not significantly change the attenuation of near-IR radiation from the iodine laser and allows the surface site concentration to be varied so that adsorption and desorption rates can be determined. The temperature jump relaxation method was used to investigate the effect of electrolyte on adsorption of a charged solute (ANS) on a C1 silica surface. Adsorption equilibrium conditions were optimized to observe a maximum relaxation signal. Without electrolyte, the relaxation signal is biexponential, which is also reflected in a broad chromatographic peak shape and a two-site sorption isotherm. When electrolyte is added, the relaxation signal is primarily single exponential, which agrees with the Linear adsorption isotherm. The adsorption rate and equilibrium constant were found to increase significantly with added electrolyte, which showed that adsorption kinetics can influence both band broadening and retention.

Methylated cyclodextrins were investigated to shed light on their solubility behavior in water where they exhibit a negative temperature coefficient, contrasting the positive temperature coefficient of unmodified cyclodextrins. Both heptakis(2,6-di-O-methyl)-beta-cyclodextrin (DIMEB) and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TRIMEB) show two different conditions of crystallization. At low temperatures around IS degreesC, highly hydrated clathrates are formed, whereas, at high temperatures around 60-70 degreesC, DIMEB crystallizes as an anhydrate and TRIMEB as a monohydrate. The crystallization at high temperature is driven by entropy gain of water to compensate for the positive enthalpy change associated with this crystallization process, as could be shown by differential scanning calorimetry experiments in H2O and D2O.

We applied isothermal titration calorimetry (ITC) and surface tension (ST) and electromotive force (emf) measurements using a coated wire sodium dodecyl sulfate membrane-selective electrode to measure the mixed micellar composition of various mixtures of the triblock copolymer EO97PO69EO97, a nonionic surfactant code-named Pluronic F127, with sodium dodecyl sulfate (SDS). In the region where mixed micelles are formed, the interaction between the two surfactants showed synergistic behavior and interaction parameters β, which characterize the nonideal interaction in the mixed micelles, could be calculated over a range of mole ratios. For several compositions, the critical micelle concentrations of the mixed micelles were determined using ITC and ST measurements. In addition, small-angle neutron scattering (SANS) experiments were carried out in order to investigate the structure and provide additional information about the composition of the mixed micelles, taking advantage of contrast variation between SDS-...

It has been established that sodium dodecyl sulfate (SDS) binds to the micelles and monomers of the block copolymer F127. SDS binds to the monomeric unassociated F127 in the form of polymer/bound SDS micellar complexes. SDS binds to F127 micelles first forming mixed micelles, which dissociate into smaller mixed aggregates and then to single F127 unassociated monomers. A third interaction of SDS, which involves promotion of F127 micelles at concentrations up to 3 degreesC below the critical micellar temperature of pure F127, was identified ana is investigated in the present work. The formation of such SDS-induced mixed micelles was monitored using differential scanning calorimetry, light scattering, isothermal titration calorimetry, and a SDS selective electrode for electromotive force measurements. These investigations have shown how the different binding and aggregation processes between SDS and F127 involving induced micellization, growth of mixed micelles, breakdown of mixed micelles, and binding of SD'S to monomeric F127 can be identified and characterized.

Fluorescence correlation spectroscopy (FCS) was used to characterize the interaction of fluorescence labeled single-stranded DNA (ssDNA) with hexameric RepA DNA-helicase (hRepA) encoded by plasmid RSF1010. The apparent dissociation constants, Kd(app), for the equilibrium binding of 12mer, 30mer, and 45mer ssDNA 5'-labeled with BFL to hRepA dimer in the presence of 0.5 mM ATPgammaS at pH 5.8 and 25 degrees C were determined to be 0.58 +/- 0.12, 0.52 +/- 0.07, and 1.66 +/- 0.32 microM, respectively. Binding curves are compatible with one binding site for ssDNA present on hRepA dimer, with no indication of cooperativity. At pH 7.6 in the presence of ATPgammaS and at pH 5.8 in the absence of ATPgammaS, complex formation between ssDNA and hRepA was too weak for measuring complete binding curves by FCS. Under these conditions, the dissociation constant, Kd(app), is in the range between 10 and 250 microM. The kinetics of complex formation at pH 5.8 are faster than the time resolution (approximately 10-20 s) of FCS experiments under pseudo-first-order conditions, with respect to BFL-ssDNA. Photon correlation spectroscopy (PCS) experiments yielded, within the experimental error range, the same values for the apparent hydrodynamic radii, R(h), of hRepA dimer and its complex with ssDNA as determined by FCS (R(h) = 6.6 +/- 1 nm). hRepA starts to aggregate under acidic conditions (<pH 6.0) which are optimal for ssDNA binding. CD spectra taken at pH 5.8 in the presence of ATPgammaS showed a structural change induced by ssDNA binding to hRepA which is not visible at pH 7.6 and with ADP as nucleotide cofactor.

A poly(ethylene oxide) derivative of polyethyleneimine behaves like a strong polyelectrolyte at pH = 2.5 and a neutral polymer at pH 10. Both the charged and uncharged versions of this polymer bind strongly to the surfactant sodium dodecyl sulfate (SDS) with no phase separation taking place. Binding isotherms were measured using a dodecyl sulfate electrode, and these data were complemented with isothermal titration calorimetry (ITC) measurements. Small-angle neutron scattering measurements were also carried out at some specific concentrations in the binding region at pHs 2.5, 5.5, and 10. With the exception of one measurement, bound micelles were detected and their aggregation numbers could be evaluated. For the SDS/polymer system at pH 10, the polymer/surfactant complex contains 6-8 bound SDS micelles per polymer molecule at the binding limit. In a solution of 6.5 mM SDS/0.5% w/v polymer at pH 10, bound SDS exists in a nonaggregated form. A detailed examination of the ITC data for the SDS/0.5% w/v polymer system shows at pH 10 that this spot solution occurs in a narrow SDS concentration range immediately following the onset of binding and proceeding until the formation of proper bound micellar aggregates was detectable, whose presence and growth were characterized in the ITC experiments by a steplike decrease in the enthalpy per injection as a function of increasing SDS concentration. These data suggest that the absence of proper micellar aggregates is an inherent consequence of the binding mechanism in the early stages of SDS binding to ethoxylated polyethyleneimines.