Activity and kinetics studies of yeast alcohol dehydrogenase in a reverse micelle formulated from functional surfactants

Yeast alcohol dehydrogenase (YADH) showed substantial decrease in its catalytic activity due to the strong electrostatic interaction between the head groups of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and YADH in AOT reverse micelles. However, the catalytic activity of YADH in a nonionic reverse micellar interface (GGDE/TX-100) obtained from a functional nonionic surfactant N-gluconyl glutamic acid didecyl ester (GGDE) and Triton X-100 (TX-100) was higher than that in AOT reverse micelle under the respective optimum conditions. A comparison of the kinetic parameters showed that the turnover number k_cat in GGDE/TX-100 reverse micelle was 1.4 times as large as that in AOT reverse micelle, but the Michaelis constants in AOT reverse micelle for ethanol K_m^B was twice and for coenzyme NAD⁺ K_m^A was 5 times higher than their counterparts in GGDE/TX-100 reverse micelle. For the conversion of ethanol, the smaller K_m^B and larger k_cat in GGDE/TX-100 reverse micelle resulted in higher catalytic efficiency k_cat/K_m^B. The stability of YADH in GGDE/TX-100 reverse micelle was also found to be better than that in AOT reverse micelle. They were mainly attributed to the absence of electric charge on the head groups of GGDE and TX-100 in the GGDE/TX-100 reverse micelle.      

Formation and microstructure transition of F127/TX-100 complex

Formation and structure transition of the complex composed of triblock copolymer F127 and nonionic surfactant TX-100 have been investigated by 1H NMR spectroscopy, dynamic light scattering (DLS), and isothermal titration calorimetry (ITC). Three TX-100 concentration regions are identified, within which TX-100/20 mg/mL F127 complex undergoes different temperature-induced structure transitions. In low concentration region (< 9.42 mM), F127 single molecular species (unimers) wrap around TX-100 micelles forming F127/TX-100 complex with TX-100 micelle as the skeleton at a lower temperature (5 degrees C), and the skeleton transfers to F127 micelle at higher temperature (40 degrees C); in intermediate TX-100 concentration region (9.42-94.85 mM), the skeleton of F127/TX-100 complex transfers from TX-100 micelle successively into F127 micelle and TX-100 micelle again upon heating. The interaction of F127 with TX-100 is saturated in high TX-100 concentration region (> 157.57 mM), and free TX-100 micelles coexist with larger clusters of F127/TX-100 complexes. In addition, TX-100-induced F127/TX-100 complex formation and structure transition are also investigated at constant temperatures. The results show that within 5-10 degrees C, F127 unimers mainly adsorb on the surface of TX-100 micelles just like normal water soluble polymers; in the temperature region of 15-25 degrees C, TX-100 micelles prompts F127 micelle formation. Within 30-40 degrees C, TX-100 inserts into F127 micelles leading to the breakdown of F127 aggregates at higher TX-100 concentrations, and the obtained unimers thread through TX-100 micelles forming complex with TX-100 micelle as skeleton.     

Effect of propylene carbonate on the adsorption and aggregation of surfactants

Surface tension of TX-100 and AOT was measured at 25 °C in water + propylene carbonate (PC) media containing 5, 10, 15, and 18 wt.% PC. Micellization does not take place in neat PC. Critical micelle concentration (cmc) of TX-100 increases with increase in percentage of PC, while that of AOT passes through a minimum around 5% PC. cmc values of AOT in 15% and 18% PC were obtained from the fluorescence emission spectra of pyrene, but not from surface tension data. Counter ion binding constant of AOT has two values in 5% PC as in water, whereas it has single value in mixed solvents containing 10% or more of PC. With increase in surfactant concentration, variation of aggregation numbers of TX-100 and AOT show opposite trends. The recently reported solvophobicity index works in the present system also.     

Solvent effects on AOT reverse micelles in liquid and compressed alkanes investigated by neutron spin-echo spectroscopy

Neutron Spin-Echo (NSE) spectroscopy has been employed to study the interfacial properties of reverse micelles formed with the common surfactant sodium bis-2-ethylhexyl-sulfosuccinate (AOT) in liquid alkane solvents and compressed propane. NSE spectroscopy provides a means to measure small energy transfers for incident neutrons that correspond to thermal fluctuations on the nanosecond time scale and has been applied to the study of colloidal systems. NSE offers the unique ability to perform dynamic measurements of thermally induced shape fluctuation in the AOT surfactant monolayer. This study investigates the effects of the bulk solvent properties, water content, and the addition of octanol cosurfactant on the bending elasticity of AOT reverse micelles and the reverse micelle dynamics. By altering these solvent properties, specific trends in the bending elasticity constant, k, are observed where increasing k corresponds to an increase in micelle rigidity and a decrease in intermicellar exchange rate, k(ex). The observed corresponding trends in k and k(ex) are significant in relating the dynamics of microemulsions and their application as a reaction media. Compressed propane was also examined for the first time with a high-pressure, compressible bulk solvent where variations in temperature and pressure are used to tune the properties of the bulk phase. A decrease in the bending elasticity is observed for the d-propane/AOT/W = 8 reverse micelle system by simultaneously increasing the temperature and pressure, maintaining constant density. With isopycnic conditions, a constant translational diffusion of the reverse micelles through the bulk phase is observed, conforming to the Stokes-Einstein relationship.     

Effect of hydrophobicity inside PEO-PPO-PEO block copolymer micelles on the stabilization of gold nanoparticles: experiments

In this paper we present the effect of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer micelles and their hydrophobicity on the stabilization of gold nanoparticles. Gold nanoparticles were prepared by a method developed by Sakai et al. (Sakai, T.; Alexandridis, P. Langmuir 2004, 20, 8426). An absorption centered at 300-400 nm in time-dependent UV spectra provided evidence that the very first step of the synthesis was to form primary gold clusters. Then the gold clusters grew in size and were stabilized by block copolymer micelles. The stabilization capacities of the micelles were modulated by tuning the block copolymer concentration and composition and by adding salts. With good stabilization, gold particles were spherical and uniform in size with a diameter of 5-10 nm. Otherwise they were aggregates with irregular shapes such as triangular, hexagonal, and rodlike. The presence of a small amount of NaF significantly increased the stabilization capacity of the micelles and consequently modified the quality of the gold particles. Using FTIR and 1H NMR spectroscopy, micellization of the block copolymers and hydrophobicity of the micelles were proven very important for the stabilization. A higher hydrophobicity of the micelle cores was expected to favor the entrapment of primary gold clusters and the stabilization of gold nanoparticles.     

Dissipative particle dynamics simulation of gold nanoparticles stabilization by PEO–PPO–PEO block copolymer micelles

Dissipative particle dynamics (DPD) was used to simulate the formation and stabilization of gold nanoparticles in poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymer micelles. Primary gold clusters that were experimentally observed in the early stage of gold nanoparticle formation were modeled as gold bead in DPD simulation. It showed that gold beads were wrapped by the block copolymer and aggregated into spherical particles inside the micelles and forming stable Pluronic–gold colloids with two-layer structures. Increasing Pluronic concentration, molecular weight, and PPO block length led to the formation of more uniform and more stable gold nanoparticles. Density profiles of water beads suggested that the micelles, especially the hydrophobicity of the micellar cores, played an important role in stabilizing gold nanoparticles. Dynamic process indicated that the formation of gold nanoparticles was controlled by the competition between aggregation of primary gold clusters and the stabilization by micelles of block copolymers.. The DPD simulation results of gold–copolymer–water system agree well with previous experiments, while more structure information on microscopic level could be provided.     

FT-IR AND MICROCALORIMETRY STUDIES ON WATER/TX-100/HEXANOL/OCTANE REVERSE MICROEMULSION SYSTEM

ABSTRACT

Water/TX-100/Hexanol/Octane reverse microemulsions were investigated by Microcalorimetry and FT-IR. Experiments show that the formation of this reverse microemulsion is an exothermic process, and it's a two-step reaction. The first step is action of TX-100 monomers with water to form hydrogen bond while the second is interaction between polyethylene oxide groups in reverse micelles and water. The characterizations of FT-IR and Microcalorimetry indicate that aqueous core of the microemulsion droplets are composed of bound and free water while a small amount of trapped water, OH stretching vibration peak of bound water, free water, and trapped water are at 3400 ±20 cm^?1, 3220 ± 20 cm^?1, and 3550 ± 20 cm^?1 respectively. Polyethylene oxide ether of TX-100 is prior to phenyl ether in acting with water because its polarity is stronger than phenyl ester.     

Spectroluminescent properties of cadmium selenide nanoparticles synthesized in AOT reverse micelles

Cadmium selenide nanoparticles have been synthesized in solutions of AOT/water/n-heptane reverse micelles with average micelle water pool diameters of 25, 30, and 47 Å using cadmium sulfate (CdSO₄) and sodium selenosulfate (Na₂SeSO₃) as precursors. Absorption and fluorescence spectra of the obtained nanoparticles were recorded. The picosecond dynamics of fluorescence decay over the entire range of their emission band have been investigated by time-resolved fluorescence spectroscopy. A procedure for the stabilization of nanoparticles by dodecanethiol was developed for electron microscopy analysis.     

Physicochemical investigation of the solubilization of ytterbium nitrate in AOT reverse micelles and liquid crystals

A wide investigation of the solubilization of the water-soluble salt Yb(NO₃)₃ in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles and AOT liquid crystals has been carried out. After saturation of water/AOT/organic solvent w/o microemulsions with pure Yb(NO₃)₃, the Yb(NO₃)₃/AOT composites were prepared by complete evaporation under vacuum of the volatile components (water and organic solvent) of the salt-containing microemulsions. It was observed that these composites can be totally dissolved in pure n-heptane or CCl₄, allowing the solubilization of a noticeable amount of Yb(NO₃)₃ in quite dry apolar media. By UV–vis–NIR, FT-IR, and ¹H NMR spectroscopies, some information on the state of Yb(NO₃)₃ within AOT reverse micelles were acquired, whereas by small angle X-ray scattering (SAXS), it has been ascertained that Yb(NO₃)₃ is quite homogeneously distributed as very small clusters among the reverse micelles. An analysis of SAXS and wide-angle X-ray scattering spectra of Yb(NO₃)₃/AOT composites leads to the hypothesis that, also in these systems, Yb(NO₃)₃ is dispersed in the surfactant matrix as very small clusters.     

Specific Features of the Preparation of AgI Nanocrystals in Reverse Micelles of Aerosol OT

It was shown that the interrelation between the mean size of AgI nanocrystals prepared in the water pools of reverse micelles as a result of reaction between KI and AgNO₃ and the pool diameter has a complex pattern and is determined mainly by the stability of micellar system. Microemulsions with small micelles (the pool diameter is up to 2 nm) are stable due to the presence of micelle oligomeric phase, regardless of the effect of water structurization in pools. In microemulsions with pool diameters from 2 to 6 nm, such an oligomeric phase is absent that leads to the avalanche-like growth of nanocrystals due to their aggregation in organic phase. As the diameter of micelle pools increases above 6 nm, the microemulsions are stabilized because of the limited dimensions of structured water layer and the deceleration of intermicellar exchange. When the excess amount of KI is added to microemulsion, the screening shell of ions is formed around nanocrystals, thus complicating the formation of icelike structure of water in micelle pools.     

Disruption of reverse micelles and release of trapped ribonuclease A photochemically induced by Malachite Green leuconitrile derivative

Photoinduced disruption of a sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelle is triggered by a Malachite Green leuconitrile derivative (MGL). UV irradiation of MGL solubilized in an AOT-water-chloroform mixture creates a cationic surfactant that interacts electrostatically with the anionic AOT. We investigated the disruption of the reverse micelle by using proton nuclear magnetic resonance spectroscopy and found that UV irradiation of MGL decreases the number of water molecules solubilized in the interior of the AOT reverse micelles. Furthermore, the photoinduced disruption of the reverse micelle is shown to release ribonuclease A, which is trapped in the water in the interior of the AOT reverse micelle. This photoinduced release may offer a desirable transport system of biopolymers.     

Can reverse micelle shells limit nanoparticle growth?

The possibility to limit the growth of nanoparticles synthesized in reverse microemulsions by the shells of reverse micelles is analyzed theoretically. At surface tension higher than about 30 mJ m⁻², the reverse micelle shells cannot inhibit the particle growth. At lower surface tension values, the particle growth can be limited, but the synthesized particles should be much larger than the initial micelles.     

When is water not water? Exploring water confined in large reverse micelles using a highly charged inorganic molecular probe

The interior water pool of aerosol OT (AOT) reverse micelles tends toward bulk water properties as the micelle size increases. Thus, deviations from bulk water behavior in large reverse micelles are less expected than in small reverse micelles. Probing the interior water pool of AOT reverse micelles with a highly charged decavanadate (V(10)) oligomer using (51)V NMR spectroscopy shows distinct changes in solute environment. For example, when an acidic stock solution of protonated V(10) is placed in a reverse micelle, the (51)V chemical shifts show that the V(10) is deprotonated consistent with a decreased proton concentration in the intramicellar water pool. Results indicate that a proton gradient exists inside the reverse micelles, leaving the interior neutral while the interfacial region is acidic.     

Synthesis and Characterization of Bimetallic Copper-Gold Nanoparticles

We have synthesized copper-gold, core-shell nanoparticles by the microemulsion method. The particles were prepared in two steps, by first reducing copper ions and then gold ions in the aqueous domains of anionic microemulsions. Two surfactants have been used as emulsifiers, AOT and Cu(AOT)₂. The latter is the source of copper ions. Gold ions come from aqueous solutions of HAuCl₄. Ultraviolet-visible spectroscopy experiments show that copper nanoparticles are created in the first step of the synthesis, and that a gold layer covers them in the second step. Transmission electron microscopy and related techniques confirm the formation of copper (core)-gold (shell) nanocrystals.     

Electrical conductivity and permittivity of water-AOT-n-heptane microemulsions

Measurements of the electrical conductivity and of the complex permittivity of water-sodium bis(2-ethylhexyl) sulfosuccinate (AOT)-n-heptane microemulsions are reported. The experimental results are rationalized in terms of a hopping mechanism of AOT anions within clusters of reversed micelles. The dependence of the hopping rate and of the cluster dimensions upon the ratio [water]/[AOT] and temperature is discussed.     

Growth kinetics for AgI nanoparticles in AOT reverse micelles: effect of molecular length of hydrocarbon solvents

The growth kinetics for AgI nanoparticles formed in the solutions of water/AOT reverse micelles in n-hexane, n-octane, n-decane, and n-dodecane were investigated. In small micelles, the rate of nanoparticles growth was found to be independent of the type of solvent, while in large micelles the growth rate grew with increasing length of solvent molecules. The effect was explained by a different amount of free water in the micelle pools of the same size.     

Effects of surfactant/water ratio and dye amount on the fluorescent silica nanoparticles

Effects of surfactant/water volume ratios and dye amounts on the properties of micelles and fluorescence silica nanoparticles were studied in microemulsions containing nonionic surfactant Triton X-100, hexanol as co-surfactant, cyclohexane as organic solvent, and metal organic dye (tris(2,2′-bipyridyl)dichlororuthenium) via fluorescence probe technique, TEM, and XPS. Fluorescence probe measurements show that the micelle microenvironment becomes stable at the surfactant/water volume ratio higher than 3.5 and the incubation time longer than 10 h. The data suggest that the silica shell, which is formed at the surfactant/water ratio of 3.5, yields an efficient protection of dye molecules against the e-beam irradiation and result in high photostability of fluorescent silica. We pioneered the localization of dye molecules on the surface of dye-doped silica and found that an increase of dye amounts, beyond a threshold, in the microemulsion cannot enhance the fluorescence intensity of dye-doped nanoparticles. These results are of significant importance for optimizing the synthesis of fluorescent nanoparticles with high photostability and low cost.     

Excited-state proton transfer of 2-(2'-pyridyl)benzimidazole in microemulsions: selective enhancement and slow dynamics in aerosol OT reverse micelles with an aqueous core

Excited-state proton transfer (ESPT) of 2-(2'-pyridyl)benzimidazole (2PBI) in reverse micelles has been studied by steady-state and time-resolved fluorescence spectroscopy. The nanometer sized water pool in the n-heptane/Aerosol OT (AOT)/water microemulsion is found to promote tautomer emission of this probe, as is evident from the emergence of a Stokes shifted band at 450 nm at the expense of the normal emission band on increasing the water content of the system. In the nonaquous microemulsion with a methanol core, the normal emission is quenched but no tautomer emission is obtained. With an acetonitrile core, there is no change in emission properties. Similarly, there is no evidence of ESPT in Triton X-100 reverse micelles. This indicates the requirement of ESPT to occur in microheterogeneous media; the medium should be a ternary system comprised of water and a hydrophobic phase separated by a negatively charged interface. In the microemulsions with an aqueous core, the fluorescence decays of 2PBI at the red end exhibit rise times of 0.8 ns and the time-resolved area-normalized emission spectra (TRANES) exhibit an isoemissive point, indicating slow dynamics of the two-state ESPT of 2PBI in aqueous AOT reverse micelles. The origin of the selective enhancement in AOT microemulsions as well as the slow dynamics is explored using fluorescence spectroscopic techniques, with support from quantum chemical calculation.     

An example of how to use AOT reverse micelle interfaces to control a photoinduced intramolecular charge-transfer process

6-Propionyl-2-(N,N-dimethyl)aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe due to its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer (ICT) states which can be particularly useful as sensors. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES), and time-resolved area normalized emission (TRANES) spectroscopies on PRODAN dissolved in nonaqueous reverse micelles. The reverse micelles are composed of polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane. Sequestered polar solvents included ethylene glycol (EG), propylene glycol (PG), glycerol (GY), formamide (FA), dimethylformamide (DMF), and dimethylacetamide (DMA). The experiments were performed with varying surfactant concentrations at a fixed molar ratio W(S) = [polar solvent]/[AOT]. In every reverse micelle studied, the results show that PRODAN undergoes a partition process between the external solvent and the reverse micelle interface. The partition constants, K(p), are quantified from the changes in the PRODAN emission and/or absorption spectra with the surfactant concentration. The K(p) values depend strongly on the encapsulated polar solvent and correlate quite well with the AOT reverse micelle interface's zones where PRODAN can exist and emits. Thus, the partition toward the reverse micelle interface is strongly favored in DMF and DMA containing micelles where the PRODAN emission comes only from an ICT state. For GY/AOT reverse micelles, the K(p) value is the lowest and only emission from the local excited (LE) state is observed. On the other hand, for EG/AOT, PG/AOT, and water/AOT reverse micelles, the K(p) values are practically the same and emission from both states (LE and ICT) is simultaneously detected. We show here that it is possible to control the PRODAN state emission by simply changing the properties of the AOT reverse micelle interfaces by choosing the appropriate polar solvent to make the reverse micelle media. Indeed, we present experimental evidence with the answer to the long time question about from which state does PRODAN emit, a process that can be controlled using the unique reverse micelle interfaces properties.     

Interaction between hydrogen peroxide and cobalt(II) acetylacetonate in the system of reverse micelles of triton X-100 nonionic surfactant in cyclohexane

The yield of free radicals upon the decomposition of hydrogen peroxide catalyzed by cobalt acetylacetonate (Co(acac)₂) in the systems of reverse micelles of TX-100/n-hexanol and AOT in cyclohexane at 37°C was studied with the inhibitor method using a stable nitroxyl radical as a spin trap. It is shown that, in micellar AOT solutions in cyclohexane as well as in n-decane, H₂O₂ and Co(acac)₂ in practice do not react, because H₂O₂ is localized in a micelle water pool and Co(acac)₂, in the organic phase. Therefore, the generation of radicals is not observed in AOT solutions in cyclohexane, whereas, in aqueous solution, Co(acac)₂ catalyzes the radical decomposition of H₂O₂. In the system of mixed reverse micelles of TX-100 and n-hexanol in cyclohexane, at equal overall concentrations of H₂O₂ and Co(acac)₂, the rate of radical formation is much higher than in aqueous solution; i.e., the micellar catalysis of the radical decomposition of H₂O₂ takes place. It follows from measurements of UV and ESR spectra and the kinetics of changes in the content of peroxides in the reaction mixture that TX-100 and n-hexanol react with free radicals formed upon H₂O₂ decomposition and with atmospheric oxygen.