Characterization of Cetyltrimethylammonium Bromide/Hexanol Reverse Micelles by Experimentally Benchmarked Molecular Dynamics Simulations

Characterization of 10MAG/LDAO reverse micelles: Understanding versatility for protein encapsulation

Reverse micelles (RMs) are spontaneously organizing nanobubbles composed of an organic solvent, surfactants, and an aqueous phase that can encapsulate biological macromolecules for various biophysical studies. Unlike other RM systems, the 1-decanoyl-rac-glycerol (10MAG) and lauryldimethylamine-N-oxide (LDAO) surfactant system has proven to house proteins with higher stability than other RM mixtures with little sensitivity to the water loading (W0, defined by the ratio of water to surfactant). We investigated this unique property by encapsulating three model proteins - cytochrome c, myoglobin, and flavodoxin - in 10MAG/LDAO RMs and applying a variety of experimental methods to characterize this system's behavior. We found that this surfactant system differs greatly from the traditional, spherical, monodisperse RM population model. 10MAG/LDAO RMs were discovered to be oblate ellipsoids at all conditions, and as W0 was increased, surfactants redistributed to form a greater number of increasingly spherical ellipsoidal particles with pools of more bulk-like water. Proteins distinctively influence the thermodynamics of the mixture, encapsulating at their optimal RM size and driving protein-free RM sizes to scale accordingly. These findings inform the future development of similarly malleable encapsulation systems and build a foundation for application of 10MAG/LDAO RMs to analyze biological and chemical processes under nanoscale confinement.

Olive Oil-Based Reverse Microemulsion for Stability and Topical Delivery of Methotrexate: In Vitro

Hydrolysis of pharmaceutically active molecules can be in control under a confined environment of water-in-oil microemulsion. Stability of model drug methotrexate (MTX) in a sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and olive oil microemulsion system has been evaluated. The physicochemical properties of AOT-MTX-water-olive oil reverse microemulsion (MTX-RM) were examined by UV-vis, Fourier transform infrared, and X-ray diffraction techniques, and the hydrodynamic size was determined by dynamic light scattering techniques and morphologies were characterized by a transmission electron microscope and atomic force microscope. In vitro permeation of MTX-RM through treated skin and its mechanism are evaluated by a UV-visible spectrophotometer, confocal laser scanning microscope, differential scanning calorimeter, and attenuated total reflecting infrared spectroscopy (ATR). The interaction of MTX with the AOT headgroup in confined environment RM enhanced the stability of MTX without affecting the molecular integrity at room temperature. Chemical stability of MTX in MTX-RM (W0 = 5) is significantly higher (∼97%) at room temperature for the study period of 1 year than in MTX-RM (W0 = 15) (∼72%). Interaction of MTX with the AOT headgroup is also visualized by a high-resolution transmission electron microscope and is in correlation with FT-IR data of MTX-RM. The skin fluxes of MTX are 15.1, 19.75, and 22.75 times higher at water content (W0) of 5, 10, and 15, respectively, in MTX-RM in comparison to aqueous solution of MTX. The enhanced amounts of the MTX were detected using CLSM in hair follicles, sweat glands, and epidermis layer of the skin. Merging of T2, T3, and T4 thermal peaks in one broad peak in treated skin endothermograph shows that carrier MTX-RM affects the lipid as well protein structure of the treated skin. ATR data of treated skin showed an increase in the intensity of the carbonyl peak at 1750 cm-1 (lipid), shifting of the amide II peaks, and separation of peaks in the range of 1060 to 1000 cm-1 (vibration mode of -CH2OH, C-O stretching, and C-OH bending peak of the carbohydrate) in comparison to control skin, which indicates that MTX-RM interacts with glycolipid and glycoprotein through carbohydrate hydroxy groups.

Membrane-Mimicking Reverse Micelles for High-Resolution Interfacial Study of Proteins and Membranes

Despite substantial advances, the study of proteins interacting with membranes remains a significant challenge. While integral membrane proteins have been a major focus of recent efforts, peripheral membrane proteins (PMPs) and their interactions with membranes and lipids have far less high-resolution information available. Their small size and the dynamic nature of their interactions have stalled detailed interfacial study using structural methods like cryo-EM and X-ray crystallography. A major roadblock for the structural analysis of PMP interactions is limitations in membrane models to study the membrane recruited state. Commonly used membrane mimics such as liposomes, bicelles, nanodiscs, and micelles are either very large or composed of non-biological detergents, limiting their utility for the NMR study of PMPs. While there have been previous successes with integral and peripheral membrane proteins, currently employed reverse micelle (RM) compositions are optimized for their inertness with proteins rather than their ability to mimic membranes. Applying more native, membrane-like lipids and surfactants promises to be a valuable advancement for the study of interfacial interactions between proteins and membranes. Here, we describe the development of phosphocholine-based RM systems that mimic biological membranes and are compatible with high-resolution protein NMR. We demonstrate new formulations that are able to encapsulate the model soluble protein, ubiquitin, with minimal perturbations of the protein structure. Furthermore, one formula, DLPC:DPC, allowed the encapsulation of the PMPs glutathione peroxidase 4 (GPx4) and phosphatidylethanolamine-binding protein 1 (PEBP1) and enabled the embedment of these proteins, matching the expected interactions with biological membranes. Dynamic light scattering and small-angle X-ray scattering characterization of the RMs reveals small, approximately spherical, and non-aggregated particles, a prerequisite for protein NMR and other avenues of study. The formulations presented here represent a new tool for the study of elusive PMP interactions and other membrane interfacial investigations.

Hydration of bromide at reverse micelle interfaces studied by X-ray absorption fine structure

Nanoconfined water exhibits various interesting properties, which are not only of fundamental importance but also of practical use. Because reverse micelles (RMs) provide versatile ways to prepare nanoconfined water, the understanding of their physicochemical properties is essential for developing efficient applications. Although the water properties in the RMs could be affected by its interaction with the RM interface, the details have not been well understood. This study focuses on the local structures of Br^- in hexadecyltrimethylammonium bromide (HTAB) RMs formed in chloroform and 10% hexanol/heptane. The dependence in Br^- hydration on the molar ratio of water to HTAB (w) is investigated using X-ray absorption fine structure (XAFS). These systems cover a wide range of w values (0-30) and allow us to study the impact of this parameter on the local structure of Br^- at the RM interface, which comprises water, surfactant headgroups, and organic solvent components. The presence of multiple scattering paths complicates the XAFS spectra and makes it difficult to analyze them using standard fitting methods. The linear combination of the spectra corresponding to the individual scattering paths captures the molecular processes that occur at the RM interface upon increasing w. The maximum hydration number of Br^- is found to be 4.5 at w > 15, suggesting that although most of the ions remain at the interface as partly hydrated ions, some of them dissociate as completely hydrated ones.

Takes Two to Tango: Choreography of the Coadsorption of CTAB and Hexanol at the Oil-Water Interface

Mixed surfactant systems at the oil-water interface play a vital role in applications ranging widely from drug delivery to oil-spill remediation. Synergistic mixtures are superior emulsifiers and more effective at modifying surface tension than either component alone. Mixtures of surfactants with dissimilar polar head groups are of particular interest because of the additional degree of control they offer. The interplay of hydrophobic and electrostatic effects in these systems is not well understood, in part because of the difficulty in examining their behavior at the buried oil-water interface where they reside. Here, surface-specific vibrational sum frequency spectroscopy is utilized in combination with surface tensiometry and computational methods to probe the cooperative molecular interactions between a cationic surfactant cetyltrimethylammonium bromide (CTAB) and a nonionic alcohol (1-hexanol) that induce the two initially reluctant surfactants to coadsorb synergistically at the interface. A careful deuteration study of CTAB reveals that hexanol cooperates with CTAB such that both molecules preferentially orient at the interface for sufficiently large enough concentrations of hexanol. This work's methodology is unique and serves as a guide for future explorations of macroscopic properties in these complex systems. Results from this work also provide valuable insights into how interfacial ordering impacts surface tensiometry measurements for nonionic surfactants.

Site-Resolved and Quantitative Characterization of Very Weak Protein-Ligand Interactions

Very weak interactions between small organic molecules and proteins have long been predicted and are expected to have dissociation constants of hundreds of millimolar and above. Unfortunately, quantitative evaluation of binding in a high-resolution structural context for this affinity regime is particularly difficult and often impossible using existing experimental approaches. Here, we show that nanoscale encapsulation of single protein molecules within the water core of reverse micelles enables the detection and evaluation of weak binding interactions at atomic resolution using solution NMR spectroscopy. This strategy is used to survey the interactions of a set of small molecules with the cytokine interleukin-1β (IL-1β). The interaction of IL-1β with these molecules is found to vary from more diffuse and weak binding modes to more specific and with a relatively higher affinity. The interactions detected here cover a large portion of the protein surface and have dissociation constants mostly in the low molar range. These results illustrate the ability of a protein to interact productively with a variety of small molecule functional groups and point to a broader potential to target even relatively featureless protein surfaces for applications in chemical biology and drug discovery.

Water loading driven size, shape, and composition of cetyltrimethylammonium/hexanol/pentane reverse micelles

Cetyltrimethylammonium bromide (CTAB)/hexanol reverse micelles have found a variety of applications that demand control over physical parameters. Water content or loading is among the most basic tunable components and is the major driver of the physical properties of these systems. This study uses small-angle scattering with contrast variation to characterize these systems as a function of water loading. The scattering data were analyzed with a variety of approaches, resulting in converging physical specifications. Equations that describe basic physical parameters were determined that allow for characterization and manipulation of the CTAB/hexanol reverse micelle surfactant system. The shape of the reverse micelles was revealed to be slightly ellipsoidal and varies slightly through the water loading range. The surfactant shell is shown to contain a higher fraction of hexanol upon addition of water. Analysis reveals that the size, shape, and surfactant/cosurfactant composition are directly tunable by variation of the water content and that these properties are consequences of the balance of forces present in the reverse micelles.

Reverse Micelle Encapsulation of Proteins for NMR Spectroscopy

Reverse micelle (RM) encapsulation of proteins for NMR spectroscopy has many advantages over standard NMR methods such as enhanced tumbling and improved sensitivity. It has opened many otherwise difficult lines of investigation including the study of membrane-associated proteins, large soluble proteins, unstable protein states, and the study of protein surface hydration dynamics. Recent technological developments have extended the ability of RM encapsulation with high structural fidelity for nearly all proteins and thereby allow high-quality state-of-the-art NMR spectroscopy. Optimal conditions are achieved using a streamlined screening protocol, which is described here. Commonly studied proteins spanning a range of molecular weights are used as examples. Very low-viscosity alkane solvents, such as propane or ethane, are useful for studying very large proteins but require the use of specialized equipment to permit preparation and maintenance of well-behaved solutions under elevated pressure. The procedures for the preparation and use of solutions of RMs in liquefied ethane and propane are described. The focus of this chapter is to provide procedures to optimally encapsulate proteins in reverse micelles for modern NMR applications.

Characterizing Protein Hydration Dynamics Using Solution NMR Spectroscopy

Protein hydration is a critical aspect of protein stability, folding, and function and yet remains difficult to characterize experimentally. Solution NMR offers a route to a site-resolved view of the dynamics of protein-water interactions through the nuclear Overhauser effects between hydration water and the protein in the laboratory (NOE) and rotating (ROE) frames of reference. However, several artifacts and limitations including contaminating contributions from bulk water potentially plague this general approach and the corruption of measured NOEs and ROEs by hydrogen exchange-relayed magnetization. Fortunately, encapsulation of single protein molecules within the water core of a reverse micelle overcomes these limitations. The main advantages are the suppression hydrogen exchange and elimination of bulk water. Here we detail guidelines for the preparation solutions of encapsulated proteins that are suitable for characterization by NOE and ROE spectroscopy. Emphasis is placed on understanding the contribution of detected NOE intensity arising from magnetization relayed by hydrogen exchange. Various aspects of fitting obtained NOE, selectively decoupled NOE, and ROE time courses are illustrated.

Local chemistry of the surfactant's head groups determines protein stability in reverse micelles

Protein stability in reverse micelles is determined by local chemical interactions between the surfactant molecules and the protein groups.

Aggregation response of triglyceride hydrolysis products in cyclohexane and triolein

Aggregation mechanism and the existence of cmc depend on apolar solvent quality and surfactant head group polarity.

Photoswitching of Azobenzene-Based Reverse Micelles above and at Subzero Temperatures As Studied by NMR and Molecular Dynamics Simulations

We designed and studied the structure, dynamics, and photochemistry of photoswitchable reverse micelles (RMs) composed of azobenzene-containing ammonium amphiphile 1 and water in chloroform at room and subzero temperatures by NMR spectroscopy and molecular dynamics simulations. The NMR and diffusion coefficient analyses showed that micelles containing either the E or Z configuration of 1 are stable at room temperature. Depending on the water-to-surfactant molar ratio, the size of the RMs remains unchanged or is slightly reduced because of the partial loss of water from the micellar cores upon extensive E → Z or Z → E photoisomerization of the azobenzene group in 1. Upon freezing at 253 or 233 K, E-1 RMs partially precipitate from the solution but are redissolved upon warming whereas Z-1 RMs remain fully dissolved at all temperatures. Light-induced isomerization of 1 at low temperatures does not lead to the disintegration of RMs remaining in the solution; however, its scope is influenced by a precipitation process. To obtain a deeper molecular view of RMs, their structure was characterized by MD simulations. It is shown that RMs allow for amphiphile isomerization without causing any immediate significant structural changes in the micelles.

The Size of AOT Reverse Micelles

Reverse micelles (RMs) made from water and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) are commonly studied experimentally as models of aqueous microenvironments. They are small enough for individual RMs to also be studied by molecular dynamics (MD) simulation, which yields detailed insight into their structure and properties. Although RM size is determined by the water loading ratio (i.e., the molar ratio of water to AOT), experimental measurements of RM size are imprecise and inconsistent, which is problematic when seeking to understand the relationship between water loading ratio and RM size, and when designing models for study by MD simulation. Therefore, a systematic study of RM size was performed by MD simulation with the aims of determining the size of an RM for a given water loading ratio, and of reconciling the results with experimental measurements. Results for a water loading ratio of 7.5 indicate that the interaction energy between AOT anions and other system components is at a minimum when there are 62 AOT anions in each RM. The minimum is due to a combination of attractive and repulsive electrostatic interactions that vary with RM size and the dielectric effect of available water. Overall, the results agree with a detailed analysis of previously published experimental data over a wide range of water loading ratios, and help reconcile seemingly discrepant experimental results. In addition, water loss and gain from an RM is observed and the mechanism of water exchange is outlined. This kind of RM model, which faithfully reproduces experimental results, is essential for reliable insights into the properties of RM-encapsulated materials.

Effect of Cosurfactants on the Interfacial Hydration of CTAB Quaternary Reverse Micelle Probed Using Excited State Proton Transfer

It has been proven previously that the negatively charged photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) resides at the interface of the cationic reverse micelle (RM) cetyltrimethylammonium bromide (CTAB)/octanol/water/cyclohexane and is a potential reporter of hydration through the excited state proton transfer (ESPT) process. However, the ESPT dynamics monitored by the pump-probe study was limited to the ultrafast timescale and hence did not report any discernible ESPT signature. Herein, we reinvestigate the ESPT behavior using fluorescence spectroscopy in the nanosecond timescale and at different values of w₀ (=[water]/[surfactant]). We clearly observed distinct w₀-dependent ESPT signatures analogous to conventional ternary cationic RMs implying considerable interfacial hydration. The results agree with a recent molecular simulation study, where significant penetration of water molecules into the interface was predicted for the CTAB quaternary RM. Moreover, we also found that the ESPT dynamics and the fluorescence anisotropy decay of HPTS depend differentially on the octanol/CTAB ratio (p₀). The ESPT process was found to be disfavored, whereas the anisotropy decay accelerates upon the increase in p₀ values. Our analysis indicates that with the increase in the octanol concentration, dehydrated regions enrich gradually at the interface. However, the increase in octanol concentration may reduce the effective electrostatic potential experienced by the probe and thus may result in faster rotational relaxation.