Aerosol OT Reverse Micelles Research Articles

How to Characterize Amorphous Shapes: The Tale of a Reverse Micelle.

Aerosol-OT reverse micelles represent a chemical construct where surfactant molecules self-assemble to stabilize water nanodroplets 1-10 nm in diameter. Although commonly assumed to adopt a spherical shape, all-atom molecular dynamics simulations and some experimental studies predict a nonspherical shape. If these aggregates are not spherical, then what shape do they take? Because the tools needed to evaluate the shape of something that lacks regular structure, order, or symmetry are not well developed, we present a set of three intuitive metrics─coordinate-pair eccentricity, convexity, and the curvature distribution─that estimate the shape of an amorphous object, and we demonstrate their use on a simulated aerosol-OT reverse micelle. These metrics are all well-established methods and principles in mathematics, and each provides unique information about the shape. Together, these metrics provide intuitive descriptions of amorphous shapes, facilitate ways to quantify those shapes, and follow their changes over time.

Influence of nanoenvironment in reverse micelles on the fluorescence of flavins

Riboflavin (RF) and flavin adenine dinucleotide (FAD) are two chromophores in ubiquitous blue-light photoreceptors where molecular motions are partially constrained. However, most previous studies on their fluorescence were performed in solutions. In this work, we studied their fluorescence in Aerosol-OT (AOT) and cetyltrimethylammonium bromide (CTAB) reverse micelles (RMs) by using steady-state UV–visible spectroscopy and femtosecond transient absorption spectroscopy. Compared with in aqueous solution, fluorescence of RF in AOT RMs remains nearly constant, but it is quenched in CTAB RMs induced by intersystem crossing promoted by bromide anion. In addition, fluorescence of FAD in AOT RMs is strengthened because the stacked conformation is less favorable in AOT RMs than in aqueous solution. Moreover, the fluorescence quenching of FAD in CTAB RMs by intersystem crossing mainly affects the unstacked FAD because the intramolecular charge transfer in stacked FAD is faster than intersystem crossing by more than 40 folds. These results determine the dependence of fluorescence of flavins on the ions, polarity and steric constraints in nanoenvironment.

Acid–base reaction of a cationic hydration probe in vicinity of anionic interface of AOT reverse micelles

The excited-state proton transfer (ESPT) of a cationic super-photoacid, N-methyl-7-hydroxyquinolium, has been studied within the water pool of an aerosol-OT reverse micelle (RM). Time-resolved emission spectra were obtained and analyzed to reveal the effect of environmental heterogeneity on the ESPT process. The ESPT was found to involve two main emissive species, namely the excited cationic and keto forms of the photoacid and was facilitated by scenarios that expose them to an abundance of water molecules. The dynamic charge-state transition (from positive to neutral) induced by the ESPT dictated the location and diffusion of the probe with the modulation of the Coulombic interaction between the probe and anionic interface of the RM. The non-exponential behavior of the ESPT was observed using time-resolved intensity profiles and rationalized in terms of the heterogeneity along the reaction coordinates involving the hydrogen-bond network, polarity, and viscosity of the confined water pool within the RM. It was found that the disruption of the hydrogen-bond network in the vicinity of the interface resulted in the decrease of polarity that retarded the overall ESPT by modulating the energetics of the reactant and the product in the ESPT. Solvation dynamics was also considered and found to occur on timescales faster than the ESPT process, profoundly for larger water pools.

The Interfacial Interactions of Glycine and Short Glycine Peptides in Model Membrane Systems.

The interactions of amino acids and peptides at model membrane interfaces have considerable implications for biological functions, with the ability to act as chemical messengers, hormones, neurotransmitters, and even as antibiotics and anticancer agents. In this study, glycine and the short glycine peptides diglycine, triglycine, and tetraglycine are studied with regards to their interactions at the model membrane interface of Aerosol-OT (AOT) reverse micelles via 1H NMR spectroscopy, dynamic light scattering (DLS), and Langmuir trough measurements. It was found that with the exception of monomeric glycine, the peptides prefer to associate between the interface and bulk water pool of the reverse micelle. Monomeric glycine, however, resides with the N-terminus in the ordered interstitial water (stern layer) and the C-terminus located in the bulk water pool of the reverse micelle.

Residence and diffusion of a dynamically prototropic hydration probe in AOT reverse micelles

We investigated the hydration dynamics in the nanosized water pool of an Aerosol-OT (AOT) reverse micelle (RM) using a prototropic (proton-donating) fluorescent dye, N-methyl-7-hydroxyquinolinium (NM7HQ+), as the probe. NM7HQ+ is a photoacid, and the deprotonation of its excited-state cationic form (C*) gives rise to a neutral keto species (K*). In the present study, the residence sites of C* and K* in water pools of various sizes were determined by monitoring their time-resolved fluorescence anisotropy. Additionally, the evolutions of the C* and K* bands in the time-resolved fluorescence spectra were analyzed by fitting the spectra to the sum of two lognormal peak functions. The time-dependent spectral shifts of the species with different formal charges revealed the characteristic hydration behavior at the interface of the anionic AOT headgroups and the bound water region, as well as the rotational and translational diffusion of the probe within the RM.

Bulk-like and Interfacial Water Dynamics in Nafion Fuel Cell Membranes Investigated with Ultrafast Nonlinear IR Spectroscopy.

The water confined in the hydrophilic domains of Nafion fuel cell membranes is central to its primary function of ion transport. Water dynamics are intimately linked to proton transfer and are sensitive to the structural features and length scales of confinement. Here, ultrafast polarization-selective pump-probe and two-dimensional infrared vibrational echo (2D IR) experiments were performed on fully hydrated Nafion membranes with sodium counterions to explicate the water dynamics. Like aerosol-OT reverse micelles (AOT RMs), the water dynamics in Nafion are attributed to bulk-like core water in the central region of the hydrophilic domains and much slower interfacial water. Population and orientational dynamics of water in Nafion are slowed by polymer confinement. Comparison of the observed dynamics to those of AOT RMs helps identify local interactions between water and sulfonate anions at the interface and among water molecules in the core. This comparison also demonstrates that the well-known spherical cluster morphology of Nafion is not appropriate. Spectral diffusion of the interfacial water, which arises from structural dynamics, was obtained from the 2D IR experiments taking the core water to have dynamics similar to bulk water. Like the orientational dynamics, spectral diffusion was found to be much slower at the interface compared to bulk water. Together, the dynamics indicate slow reorganization of weakly hydrogen-bonded water molecules at the interface of Nafion. These results provide insights into proton transport mechanisms in fuel cell membranes, and more generally, water dynamics near the interface of confining systems.

Investigating Substrate Analogues for Mycobacterial MenJ: Truncated and Partially Saturated Menaquinones.

Menaquinones (MKs) are essential for electron transport in prokaryotes, and importantly, partially saturated MKs represent a novel virulence factor. However, little is known regarding how the degree of saturation in the isoprenyl side chain influences conformation or quinone redox potential. MenJ is an enzyme that selectively reduces the second isoprene unit on MK-9 and is contextually essential for the survival of Mycobacterium tuberculosis in J774A.1 macrophage-like cells, suggesting that MenJ may be a conditional drug target for pathogenic mycobacteria. Therefore, fundamental information about the properties of this system is important, and we synthesized the simplest MKs, unsaturated MK-1 and the saturated analogue, MK-1(H2). Using two-dimensional nuclear magnetic resonance spectroscopy, we established that MK-1 and MK-1(H2) adopted similar folded-extended conformations (i.e., the isoprenyl side chain folds upward) in each solvent examined but the folded-extended conformations differed slightly between organic solvents. Saturation of the isoprenyl side chain slightly altered the MK-1 analogue conformation in each solvent. We used molecular mechanics to illustrate the MK-1 analogue conformations. The measured quinone redox potentials of MK-1 and MK-1(H2) differed between organic solvents (presumably due to differences in dielectric constants), and remarkably, an ∼20 mV semiquinone redox potential difference was observed between MK-1 and MK-1(H2) in pyridine, acetonitrile, and dimethyl sulfoxide, demonstrating that the degree of saturation in the isoprenyl side chain of MK-1 influences the quinone redox potential. Finally, MK-1 and MK-1(H2) interacted with Langmuir phospholipid monolayers and Aerosol-OT reverse micelle (RM) model membrane interfaces, where MK-1 adopted a slightly different folded conformation within the RM model membrane interface.

Structure Dependence of Pyridine and Benzene Derivatives on Interactions with Model Membranes.

Pyridine-based small-molecule drugs, vitamins, and cofactors are vital for many cellular processes, but little is known about their interactions with membrane interfaces. These specific membrane interactions of these small molecules or ions can assist in diffusion across membranes or reach a membrane-bound target. This study explores how minor differences in small molecules (isoniazid, benzhydrazide, isonicotinamide, nicotinamide, picolinamide, and benzamide) can affect their interactions with model membranes. Langmuir monolayer studies of dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylethanolamine (DPPE), in the presence of the molecules listed, show that isoniazid and isonicotinamide affect the DPPE monolayer at lower concentrations than the DPPC monolayer, demonstrating a preference for one phospholipid over the other. The Langmuir monolayer studies also suggest that nitrogen content and stereochemistry of the small molecule can affect the phospholipid monolayers differently. To determine the molecular interactions of the simple N-containing aromatic pyridines with a membrane-like interface, 1H one-dimensional NMR and 1H-1H two-dimensional NMR techniques were utilized to obtain information about the position and orientation of the molecules of interest within aerosol-OT (AOT) reverse micelles. These studies show that all six of the molecules reside near the AOT sulfonate headgroups and ester linkages in similar positions, but nicotinamide and picolinamide tilt at the water-AOT interface to varying degrees. Combined, these studies demonstrate that small structural changes of small N-containing molecules can affect their specific interactions with membrane-like interfaces and specificity toward different membrane components.

6,7-dimethoxy-coumarin as a probe of hydration dynamics in biologically relevant systems

Coumarin derivatives are well known fluorescence reporters for investigating biological systems due to their strong micro-environment sensitivity. Despite having wide range of environment sensitive fluorescence probes, the potential of 6,7-dimethoxy-coumarin has not been studied extensively so far. With a perspective of its use in protein studies, namely using the unnatural amino acid technology or as a substrate for hydrolase enzymes, we study acetyloxymethyl-6,7-dimethoxycoumarin (Ac-DMC). We investigate the photophysics and hydration dynamics of this dye in aerosol-OT (AOT) reverse micelles at various water contents using the time dependent fluorescence shift (TDFS) method. The TDFS response in AOT reverse micelles from water/surfactant ratio of 0 to 20 confirms its sensitivity towards the hydration and mobility of its microenvironment. Moreover, we show that the fluorophore can be efficiently quenched by halide ions. Hence, we conclude that the 6,7-dimethoxy-methylcoumarin fluorophore is useful for studying hydration parameters in biologically relevant systems.

How Does Nanoconfinement within a Reverse Micelle Influence the Interaction of Phenazinium-Based Photosensitizers with DNA?

The major focus of the present work lies in exploring the influence of nanoconfinement within aerosol-OT (AOT) reverse micelles on the binding interaction of two phenazinium-based photosensitizers, namely, phenosafranin (PSF) and safranin-O (SO), with the DNA duplex. Circular dichroism and dynamic light-scattering studies reveal the condensation of DNA within the reverse micellar interior (transformation of the B-form of native DNA to ψ-form). Our results unveil a remarkable effect of the degree of hydration of the reverse micellar core on the stability of the stacking interaction (intercalation) of the drugs (PSF and SO) into DNA; increasing size of the water nanopool (that is, w0) accompanies decreasing curvature of the DNA duplex structure with the consequent effect of increasing stabilization of the drug:DNA intercalation. The marked differences in the dynamical aspects of the interaction scenario following encapsulation within the reverse micellar core and the subsequent dependence on the size of the water nanopool are also meticulously explored. The differential degrees of steric interactions offered by the drug molecules (presence of methyl substitutions on the planar phenazinium ring in SO) are also found to affect the extent of intercalation of the drugs to DNA. In this context, it is imperative to state that the water pool of the reverse micellar core is often argued to approach bulk-like properties of water with increasing micellar size (typically w0 ≥ 10), so that deviation from the bulk water properties is likely to be minimized in large reverse micelles (w0 ≥ 10). On the contrary, our results (particularly quantitative elucidation of micropolarity and dynamical aspects of the interaction) explicitly demonstrate that the bulk-like behavior of the nanoconfined water is not truly achieved even in large reverse micelles.

A Synthetic Isoprenoid Lipoquinone, Menaquinone-2, Adopts a Folded Conformation in Solution and at a Model Membrane Interface.

Menaquinones (naphthoquinones, MK) are isoprenoids that play key roles in the respiratory electron transport system of some prokaryotes by shuttling electrons between membrane-bound protein complexes acting as electron acceptors and donors. Menaquinone-2 (MK-2), a truncated MK, was synthesized, and the studies presented herein characterize the conformational and chemical properties of the hydrophobic MK-2 molecule. Using 2D NMR spectroscopy, we established for the first time that MK-2 has a folded conformation defined by the isoprenyl side-chain folding back over the napthoquinone in a U-shape, which depends on the specific environmental conditions found in different solvents. We used molecular mechanics to illustrate conformations found by the NMR experiments. The measured redox potentials of MK-2 differed in three organic solvents, where MK-2 was most easily reduced in DMSO, which may suggest a combination of solvent effect (presumably in part because of differences in dielectric constants) and/or conformational differences of MK-2 in different organic solvents. Furthermore, MK-2 was found to associate with the interface of model membranes represented by Langmuir phospholipid monolayers and Aerosol-OT (AOT) reverse micelles. MK-2 adopts a slightly different U-shaped conformation within reverse micelles compared to within solution, which is in sharp contrast to the extended conformations illustrated in literature for MKs.

Organized assemblies of AOT determine nanoparticle characteristics and their performance as FRET donors

CdS nanoparticles (NPs) have been synthesized in three organized assemblies formed by the surfactant Aerosol-OT (AOT). Although NP synthesis in AOT reverse micelles is well documented, but AOT regular micelles and gels have not been commonly used for NP synthesis in the past. The NP shape, size and particularly crystallinity are found to bear direct correlation to the type of AOT assembly. These CdS NPs were found to be photoluminescent and participate in very effective fluorescence resonance energy transfer (FRET) with a dye, Ethidium bromide (EB). The efficiency of FRET between CdS NPs and EB is found to depend largely on the location of dye in the surfactant assembly and more significantly on the NP characteristics. This is the first report of preparation of CdS NPs in AOT regular micelles and gels and the first report of FRET between CdS NPs and dye in AOT based organized assemblies.

Dynamics in a Water Interfacial Boundary Layer Investigated with IR Polarization-Selective Pump-Probe Experiments.

The dynamics of water molecules near the surfactant interface in large Aerosol-OT reverse micelles (RMs) (w0 = 16-25) was investigated with IR polarization-selective pump-probe experiments using the SeCN- anion as a vibrational probe. Linear absorption spectra of RMs (w0 = 25-2) can be decomposed into the weighted sum of the SeCN- spectra in bulk water and the spectrum of the SeCN- anion interacting with the interfacial sulfonate head groups (w0 = 1). The spectra of the large RMs, w0 ≥ 16, are overwhelmingly dominated by the bulk water component. Anisotropy decays (orientational relaxation) of the anion for w0 ≥ 16 displayed bulk water relaxation (1.4 and 4.5 ps) plus an additional slow decay with a time constant of ∼13 ps. The amplitude of the slow decay was too large to be associated with SeCN- in contact with the interface on the basis of the linear spectrum decomposition. The results indicate that the observed slow components arise from SeCN- in a water boundary layer, in which water molecules are perturbed by the interface but are not directly associated with it. This layer is the transition between water in direct contact with the interface and bulk water in the large RM cores. In the boundary layer, the water dynamics is slow compared to that in bulk water.

Formose reaction accelerated in aerosol-OT reverse micelles.

The formose reaction in reverse micelles of aerosol-OT (AOT), triton X-100 (TX), and hexadecyltrimethylammonium bromide (CTAB) was investigated. Time–conversion data have indicated that the interfacial water layer of AOT reverse micelles is a medium that accelerates formation of glycolaldehyde in the formose reaction. The 13C NMR spectra for the products of the formose reaction using formaldehyde-13C as starting material are indicative of the formation of ethylene glycol as a major product.

Photophysics and Rotational Dynamics of a Fluoroquinolone Drug Norfloxacin in Biomimetic Reverse Micellar Nanocavities

The present work describes the study of the photophysics and dynamics of a prospective anti-cancer antibacterial drug norfloxacin (NFX) within the micro-heterogeneous environment of Aerosol OT (AOT) reverse micelle (RM). The environment sensitive fluorescence spectroscopic behavior of NFX is found to exhibit a significant red-shift with increasing water pool size (w 0 = [water]/[surfactant]) of the RM. A prominent decrease of fluorescence anisotropy of NFX with increasing w 0 demonstrates a relaxation of the motional restrictions imposed on the bound drug molecules with enhanced hydration. These results are further substantiated from our time-resolved measurements. Study of the rotational dynamics of NFX within the RM as a function of w 0 exhibits a typical biexponential decay behavior which has been adequately described on the basis of the wobbling-in-cone model. The rotational dynamical parameters are also calculated for wobbling motion of the drug within the RM as a function of w 0 .

Copper nanoparticles in AOT “revisited”-direct micelles versus reverse micelles

For the first time, direct micelles of the surfactant Aerosol-OT (AOT) have been used to synthesize copper nanoparticles. The synthesized nanoparticles are spherical and highly crystalline with diameter 5–10 nm. They show the prominent surface plasmon resonance typical of copper nanoparticles. AOT plays a dual role-initially it provides a site for initiation of reduction and subsequently it acts as a stabilizer for controlled nucleation. For comparison, copper nanoparticles were also synthesized in the well known AOT reverse micelles. Small sized nanoparticles (diameter < 4 nm) were formed. Unlike previous works on copper nanoparticle synthesis in reverse micelles using substituted AOT i.e Cu(AOT)2, in this work unsubstituted Na-AOT was used. This minimizes the adverse effects on stability and shape which arise due to surfactant functionalization. Finally, the most important finding of the present work is that AOT direct micelles are better substrates than reverse micelles for synthesis of stable, spherical and crystalline copper nanoparticles with distinct surface plasmon resonance spectra.

Kinetic Evidence for Reactive Dimeric TAML Iron Species in the Catalytic Oxidation of NADH and a Dye by O2 in AOT Reverse Micelles

The efficacy of the iron(III) TAML activator [Fe{C6H4-1,2-(N1COCMe2N2CO)2CMe2(Fe–N1)(Fe–N2)}(OH2)]− (1) for the catalytic activation of dioxygen in reverse micelles of Aerosol OT (AOT) in n-octane has been studied. Kinetic evidence is presented for the existence of unusual second-order catalytic pathways in the oxidation of NADH to NAD+ and the bleaching of blue pinacyanol chloride (PNC) dye. Depending on the substrate and reaction conditions, a second-order pathway in [1] either dominates or proceeds in obvious combination with a first-order pathway in [1]. Detailed kinetic analysis of the experimental data supports the previously made hypothesis that the reactive intermediate is associated with the mixed-valent dimer system [LFeIIIOFeIVL]3–/[LFeIII(OH)FeIVL]2–.

Photophysics and Rotational Dynamics of a Fluoroquinolone Drug Norfloxacin in Biomimetic Reverse Micellar Nanocavities

The present work describes the study of the photophysics and dynamics of a prospective anti-cancer antibacterial drug norfloxacin (NFX) within the micro-heterogeneous environment of Aerosol OT (AOT) reverse micelle (RM). The environment sensitive fluorescence spectroscopic behavior of NFX is found to exhibit a significant red-shift with increasing water pool size (w 0 = [water]/[surfactant]) of the RM. A prominent decrease of fluorescence anisotropy of NFX with increasing w 0 demonstrates a relaxation of the motional restrictions imposed on the bound drug molecules with enhanced hydration. These results are further substantiated from our time-resolved measurements. Study of the rotational dynamics of NFX within the RM as a function of w 0 exhibits a typical biexponential decay behavior which has been adequately described on the basis of the wobbling-in-cone model. The rotational dynamical parameters are also calculated for wobbling motion of the drug within the RM as a function of w 0 .

Activation of Dioxygen by a TAML Activator in Reverse Micelles: Characterization of an Fe(III)Fe(IV) Dimer and Associated Catalytic Chemistry.

Iron TAML activators of peroxides are functional catalase-peroxidase mimics. Switching from hydrogen peroxide (H2O2) to dioxygen (O2) as the primary oxidant was achieved by using a system of reverse micelles of Aerosol OT (AOT) in n-octane. Hydrophilic TAML activators are localized in the aqueous microreactors of reverse micelles where water is present in much lower abundance than in bulk water. n-Octane serves as a proximate reservoir supplying O2 to result in partial oxidation of Fe(III) to Fe(IV)-containing species, mostly the Fe(III)Fe(IV) (major) and Fe(IV)Fe(IV) (minor) dimers which coexist with the Fe(III) TAML monomeric species. The speciation depends on the pH and the degree of hydration w0, viz., the amount of water in the reverse micelles. The previously unknown Fe(III)Fe(IV) dimer has been characterized by UV-vis, EPR, and Mössbauer spectroscopies. Reactive electron donors such as NADH, pinacyanol chloride, and hydroquinone undergo the TAML-catalyzed oxidation by O2. The oxidation of NADH, studied in most detail, is much faster at the lowest degree of hydration w0 (in "drier micelles") and is accelerated by light through NADH photochemistry. Dyes that are more resistant to oxidation than pinacyanol chloride (Orange II, Safranine O) are not oxidized in the reverse micellar media. Despite the limitation of low reactivity, the new systems highlight an encouraging step in replacing TAML peroxidase-like chemistry with more attractive dioxygen-activation chemistry.

An AOT reverse micelle in a medium of supercritical carbon dioxide

Reverse micelles of Aerosol OT in the medium of supercritical carbon dioxide have been simulated by the molecular dynamics method. For the first time, a micelle in ternary water-Aerosol OT-supercritical CO2 has been simulated by describing the surfactant and the supercritical liquid in terms of the coarse-grained approach. It has been shown that the structural and energetic characteristics of the micelle are, with a good accuracy, described within the framework of simplified models.