Correlating Proton Transfer Dynamics To Probe Location in Confined Environments

Ligandability at the Membrane Interface of GPx4 Revealed through a Reverse Micelle Fragment Screening Platform

While they account for a large portion of drug targets, membrane proteins present a unique challenge for drug discovery. Peripheral membrane proteins (PMPs), a class of water-soluble proteins that bind to membranes, are also difficult targets, particularly those that function only when bound to membranes. The protein-membrane interface in PMPs is often where functional interactions and catalysis occur, making it a logical target for inhibition. However, protein-membrane interfaces are underexplored spaces in inhibitor design, and there is a need for enhanced methods for small-molecule ligand discovery. In an effort to better initiate drug discovery efforts for PMPs, this study presents a screening methodology using membrane-mimicking reverse micelles (mmRM) and NMR-based fragment screening to assess ligandability at the protein-membrane interface. The proof-of-principle target, glutathione peroxidase 4 (GPx4), is a lipid hydroperoxidase that is essential for the oxidative protection of membranes and thereby the prevention of ferroptosis. GPx4 inhibition is promising for therapy-resistant cancer therapy, but current inhibitors are generally covalent ligands with limited clinical utility. Presented here is the discovery of noncovalent small-molecule ligands for membrane-bound GPx4 revealed through the mmRM fragment screening methodology. The fragments were tested against GPx4 under bulk aqueous conditions and displayed little to no binding to the protein without embedment into the membrane. The 9 hits had varying affinities and partitioning coefficients and revealed properties of fragments that bind within the protein-membrane interface. Additionally, a secondary screen confirmed the potential to progress the fragments by enhancing the affinity from >200 to ∼15 μM with the addition of certain hydrophobic groups. This study presents an advancement of screening capabilities for membrane-associated proteins, reveals ligandability within the GPx4 protein-membrane interface, and may serve as a starting point for developing noncovalent inhibitors of GPx4.

Ligandability at the membrane interface of GPx4 revealed through a reverse micelle fragment screening platform

While they account for a large portion of drug targets, membrane proteins (MPs) present a unique challenge for drug discovery. Peripheral membrane proteins (PMPs), a class of proteins that bind reversibly to membranes, are also difficult targets, particularly those that function only while bound to membranes. The protein-membrane interface in PMPs is often where functional interactions and catalysis occur, making it a logical target for inhibition. However, interfaces are underexplored spaces in inhibitor design and there is a need for enhanced methods for small-molecule ligand discovery. In an effort to better initiate drug discovery efforts for PMPs, this study presents a screening methodology using membrane-mimicking reverse micelles (mmRM) and NMR-based fragment screening to assess ligandability in the protein-membrane interface. The proof-of-principle target, glutathione peroxidase 4 (GPx4), is a lipid hydroperoxidase which is essential for the oxidative protection of membranes and thereby the prevention of ferroptosis. GPx4 inhibition is promising for therapy-resistant cancer therapy, but current inhibitors are generally covalent ligands with limited clinical utility. Presented here is the discovery of non-covalent small-molecule ligands for membrane-bound GPx4 revealed through the mmRM fragment screening methodology. The fragments were tested against GPx4 in bulk aqueous conditions and displayed little to no binding to the protein without embedment into the membrane. The 9 hits had varying affinities and partitioning coefficients and revealed properties of fragments that bind within the protein-membrane interface. Additionally, a secondary screen confirmed the potential to progress the fragments by enhancing the affinity from > 200 μM to ~15 μM with the addition of certain hydrophobic groups. This study presents an advancement of screening capabilities for membrane associated proteins, reveals ligandability within the GPx4 protein-membrane interface, and may serve as a starting point for developing non-covalent inhibitors of GPx4.

Anomalous proton transfer of a photoacid HPTS in nonaqueous reverse micelles

The proton transfer reaction is one of the fundamental chemical reactions where the reaction dynamics strongly depend on solvent properties such as acidity or basicity. A photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) shows a sharp decrease of pKa (7.7 → 0.5) upon photoexcitation, and the excited-state proton transfer (ESPT) occurs with ultrafast time constants of 2.5 and 89 ps in bulk aqueous solution. However, the two-step proton transfers via the contact ion pair formation and the proton diffusion are strongly limited inside the nanopools of reverse micelles (RMs). The confinement in small RMs strongly impeded the proton transfer reactions. In this work, we report the ESPT of HPTS confined in methanol-in-oil RMs by steady-state and time-resolved electronic spectroscopy. Interestingly, HPTS shows substantial deprotonation in the excited state only in small RMs, while the ESPT of HPTS does not occur in bulk methanol solution due to the low basicity of aliphatic alcohols. The kinetic analysis of time-resolved fluorescence and transient absorption measurements will compare the proton transfer dynamics of HPTS in the water-in-oil and methanol-in-oil RMs. The ESPT of photoacids, especially in the nonaqueous RMs, can be crucial in understanding many important chemical reactions involving proton transfer in the confined environments of cells and membranes.

Proton coupled isomerization in double-hydrogen-bonding-center salicylaldehyde azine

The proton transfer dynamics of Schiff bases are of great importance due to the application potential. The excited state intramolecular proton transfer (ESIPT) of photochromic salicylaldehyde azine (SAA) in solutions was investigated by experimental and computational methods. Two distinguished spectral bands are observed in transient absorption spectra upon the photoexcitation with 400 nm. From the delayed stimulated emission signals, the ESIPT process is determined to be within <200 fs. Subsequently, the photoprotonated products undergo the vibrational relaxation with several picoseconds and the following isomerization with tens of picoseconds. These processes are significantly affected by the polarity of the solvents. The longest component with nanosecond scale can be explained to the relaxation to the enol structure. According to our observations, the SAA molecules with symmetric double-hydrogen-bonding-centers undergo single proton transfer rather than double proton transfer and subsequent intramolecular isomerization.

Highly Altered State of Proton Transport in Acid Pools in Charged Reverse Micelles

Transport mechanisms of solvated protons of 1 M HCl acid pools, confined within reverse micelles (RMs) containing the negatively charged surfactant sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) or the positively charged cetyltrimethylammonium bromide (CTABr), are analyzed with reactive force field simulations to interpret dynamical signatures from TeraHertz absorption and dielectric relaxation spectroscopy. We find that the forward proton hopping events for NaAOT are further suppressed compared to a nonionic RM, while the Grotthuss mechanism ceases altogether for CTABr. We attribute the sluggish proton dynamics for both charged RMs as due to headgroup and counterion charges that expel hydronium and chloride ions from the interface and into the bulk interior, thereby increasing the pH of the acid pools relative to the nonionic RM. For charged NaAOT and CTABr RMs, the localization of hydronium near a counterion or conjugate base reduces the Eigen and Zundel configurations that enable forward hopping. Thus, localized oscillatory hopping dominates, an effect that is most extreme for CTABr in which the proton residence time increases dramatically such that even oscillatory hopping is slow.

The Dual Use of the Pyranine (HPTS) Fluorescent Probe: A Ground-State pH Indicator and an Excited-State Proton Transfer Probe

Molecular fluorescent probes are an essential experimental tool in many fields, ranging from biology to chemistry and materials science, to study the localization and other environmental properties surrounding the fluorescent probe. Thousands of different molecular fluorescent probes can be grouped into different families according to their photophysical properties. This Account focuses on a unique class of fluorescent probes that distinguishes itself from all other probes. This class is termed photoacids, which are molecules exhibiting a change in their acid-base transition between the ground and excited states, resulting in a large change in their pKa values between these two states, which is thermodynamically described using the Förster cycle. While there are many different photoacids, we focus only on pyranine, which is the most used photoacid, with pKa values of ∼7.4 and ∼0.4 for its ground and excited states, respectively. Such a difference between the pKa values is the basis for the dual use of the pyranine fluorescent probe. Furthermore, the protonated and deprotonated states of pyranine absorb and emit at different wavelengths, making it easy to focus on a specific state. Pyranine has been used for decades as a fluorescent pH indicator for physiological pH values, which is based on its acid-base equilibrium in the ground state. While the unique excited-state proton transfer (ESPT) properties of photoacids have been explored for more than a half-century, it is only recently that photoacids and especially pyranine have been used as fluorescent probes for the local environment of the probe, especially the hydration layer surrounding it and related proton diffusion properties. Such use of photoacids is based on their capability for ESPT from the photoacid to a nearby proton acceptor, which is usually, but not necessarily, water. In this Account, we detail the photophysical properties of pyranine, distinguishing between the processes in the ground state and the ones in the excited state. We further review the different utilization of pyranine for probing different properties of the environment. Our main perspective is on the emerging use of the ESPT process for deciphering the hydration layer around the probe and other parameters related to proton diffusion taking place while the molecule is in the excited state, focusing primarily on bio-related materials. Special attention is given to how to perform the experiments and, most importantly, how to interpret their results. We also briefly discuss the breadth of possibilities in making pyranine derivatives and the use of pyranine for controlling dynamic reactions.

Ultrafast Excited-State Proton Transfer of a Cationic Superphotoacid in a Nanoscopic Water Pool

The excited-state proton transfer (ESPT) of a cationic superphotoacid, N-methyl-7-hydroxyquinolium, was studied within the water pool of an anionic aerosol-OT (AOT), bis(2-ethylhexyl) sulfosuccinate, reverse micelle (RM). Previously, we had found that the cationic photoacid residing at the anionic AOT interface was conducive to ESPT to the bound water having concentric heterogeneity on the time scale of hundreds of picoseconds to nanoseconds. In our present study, on the time scale of hundreds of femtoseconds to a few tens of picoseconds, the photoacid underwent an ultrafast ESPT influenced by mobile water constituting the core of the RM. The two subpopulations of the core water molecules that determine the ultrafast biphasic deprotonation of the photoacid on time scales differing by an order of magnitude were identified. The core water molecules solvating the counteranion of the photoacid showed a higher basicity than typical water clusters in bulk resulting in ESPT on a subpicosecond time scale. Bare water clusters sensed by the photoacid showed a slower ESPT, over several picoseconds, as typically limited by the rotational motion of water molecules for similar types of the photoacid.

Proton Transfer from a Photoacid to a Water Wire: First Principles Simulations and Fast Fluorescence Spectroscopy

Proton transfer reactions are ubiquitous in chemistry, especially in aqueous solutions. We investigate photoinduced proton transfer between the photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) and water using fast fluorescence spectroscopy and ab initio molecular dynamics simulations. Photoexcitation causes rapid proton release from the HPTS hydroxyl. Previous experiments on HPTS/water described the progress from photoexcitation to proton diffusion using kinetic equations with two time constants. The shortest time constant has been interpreted as protonated and photoexcited HPTS evolving into an "associated" state, where the proton is "shared" between the HPTS hydroxyl and an originally hydrogen bonded water. The longer time constant has been interpreted as indicating evolution to a "solvent separated" state where the shared proton undergoes long distance diffusion. In this work, we refine the previous experimental results using very pure HPTS. We then use excited state ab initio molecular dynamics to elucidate the detailed molecular mechanism of aqueous excited state proton transfer in HPTS. We find that the initial excitation results in rapid rearrangement of water, forming a strong hydrogen bonded network (a "water wire") around HPTS. HPTS then deprotonates in ≤3 ps, resulting in a proton that migrates back and forth along the wire before localizing on a single water molecule. We find a near linear relationship between the emission wavelength and proton-HPTS distance over the simulated time scale, suggesting that the emission wavelength can be used as a ruler for the proton distance. Our simulations reveal that the "associated" state corresponds to a water wire with a mobile proton and that the diffusion of the proton away from this water wire (to a generalized "solvent-separated" state) corresponds to the longest experimental time constant.

Structure and dynamics of nanoconfined water and aqueous solutions

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

Altered relaxation dynamics of excited state reactions by confinement in reverse micelles probed by ultrafast fluorescence up-conversion

Chemical reactions in confined environments are important in areas as diverse as heterogenous catalysis, environmental chemistry and biochemistry, yet they are much less well understood than the equivalent reactions in either the gas phase or in free solution. The understanding of chemical reactions in solution was greatly enhanced by real time studies of model reactions, through ultrafast spectroscopy (especially when supported by molecular dynamics simulation). Here we review some of the efforts that have been made to adapt this approach to the investigation of reactions in confined media. Specifically, we review the application of ultrafast fluorescence spectroscopy to measure reaction dynamics in the nanoconfined water phase of reverse micelles, as a function of the droplet radius and the charge on the interface. Methods of measurement and modelling of the reactions are outlined. In all of the cases studied (which are focused on ultrafast intramolecular reactions) the effect of confinement was to suppress the reaction. Even in the largest micelles the result in the bulk aqueous phase was not usually recovered, suggesting an important role for specific interactions between reactant and environment, for example at the interface. There was no simple one-to-one correspondence with direct measures of the dynamics of the confined phase. Thus, understanding the effect of confinement on reaction rate appears to require not only knowledge of the dynamics of the reaction in solutions and the effect of confinement on the medium, but also of the interaction between reactant and confining medium.

Confinement-Controlled Aqueous Chemistry within Nanometric Slit Pores

In this Focus Review, we put the spotlight on very recent insights into the fascinating world of wet chemistry in the realm offered by nanoconfinement of water in mechanically rather rigid and chemically inert planar slit pores wherein only monolayer and bilayer water lamellae can be hosted. We review the effect of confinement on different aspects such as hydrogen bonding, ion diffusion, and charge defect migration of H⁺(aq) and OH^-(aq) in nanoconfined water depending on slit pore width. A particular focus is put on the strongly modulated local dielectric properties as quantified in terms of anisotropic polarization fluctuations across such extremely confined water films and their putative effects on chemical reactions therein. The stunning findings disclosed only recently extend wet chemistry in particular and solvation science in general toward extreme molecular confinement conditions.

Proton Transfer Dynamics in the Aprotic Proton Accepting Solvent 1-Methylimidazole

The dynamics of proton transfer to the aprotic solvent 1-methylimidazole (MeIm, proton acceptor) from the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) was investigated using fast fluorescence measurements. The closely related molecule, 8-methoxypyrene-1,3,6-trisulfonic acid trisodium salt (MPTS), which is not a photoacid, was also studied for comparison. Following optical excitation, the wavelength-dependent population dynamics of HPTS in MeIm resulting from the deprotonation process were collected over the entire fluorescence emission window. Analysis of the time-dependent fluorescence spectra revealed four distinct fluorescence bands that appear and decay on different time scales. We label these four states as protonated (P), associated I (A_I), associated II (A_II), and deprotonated (D). We find that the simple kinetic scheme of P → A_I → A_II → D is not consistent with the data. Instead, the kinetic scheme that describes the data has P decaying into A_I, which mainly goes on to deprotonation (D), but A_I can also feed into A_II. A_II can return to A_I or decay to the ground state, but does not deprotonate within experimental error. Quantum chemistry and excited state QM/MM Born-Oppenheimer molecular dynamics simulations indicate that A_I and A_II are two H-bonding conformations of MeIm to the HPTS hydroxyl, axial, and equatorial, respectively.

Slow Proton Transfer in Nanoconfined Water

The transport of protons in nanoconfined environments, such as in nanochannels of biological or artificial proton conductive membranes, is essential to chemistry, biology, and nanotechnology. In water, proton diffusion occurs by hopping of protons between water molecules. This process involves the rearrangement of many hydrogen bonds and as such can be strongly affected by nanoconfinement. We study the vibrational and structural dynamics of hydrated protons in water nanodroplets stabilized by a cationic surfactant using polarization-resolved femtosecond infrared transient absorption spectroscopy. We determine the time scale of proton hopping in the center of the water nanodroplets from the dynamics of the anisotropy of the transient absorption signals. We find that in small nanodroplets with a diameter <4 nm, proton hopping is more than 10 times slower than in bulk water. Even in relatively large nanodroplets with a diameter of ∼7 nm, we find that the rate of proton hopping is slowed by ∼4 times compared with bulk water.

Effect of Photoacid Strength on Fluorescence Modulation of 2-Naphthol Derivatives inside β-Cyclodextrin Cavity: Insights from Fluorescence, Isothermal Calorimetry, and Molecular Dynamics Simulations

Fluorescence response of a photoacid inside a confined environment often differs markedly from the bulk response. Is there any correlation between the extent of fluorescence modulation and the strength of the photoacid? Here, we used three photoacids: 2-naphthol (2OH, pK_a* = 3.3), 6-sulfonate-2-naphthol (6SO₃-2OH, pK_a* = 3.06), and 6-cyano-2-naphthol (6CN-2OH, pK_a* = 0.6) with remarkably different excited-state acidities to investigate fluorescence modulation inside the nanocavity of β-cyclodextrin (β-CD). Interestingly, we found strong fluorescence modulation for 2OH and 6SO₃-2OH but almost none for 6CN-2OH. Isothermal calorimetry measurements showed that all three fluorophores form 1:1 inclusion complex with comparable binding constants (285, 420, and 580 M^-1 for 2OH, 6SO₃-2OH, and 6CN-2OH, respectively). Molecular dynamics simulation further revealed that binding modes are quite similar, and the distribution of water molecules around the proton-donating hydroxyl group of the photoacids are also comparable. Consequently, the difference in the fluorescence response should be accounted solely to the difference in the photoacidity strengths.

Supramolecularly Assembled Nanocomposites as Biomimetic Chloroplasts for Enhancement of Photophosphorylation

Prototypes of natural biosystems provide opportunities for artificial biomimetic systems to break the limits of natural reactions and achieve output control. However, mimicking unique natural structures and ingenious functions remains a challenge. Now, multiple biochemical reactions were integrated into artificially designed compartments via molecular assembly. First, multicompartmental silica nanoparticles with hierarchical structures that mimic the chloroplasts were obtained by a templated synthesis. Then, photoacid generators and ATPase-liposomes were assembled inside and outside of silica compartments, respectively. Upon light illumination, protons produced by a photoacid generator in the confined space can drive the liposome-embedded enzyme ATPase towards ATP synthesis, which mimics the photophosphorylation process in vitro. The method enables fabrication of bioinspired nanoreactors for photobiocatalysis and provides insight for understanding sophisticated biochemical reactions.

Modulation of the Excited-State Proton Transfer Rate of d-luciferin in Mixed Reverse Micellar Systems

The excited-state proton transfer (ESPT) rate of photo-acids in a confined medium depends on several physical parameters of the immediate environment. We introduce a new parameter in the form of charge type at the interface of reverse micellar (RM) systems to modulate the ESPT rate. We investigate the ESPT reaction of d-luciferin in mixed RM systems composed of nonionic polyoxyethylene(5)nonylphenylether (Igepal CO-520) with cationic didodecyldimethylammonium bromide (DDAB) and anionic sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in cyclohexane (Cy) at different mole fractions of Ig (X _Ig) and fixed hydration. ESPT is feeble in AOT RM, whereas it is favorable in the other two RMs. Addition of Ig is observed to facilitate ESPT in AOT RM linearly, whereas in DDAB, it shows a synergistic effect. The various physical parameters of water in the mixed RM water pool have been investigated using dynamic light scattering, Fourier transform infrared, and time-resolved fluorescence spectroscopy measurements to underline the ESPT mechanism in these mixed RMs.

Confinement Effects on Chemical Equilibria: Pentacyano(Pyrazine)Ferrate(II) Stability Changes within Nanosized Droplets of Water

Nanoscale confinement is known to impact properties of molecules and we observed changes in the reactivity of an iron coordination complex, pentacyano(pyrazine)ferrate(II). The confinement of two coordination complexes in a sodium AOT/isooctane reverse micellar (RM) water droplet was found to dramatically increase the hydrolysis rate of [Fe(CN)₅pyz]3- and change the monomer-dimer equilibria between [Fe(CN)₅pyz]3- and [Fe₂(CN)10pyz]6-. Combined UV-Vis and ¹H-NMR spectra of these complexes in RMs were analyzed and the position of the monomer-dimer equilibrium and the relative reaction times were determined at three different RM sizes. The data show that the hydrolysis rates (loss of pyrazine) are dramatically enhanced in RMs over bulk water and increase as the size of the RM decreases. Likewise, the monomer-dimer equilibrium changes to favor the formation of dimer as the RM size decreases. We conclude that the effects of the [Fe(CN)₅pyz]3- stability is related to its solvation within the RM.

Excited-State Proton Transfer on the Surface of a Therapeutic Protein, Protamine

Proton transfer reactions on biosurfaces play an important role in a myriad of biological processes. Herein, the excited-state proton transfer reaction of 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) has been investigated in the presence of an important therapeutic protein, Protamine (PrS), using ground-state absorption, steady-state, and detailed time-resolved emission measurements. HPTS forms a 1:1 complex with Protamine with a high association constant of 2.6 × 10⁴ M^-1. The binding of HPTS with Protamine leads to a significant modulation in the ground-state prototropic equilibrium causing a downward shift of 1.1 unit in the acidity constant (pK_a). In contrast to a large number of reports of slow proton transfer of HPTS on biosurfaces, interestingly, HPTS registers a faster proton transfer event in the presence of Protamine as compared to that of even the bulk aqueous buffer medium. Furthermore, the dimensionality of the proton diffusion process is also significantly reduced on the surface of Protamine that is in contrast to the behavior of HPTS in the bulk aqueous buffer medium, where the proton diffusion process is three-dimensional. The effect of ionic strength on the binding of HPTS toward PrS suggests a predominant role of electrostatic interaction between anionic HPTS and cationic Protamine, which is further supported by molecular docking simulations which predict that the most preferable binding site for HPTS on the surface of Protamine is surrounded by multiple cationic arginine residues.

Proton Transfer in Perfluorosulfonic Acid Fuel Cell Membranes with Differing Pendant Chains and Equivalent Weights

Proton transfer in the nanoscopic water channels of polyelectrolyte fuel cell membranes was studied using a photoacid, 8-hydroxypyrene-1,3,6-trisulfonic acid sodium salt (HPTS), in the channels. The local environment of the probe was determined using 8-methoxypyrene-1,3,6-trisulfonic acid sodium salt (MPTS), which is not a photoacid. Three fully hydrated membranes, Nafion (DuPont) and two 3M membranes, were studied to determine the impact of different pendant chains and equivalent weights on proton transfer. Fluorescence anisotropy and excited state population decay data that characterize the local environment of the fluorescent probes and proton transfer dynamics were measured. The MPTS lifetime and anisotropy results show that most of the fluorescent probes have a bulk-like water environment with a relatively small fraction interacting with the channel wall. Measurements of the HPTS protonated and deprotonated fluorescent bands' population decays provided information on the proton transport dynamics. The decay of the protonated band from ∼0.5 ns to tens of nanoseconds is in part determined by dissociation and recombination with the HPTS, providing information on the ability of protons to move in the channels. The dissociation and recombination is manifested as a power law component in the protonated band fluorescence decay. The results show that equivalent weight differences between two 3M membranes resulted in a small difference in proton transfer. However, differences in pendant chain structure did significantly influence the proton transfer ability, with the 3M membranes displaying more facile transfer than Nafion.

Chemistry in nanoconfined water

Nanoconfined liquids have extremely different properties from the bulk, which profoundly affects chemical reactions taking place in nanosolvation.

Nanoconfined liquids have extremely different properties from the bulk, which profoundly affects chemical reactions taking place in nanosolvation. Here, we present extensive ab initio simulations of a vast set of chemical reactions within a water lamella that is nanoconfined by mineral surfaces, which might be relevant to prebiotic peptide formation in aqueous environments. Our results disclose a rich interplay of distinct effects, from steric factors typical of reactions occurring in small spaces to a charge-stabilization effect in nanoconfined water at extreme conditions similar to that observed in bulk water when changing from extreme to ambient conditions. These effects are found to modify significantly not only the energetics but also the mechanisms of reactions happening in nanoconfined water in comparison to the corresponding bulk regime.

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.

Vibrational Relaxation of the Aqueous Proton in Acetonitrile: Ultrafast Cluster Cooling and Vibrational Predissociation

We study the ultrafast O-H stretch vibrational relaxation dynamics of protonated water clusters embedded in a matrix of deuterated acetonitrile, using polarization-resolved mid-IR femtosecond spectroscopy. The clusters are produced by mixing triflic (trifluoromethanesulfonic) acid and H2O in molar ratios of 1:1, 1:2, and 1:3, thus varying the degree of hydration of the proton. At all hydration levels the excited O-H stretch vibration of the hydrated proton shows an ultrafast vibrational relaxation with a time constant T1 < 100 fs, leading to an ultrafast local heating of the protonated water cluster. This excess thermal energy, initially highly localized to the region of the excited proton, first re-distributes over the aqueous cluster and then dissipates into the surrounding acetonitrile matrix. For clusters with a triflic acid to H2O ratio of 1:3 these processes occur with time constants of 320 ± 20 fs and 1.4 ± 0.1 ps, respectively. The cooling of the clusters reveals a long-living, underlying transient absorption change with high anisotropy. We argue that this feature stems from the vibrational predissociation of a small fraction of the proton hydration structures, directly following the ultrafast infrared excitation.

How Does Interfacial Hydration Alter during Rod to Sphere Transition in DDAB/Water/Cyclohexane Reverse Micelles? Insights from Excited State Proton Transfer and Fluorescence Anisotropy

How does microscopic organization of an organized assembly alter during macroscopic structural transition? The question may be important to ascertain driving forces responsible for such transitions. Didodecyldimethylammonium bromide (DDAB)/water/cyclohexane reverse micelle is an attractive assembly that undergoes structural transition from rod to spherical shape when the amount of water loading, w0 ([water]/[surfactant]), exceeds a particular value (w0 ∼ 8). Here, we intend to investigate the effect of the morphological change upon interfacial hydration using steady-state and time-resolved fluorescence measurements. The anionic fluorophore 8-hydroxypyrene-1,3,6-trisulfonate (HPTS or pyranine) is expected to be trapped within the positively charged RM interface. The fluorophore can undergo excited-state proton transfer (ESPT) in the presence of water and, thus, is able to provide insight on the level of hydration within the interface. The ESPT process is markedly inhibited within the interface at low w0 and gradually favored with increase of w0. The time-resolved fluorescence decays could be best analyzed by assuming distribution of HPTS over two distinct interfacial regions- partly hydrated and mostly dehydrated. The relative population of the two regions varies distinctly at low w0 (<6) and high w0 (>6) regimes. Moreover, fluorescence anisotropy (steady-state and time-resolved) varies differently with respect to w0, before and after the transition point (w0 ∼ 8).

Wet Interface of Benzylhexadecyldimethylammonium Chloride Reverse Micelle Revealed by Excited State Proton Transfer of a Localized Probe

Excited state proton transfer (ESPT) of an anionic photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS or pyranine) has been studied inside a cationic reverse micelle (RM), water/benzylhexadecyldimethylammonium chloride (BHDC)/benzene, using steady-state and time-resolved fluorescence spectroscopy. The observed ESPT behavior is found to be remarkably different from the known ESPT trend of HPTS inside anionic AOT and cationic CTAB RMs; the ESPT dynamics approaches that of bulk water at higher w0 (≥10) inside AOT RM while no ESPT was observed for CTAB reverse micelle [ Sedgwick J. Am. Chem. Soc. 2012 , 134 , 11904 - 11907 ]. The ESPT dynamics inside BHDC RM is remarkably slower compared to that of water at all w0 (= [water]/[surfactant]) values and relatively much less sensitive to w0 variation compared to AOT RM. 2D NOESY and fluorescence anisotropy measurements reveal that the probe (HPTS) is embedded inside the positive interface of BHDC RM. Despite its trapped location, HPTS is able to undergo ESPT due to significant penetration of water molecules into the interface. Furthermore, facile ESPT at higher w0 is consistent with higher degree of interface hydration as predicted by a recent MD simulation [ Agazzi Langmuir 2014 , 30 , 9643 - 9653 ]. The study shows that ESPT dynamics inside RM varies not only with the interface charge but also on the nature of the headgroup and solvation.

Insights into the Partitioning Behavior of Secondary Surfactants in a Microemulsion-Based Synthesis of Metal Nanoparticles: A DLS and 2D NMR Spectroscopic Investigation

Diffusion-ordered NMR spectroscopy (DOSY) and nuclear Overhauser effect spectroscopy (NOESY) have been used to explore the diffusion and partitioning behavior of secondary surfactants added to suspensions of reverse micelles (RMs) containing either silver or gold nanoparticles (NPs), with an aim of advancing our understanding of the mechanism of metal NP extraction and/or surface functionalization with specific capping agents as performed during a microemulsion-based synthesis. We have coupled these NMR techniques with corresponding dynamic light scattering (DLS) measurements of RMs, with and without encapsulated metal NPs, upon addition of secondary surfactants. Using oleylamine (OAm), oleic acid (OA), dodecylamine (DDAm), and dodecanethiol (DDT), we show that all four secondary surfactants can rapidly diffuse into/out of the RM environment with their head groups in close proximity to the RM interior and encapsulated water molecules; however, surfactant molecules containing a terminal -NH2 or -COOH group undergo a persistent association with the molecules of the RMs, thus solubilizing and partially sequestering a portion of the total concentration of these secondary agents within the RM interface for a lengthened period of time (in relation to the time frame of the DOSY experiments) and slowing their rate of exchange with freely diffusing molecules in the bulk solvent. The extraction of Ag or Au NPs from RMs into organic phase was determined to be critically dependent on the type and concentration of secondary surfactant added to the system, with DDT proving to be most efficient for the extraction of Ag NPs, while OA was shown to be most efficient for Au NPs. Consideration of the results obtained from this particular combination of techniques has provided new knowledge with respect to dynamic metal NP-containing microemulsion systems.

The strikingly different miscibility of n-octanol in highly-confined and quasi-confined water

n-Octanol, a water-immiscible alcohol, is found to be significantly miscible with highly-confined water, but immiscible with quasi-confined water.

Interaction of a biguanide compound with membrane model interface systems: probing the properties of antimalaria and antidiabetic compounds

Since membrane penetration is important for drug efficacy, how antimalarial precursor material 1-phenylbiguanide (PBG) interacts with an interface was characterized using a reverse micelle (RM) model system. (1)H NMR studies show that PBG partitions across the membrane interface. Specifically, the (1)H NMR studies showed that the 1-phenylbiguanide compound in an aqueous environment changed when placed near an interface. PBG is known to affect hydrogen bonding in water, and as the size of the RMs changes, the water organization in the water pool is changed. The NOESY spectrum of PBG in AOT RM contains cross-peak signals between the PBG protons and AOT protons, which is consistent with the penetration of the PBG into the interface. At the same time, there is a cross peak between the biguanide moiety and the HOD signal. This shows that these NH protons are near the HOD protons, placing the biguanide functional group in the water pool. Preliminary differential FTIR spectroscopic studies confirmed this location. In summary, we found that PBG interacts with different regions of the interface, with the phenyl group penetrating the hydrophobic interface while the biguanide remains in the water pool.

Supramolecular assemblies obtained by mixing different cyclodextrins and AOT or BHDC reverse micelles

In this contribution we show the effect of the surfactant polar head and the external solvent on the incorporation of different cyclodextrins (CDs) {α-CD, β-CD, γ-CD, decenylsuccinyl-β-CD (Mod-β-CD), and hydroxypropyl-β-CD (hp-β-CD)} in different reverse micelles (RMs) {benzene/sodium 1,4-bis(2-ethylhexyl) sulfosuccinate(AOT)/water, and benzene/benzyl-n-hexadecyldimethylammonium chloride (BHDC)/water} and compare them with previous results obtained in n-heptane/AOT/water RMs. To investigate the different systems, we have used UV-vis spectrophotometry, induced circular dichroism spectroscopy (ICD), and the achiral molecular probe methyl orange (MO). The results show dramatic differences changing the external solvent and the surfactant, which are explained by considering the differences in the RMs interface composition, the water-surfactant interaction, and the CDs' location in the different media investigated. None of the CDs were incorporated into the benzene/AOT/water RMs at any [H2O]/[surfactant] ratio studied (W0) whereas it was previously shown that Mod-β-CD and hp-β-CD could be included in n-heptane/AOT/water RMs. However, all of the CDs are incorporated in benzene/BHDC/water RMs at W0 > 10 and hp-β-CD is dissolved even at W0 = 0. Different from what was found in n-heptane/AOT RMs, in BHDC RMs MO showed ICD signals with two different CDs: Mod-β-CD and hp-β-CD. The results are explained by considering the known difference in the interfacial water structure for AOT and BHDC RMs and the electron-rich region on the secondary hydroxyl (wider side of the CDs), which helps to solubilize all CDs in BHDC. This study shows that chiral cyclodextrin could be available for a guest in an organic medium such as the RMs. Therefore we have created a potentially powerful nanoreactor with two different confined regions in the same aggregate: the polar core of the RMs and the chiral hydrophobic cavity of cyclodextrin.

Effect of confinement on excited-state proton transfer of firefly's chromophore D-luciferin in AOT reverse micelles

Excited-state intermolecular proton transfer of D-luciferin in reverse micelles has been investigated using steady-state and time-resolved fluorescence spectroscopy measurement. The different polar cores have been chosen for the study of proton transfer dynamics in aerosol-OT (AOT) reverse micelles. It is shown that aqueous reverse micelle is the suitable environment for the photoprotolytic reaction of D-luciferin. The neutral form of the chromophore is present both in ground and excited state at W0 = 0. The proton transfer in nanometer size water pool of water/AOT/n-heptane begins at W0 = 8 and increases with increasing W0 values. However, the intermolecular excited-state proton transfer (ESPT) of D-luciferin is inhibited in nonaquous reverse micelles with DMF and DMSO as a polar core. Thus, the requirement of ESPT of D-luciferin to take place in reverse micelles consists of polar protic solvent like water as a polar core.

Reverse micelles as a tool for probing solvent modulation of protein dynamics: Reverse micelle encapsulated hemoglobin

Hydration waters impact protein dynamics. Dissecting the interplay between hydration waters and dynamics requires a protein that manifests a broad range of dynamics. Proteins in reverse micelles (RMs) have promise as tools to achieve this objective because the water content can be manipulated. Hemoglobin is an appropriate tool with which to probe hydration effects. We describe both a protocol for hemoglobin encapsulation in reverse micelles and a facile method using PEG and cosolvents to manipulate water content. Hydration properties are probed using the water-sensitive fluorescence from Hb bound pyranine and covalently attached Badan. Protein dynamics are probed through ligand recombination traces derived from photodissociated carbonmonoxy hemoglobin on a log scale that exposes the potential role of both α and β solvent fluctuations in modulating protein dynamics. The results open the possibility of probing hydration level phenomena in this system using a combination of NMR and optical probes.