Nanoconfinement's Dramatic Impact on Proton Exchange between Glucose and Water

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.

Where Are Sodium Ions in AOT Reverse Micelles? Fluoride Anion Probes Nanoconfined Ions by 19F Nuclear Magnetic Resonance Spectroscopy

Confining water to nanosized spaces creates a unique environment that can change water's structural and dynamic properties. When ions are present in these nanoscopic spaces, the limited number of water molecules and short screening length can dramatically affect how ions are distributed compared to the homogeneous distribution assumed in bulk aqueous solution. Here, we demonstrate that the chemical shift observed in ¹⁹F NMR spectroscopy of fluoride anion, F^-, probes the location of sodium ions, Na⁺, confined in reverse micelles prepared from AOT (sodium dioctyl sulfosuccinate) surfactants. Our measurements show that the nanoconfined environment of reverse micelles can lead to extremely high apparent ion concentrations and ionic strength, beyond the limit in bulk aqueous solutions. Most notably, the ¹⁹F NMR chemical shift trends we observe for F^- in the reverse micelles indicate that the AOT sodium counterions remain at or near the interior interface between surfactant and water, thus providing the first experimental support for this hypothesis.

Urea Disrupts the AOT Reverse Micelle Structure at Low Temperatures

Aside from its prominent role in the excretory system, urea is also a known protein denaturant. Here, we characterize urea as it behaves in confined spaces of AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles as a model of tight, confined spaces found at the subcellular level. Dynamic light scattering revealed that low temperatures (275 K) caused the smallest of the reverse micelle sizes, w₀ = 10, to destabilize and dramatically increase in apparent hydrodynamic diameter. We attribute this to urea embedded into the surfactant interface as confirmed by 2D ¹H-NOESY NMR spectroscopy. This increase in size in turn caused the hydrogen exchange between urea and water within the nanosized reverse micelles to increase as measured by 1D EXSY-NMR. A minimal enlarging effect and no increase in hydrogen exchange were observed when aqueous urea was introduced into w₀ = 15 or 20 reverse micelles, suggesting that this effect is unique to particularly small-diameter spaces (∼7 nm).

The Role of Hydrogen Bonding in the Preferential Solvation of 5-Aminoquinoline in Binary Solvent Mixtures

5-Aminoquinoline (5AQ) has been used as a fluorescent probe of preferential solvation (PS) in binary solvent mixtures in which the nonpolar component is diethyl ether and the polar component is protic (methanol) or aprotic (acetonitrile). Hence, the roles of solvent polarity and solute-solvent hydrogen bonding have been delineated. Positive deviations of spectral shifts from a linear dependence on the concentration of the polar component, signifying PS, are markedly more pronounced in case of the protic solvent. Solvation dynamics on a nanosecond time scale mark the formation of the solvation shell around the fluorescent probe. Time-resolved area-normalized emission spectra indicate the occurrence of the continuous solvation of the excited state when the polar component is acetonitrile. In contrast, two distinct states were observed when the polar component was methanol, the second state being the hydrogen bonded one. Translational diffusion is the rate-determining step for formation of the solvation shell. The time constant associated with it has been estimated from rise times observed in fluorescence transients monitored at the red end of the fluorescence spectra and also from the time evolution of the spectral width of time-resolved emission spectra.

Nanoconfinement Raises the Energy Barrier to Hydrogen Atom Exchange between Water and Glucose

In bulk aqueous environments, the exchange of protons between labile hydroxyl groups typically occurs easily and quickly. Nanoconfinement can dramatically change this normally facile process. Through exchange spectroscopy (EXSY) NMR measurements, we observe that nanoconfinement of glucose and water within AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles raises the energy barrier to labile hydrogen exchange, which suggests a disruption of the hydrogen bond network. Near room temperature, we measure barriers high enough to slow the process by as much as 2 orders of magnitude. Although exchange rates slow with decreasing temperatures in these nanoconfined environments, the barrier we measure below ∼285 K is 3-5 times lower than the barrier measured at room temperature, indicating a change in mechanism for the process. These findings suggest the possibility of hydrogen tunneling at a surprisingly high-temperature threshold. Furthermore, differences in exchange rates depend on the hydroxyl group position on the glucose pyranose ring and suggest a net orientation of glucose at the reverse micelle interface.

Chemical Exchange of Hydroxyl Groups in Lipidic Cubic Phases Characterized by NMR

Proton transportation in proximity to the lipid bilayer membrane surface, where chemical exchange represents a primary pathway, is of significant interest in many applications including cellular energy turnover underlying ATP synthesis, transmembrane mobility, and transport. Lipidic inverse bicontinuous cubic phases (LCPs) are unique membrane structures formed via the spontaneous self-assembly of certain lipids in an aqueous environment. They feature two networks of water channels, separated by a single lipid bilayer which approximates the geometry of a triply periodic minimal surface. When composed of monoolein, the LCP bilayer features two glycerol hydroxyl groups at the lipid-water interface which undergo exchange with water. Depending on the conditions of the aqueous solution used in the formation of LCPs, both resonances of the glycerol hydroxyl groups may be observed by solution ¹H NMR. In this study, PFG-NMR and 1D EXSY were employed to gain insight into chemical exchange between the monoolein hydroxyl groups and water in LCPs. Results including the relative population of hydroxyl protons in exchange with water for a number of LCPs at different hydration levels and the exchange rate constants at 35 wt % hydration are reported. Several technical aspects of PFG-NMR and EXSY-NMR for the characterization of chemical exchange in LCPs are discussed, including an alternative way to analyze PFG-NMR data of exchange systems which overcomes the inherent low sensitivity at high diffusion encoding.

Speeding up biphasic reactions with surface nanodroplets

Biphasic chemical reactions compartmentalized in small droplets offer advantages, such as streamlined procedures for chemical analysis, enhanced chemical reaction efficiency and high specificity of conversion. In this work, we experimentally and theoretically investigate the rate for biphasic chemical reactions between acidic nanodroplets on a substrate surface and basic reactants in a surrounding bulk flow. The reaction rate is measured by droplet shrinkage as the product is removed from the droplets by the flow. In our experiments, we determine the dependence of the reaction rate on the flow rate and the solution concentration. The theoretical analysis predicts that the life time τ of the droplets scales with Peclet number Pe and the reactant concentration in the bulk flow c_re,bulk as τ∝ Pe^-3/2c_re,bulk^-1, in good agreement with our experimental results. Furthermore, we found that the product from the reaction on an upstream surface can postpone the droplet reaction on a downstream surface, possibly due to the adsorption of interface-active products on the droplets in the downstream. The time of the delay decreases with increasing Pe of the flow and also with increasing reactant concentration in the flow, following the scaling same as that of the reaction rate with these two parameters. Our findings provide insight for the ultimate aim to enhance droplet reactions under flow conditions.

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.

Toward comprehensive measurement of protein hydration dynamics: Facilitation of NMR-based methods by reverse micelle encapsulation

Protein-water interactions are a fundamental determinant of protein structure and function. Despite their importance, the molecular details of water orientations and dynamics near protein surfaces remain poorly understood, largely due to the difficulty of measuring local water mobility near the protein in a site-resolved fashion. Solution NMR-based measurement of water mobility via the nuclear Overhauser effect was presented as a method for performing comprehensive, site-resolved measurements of water dynamics many years ago. Though this approach yielded extensive insight on the dynamics and locations of waters buried within proteins, its promise for measuring surface hydration dynamics was impeded by various technical barriers. Over the past several years, however, this approach has been pursued anew with the aid of reverse micelle encapsulation of proteins of interest. The confined environment of the reverse micelle resolves many of these barriers and permits site-resolved measurement of relative water dynamics across much of the protein surface. Here, the development of this strategy for measuring hydration dynamics is reviewed with particular focus on the important remaining challenges to its widespread application.