Solvation Dynamics Experiments Research Articles

Triplet Solvation Dynamics of Hydrogen Bonding Liquids in Confinement

Abstract We have developed a flexible experimental setup to conduct triplet solvation dynamics (TSD) experiments. The setup is capable of exciting dyes at 355, 320 and 266 nm. Phosphorescence spectra can be recorded up to a 10 ns-resolution usually covering three decades using a grating spectrograph and a CCD camera. In this contribution, we describe the experimental setup as well as first investigations on water-alcohol mixtures, microemulsions and new dyes for TSD, i.e. naphthalene derivates, which take full advantage of this experimental method sensitive to the local environment of the dyes.

Computational Study of Phosphate Vibrations as Reporters of DNA Hydration

The sensitivity of the phosphate asymmetric stretch vibrational frequency to DNA hydration was investigated with molecular dynamics (MD) simulations and a spectroscopic map relating the vibrational frequency to the electrostatics of its environment. 95% of the phosphate vibrational frequency shift in fully hydrated DNA was due to water within two hydration layers. The phosphate vibration was relatively insensitive to water in the major and minor grooves and to the sodium counterions but was enormously sensitive to water interacting with the DNA backbone. Comparisons to experimental measurements on DNA as a function of relative humidity suggest that one water molecule per phosphate group likely persists at the lowest values of the relative humidity. Finally, the calculated spectral diffusion dynamics show that water in the vicinity of the DNA backbone is slowed by a factor of ∼5, in agreement with NMR and solvation dynamics experiments, as well as previous MD simulations.

An Interesting Case Where Water Behaves as a Unique Solvent. 4-Aminophthalimide Emission Profile to Monitor Aqueous Environment

The behavior of 4-aminophthalimide (4-AP), a common molecular probe utilized in solvation dynamics experiments, was revisited in polar aprotic and protic solvents using absorption, steady-state, and time-resolved fluorescence (TRES) techniques. Also, the deuterium isotope effect was investigated using D(2)O as solvent. The absorption spectra of 4-AP consist of two absorption bands with maxima around 300 nm (B2 band) and 370 nm (B1 band) depending on the environment, while the emission feature consists of a single band. In all the solvents investigated (excluding water), the 4-AP photophysics is similar and the emission spectra are independent of the excitation wavelength used. In water the behavior is unique and the emission spectra maximum is different depending on the excitation wavelength used. The emission maximum is 561.7 nm using the excitation wavelength that correspond to the B2 absorption band maximum (λ(excB2) = 303.4 nm) but is 545.7 nm when the excitation wavelength that correspond to the B1 absorption maximum (λ(excB1) = 370.0 nm) is used. Moreover, while the fluorescence decays of 4-AP in water exhibit no emission wavelength dependence at λ(excB2), the situation is quite different when λ(excB1) is used. In this case, we found a time-dependent emission spectrum that shifts to the blue with time. Our results show that the solvent-mediated proton transfer process displays a fundamental role in the 4-AP emission profile and for the first time a mechanism was proposed that fully explains the 4-AP behavior in every solvent including water. The deuterium isotope effect confirms the assumption because the proton-transfer process is dramatically retarded in this solvent. Consequently, we were able to elucidate not only why in water the emission spectra depend on the excitation wavelength but also why the time-dependent emission spectra shift to the blue with time. Thus, our work reveals the importance that the medium has on the behavior of a widespread dye used as chromophore. This is significant since the use of chromophores without understanding its chemistry can induce artifacts into the interpretation of solvation dynamics in heterogeneous environments, in particular, those provided by aqueous biological systems.

C343 behavior in benzene/AOT reverse micelles. The role of the dye solubilization in the non-polar organic pseudophase

Abstract The behavior of coumarin 343 (C343), a common molecular probe utilized in solvation dynamics experiments, was studied in water/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/benzene reverse micelles (RMs). In all the studies performed until now, C343 was not soluble in the organic solvent used to create the RM systems (namely different alkanes such as n- heptane, cyclohexane or isooctane). In this work we have chosen benzene as the organic solvent because C343 is completely soluble at the work concentration used (∼10 −6 M). Thus, a well known AOT RM system: benzene/AOT/water has been used in order to gain insights, for the first time, on how the RMs’ formation can affect the C343 spectroscopic behavior. Dissolved in pure benzene C343 exists as a dimer since the intermolecular H-bond interactions are very strong in this solvent. When introduced to the AOT RMs, C343 resides in the RMs’ interface and, at low occupation number the probability of finding more than one C343 molecule is reduced and only C343 monomer species is detected. From the spectral changes it was possible to determine the critical micelle concentration at W 0 = 0 and 10 and the C343 partition constants between two pseudophases. In summary, in this work we have shown how the spectroscopic behavior of C343 is dramatically altered because of the partition of the dye to the AOT RMs’ interfaces.

Hydration dynamics of protein molecules in aqueous solution: Unity among diversity#

Dielectric dispersion and NMRD experiments have revealed that a significant fraction of water molecules in the hydration shell of various proteins do not exhibit any slowing down of dynamics. This is usually attributed to the presence of the hydrophobic residues (HBR) on the surface, although HBRs alone cannot account for the large amplitude of the fast component. Solvation dynamics experiments and also computer simulation studies, on the other hand, repeatedly observed the presence of a non-negligible slow component. Here we show, by considering three well-known proteins (lysozyme, myoglobin and adelynate kinase), that the fast component arises partly from the response of those water molecules that are hydrogen bonded with the backbone oxygen (BBO) atoms. These are structurally and energetically less stable than those with the side chain oxygen (SCO) atoms. In addition, the electrostatic interaction energy distribution (EIED) of individual water molecules (hydrogen bonded to SCO) with side chain oxygen atoms shows a surprising two peak character with the lower energy peak almost coincident with the energy distribution of water hydrogen bonded to backbone oxygen atoms (BBO). This two peak contribution appears to be quite general as we find it for lysozyme, myoglobin and adenylate kinase (ADK). The sharp peak of EIED at small energy (at less than 2 k B T) for the BBO atoms, together with the first peak of EIED of SCO and the HBRs on the protein surface, explain why a large fraction (~ 80%) of water in the protein hydration layer remains almost as mobile as bulk water. Significant slowness arises only from the hydrogen bonds that populate the second peak of EIED at larger energy (at about 4 kBT). Thus, if we consider hydrogen bond interaction alone, only 15–20% of water molecules in the protein hydration layer can exhibit slow dynamics, resulting in an average relaxation time of about 5–10 ps. The latter estimate assumes a time constant of 20–100 ps for the slow component. Interestingly, relaxation of water molecules hydrogen bonded to back bone oxygen exhibit an initial component faster than the bulk, suggesting that hydrogen bonding of these water molecules remains frustrated. This explanation of the heterogeneous and non-exponential dynamics of water in the hydration layer is quantitatively consistent with all the available experimental results, and provides unification among diverse features. This study reveals a bimodal electrostatic energy distribution of protein–water hydrogen bonds involving side chain oxygen and faster than bulk water hydrogen bond breaking dynamics of protein–water hydrogen bond involving backbone oxygen.

Role of the Medium on the C343 Inter/Intramolecular Hydrogen Bond Interactions. An Absorption, Emission, and 1HNMR Investigation of C343 in Benzene/n-Heptane Mixtures

C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media. Absorption, emission, and (1)HNMR spectroscopies were used to investigate the behavior of C343 in benzene and in benzene/n-heptane mixtures. We demonstrate the implications of the medium polarity, measured as the Kamlet-Taft polarity-polarizability (pi*) parameter, in the C343 inter/intramolecular hydrogen bond (H-bond) interactions and the role that this interaction plays in the dimerization process of the dye. In pure benzene, the dimer prevails because the intermolecular H-bond interaction is favored. On the other hand, as the n-heptane content increases the intramolecular H-bond is the strongest and the C343 monomer is favored. As the polarity of the medium decreases, the solvophobic interaction makes that C343 monomer species experiences a more complicated aggregation process beyond the simple monomer dimer equilibrium present in pure benzene. Thus, the addition of n-heptane to the mixture yields a C343 higher-order aggregates species. Thus, our work reveals the importance that the medium has on the behavior of a widespread dye used as chromophore for very different systems such as homogeneous and microheterogenous media. This is very important since the use of chromophores without understanding its chemistry can induce artifacts into the interpretation of solvation dynamics in heterogeneous environments, in particular, those provided by biological systems such as proteins. Considerable care in choosing and characterizing the system is required to analyze the results fully.

Ultrafast reaction dynamics in nanoscale water droplets confined by ionic surfactants

The excited state dynamics of the dye molecule auramine O have been measured in nanoscale size water droplets stabilized by ionic surfactants using ultrafast fluorescence up-conversion. Specifically, the reaction dynamics have been measured as a function of the size of the water droplet in the range 1–10 nm and as a function of the counterion charge. The data are analysed quantitatively using a generalized Smoluchowski equation. The rate of the reaction in the reverse micelle is dramatically decreased compared to the bulk water environment. It increases as nanodroplet size increases, but never attains the bulk value. The data are not well described by a ‘core-shell’ model, i.e. by assuming a slow reaction in the interface (shell) and a bulk-like decay in the core. The effect of changing the counterion is small, with three different singly charged counterions showing essentially identical reaction dynamics, while the double charged Ca2+ counterion only slightly slows the reaction. This insensitivity to counterion is in contrast to the observations of solvation dynamics experiments and molecular dynamics simulation. The origins of these differences are discussed in terms of the solute reaction coordinate being more sensitive to different motions of the confined aqueous solvent than solvation dynamics and to more delocalised properties of the dispersed phase than are detected in simulations.

Solvation Dynamics of Dipolar Probes in Dipolar Room Temperature Ionic Liquids: Separation of Ion−Dipole and Dipole−Dipole Interaction Contributions

Recent solvation dynamics experiments with common dipolar solvation probes in imidazolium room temperature ionic liquids (RTIL) have revealed large dynamic Stokes' shifts and biphasic solvation energy relaxations. Because of the dipolar nature of the imidazolium cations, the solute-cation (dipole-dipole) interaction may, in addition to the ion-solute (ion-dipole) interaction, contribute significantly to the observed Stokes' shift and its dynamics. Conventional time-resolved measurements, however, cannot separate out these contributions. A simple semimolecular theory is described here which, upon separation of the dipolar part of the solvation energy from the ion-dipole part, allows estimation of these two components in the measured shifts. While the sum-total of these separated out components agree well with experiments, the ion-dipole interaction is found to contribute approximately 60% of the measured shift in each of these dipolar RTILs. In addition, the calculated solvation response functions, as observed in experiments, are characterized by a fast exponential component (approximately 15-20%) with a time constant in the subpicosecond regime and a slow nonexponential component with a time constant in the subnanosecond regime. Interestingly, the present theory finds that the fast component of the solvation response function in imidazolium ionic liquids originates from the rapid orientational relaxation involving the dipolar species in these liquids, whereas the relaxation of the ion dynamic structure factor via ion translation produces the observed slow nonexponential component. In addition, calculations presented here explain why the continuum model based theories of solvation dynamics do not work for these liquids. For alkylphosphonium ionic liquids, the ion-dipole interaction accounts for nearly 75% of the measured shifts. The present theory also explains why the experimentally observed solvation response function in these liquids does not contain any subpicosecond component and decays nonexponentially with only a single relaxation time constant.

The Dynamics of Water at DNA Interfaces: Computational Studies of Hoechst 33258 Bound to DNA

Together, spectroscopy combined with computational studies that relate directly to the experimental measurements have the potential to provide unprecedented insight into the dynamics of important biological processes. Recent time-resolved fluorescence experiments have shown that the time scales for collective reorganization at the interface of proteins and DNA with water are more than an order of magnitude slower than in bulk aqueous solution. The molecular interpretation of this change in the collective response is somewhat controversial some attribute the slower reorganization to dramatically retarded water motion, while others describe rapid water dynamics combined with a slower biomolecular response. To connect directly to solvation dynamics experiments of the fluorescent probe Hoechst 33258 (H33258) bound to DNA, we have generated 770 ns of molecular dynamics (MD) simulations and calculated the equilibrium and nonequilibrium solvation response to excitation of the probe. The calculated time scales for the solvation response of H33258 free in solution (0.17 and 1.4 ps) and bound to DNA (1.5 and 20 ps) are highly consistent with experiment (0.2 and 1.2 ps, 1.4 and 19 ps, respectively). Decomposition of the calculated response revealed that water solvating the probe bound to DNA was still relatively mobile, only slowing by a factor of 2-3, while DNA motion was responsible for the long-time component (approximately 20 ps).

Intramolecular Charge Transfer Reaction, Polarity, and Dielectric Relaxation in AOT/Water/Heptane Reverse Micelles: Pool Size Dependence

Intramolecular charge transfer (ICT) reaction in a newly synthesized molecule, of 4-(1-morpholenyl) benzonitrile (M6C), in AOT/water/heptane reverse micelles at different pool sizes has been studied by using steady-state and time-resolved fluorescence emission spectroscopy. The pool size dependences of the reaction equilibrium constant and reaction rate have been explained in terms of the average polarity of the confined solvent pools estimated from the fluorescence emission Stokes shift of a nonreactive probe, coumarin 153, dissolved in these microemulsions. The complex permittivity measurements in the frequency range 0.01<or=nu/GHz<or=2 for these microemulsions at different pool sizes (0<or=w0<or=40) and AOT concentrations (0.1<or=c/M<or=0.5) at 298.15 K have also been performed. At sufficient water content, a large dispersion with a relaxation time of approximately 600 ps has been observed at approximately 300 MHz and attributed to the average reorientation of water molecules residing in the close vicinity of the polar interface of the AOT headgroup and n-heptane. The reorientation of these interfacial water molecules is probably responsible for the nanosecond component observed in numerous polar solvation dynamics experiments in these reverse micelles. Subsequently, the estimated polarity and the measured reorientational time scale have been used to explain the dramatic slowing down of the ICT reaction rate and its dependence on pool size in these confined environments.

Dynamics of Water in the Hydration Layer of a Partially Unfolded Structure of the Protein HP-36

Atomistic molecular dynamics simulations of the folded native structure and a partially unfolded molten globule structure of the protein villin headpiece subdomain or HP-36 have been carried out with explicit solvent to explore the effects of unfolding on the dynamical behavior of water present in the hydration layers of different segments (three alpha-helices) of the protein. The calculations revealed that the unfolding of helix-2 influences the translational and rotational motions of water present in the hydration layers of the three helices in a heterogeneous manner. It is observed that a correlation exists between the unfolding of helix-2 and the microscopic kinetics of protein-water hydrogen bonds formed by its residues. This in turn has an influence on the rigidity of the hydration layers of the helices in the unfolded structure versus that in the folded native structure. These results should provide a microscopic explanation to recent solvation dynamics experiments on folded native and unfolded structures of proteins.

Confined viscous liquids: Interfacial versus finite size effects

Confining a supercooled liquid to spaces of several nanometer in diameter can lead to dramatic changes in the relaxation dynamics of the material. In many cases, the effect is reported as a confinement induced shift of the glass transition temperature Tg. Both positive and negative values for ΔT g have been observed and the length scale of the confining geometry is considered the main variable. We review the dynamics of glass-forming liquids in both hard and soft confinement of <10 nm spaces, with focus on results from solvation dynamics experiments. It is shown that the interface is instrumental in determinig the dynamics, giving rise to reaxation time gradients across the cooperativity length scale of the liquid. Depending on the interfacial conditions, dynamics can become faster or slower for the same liquid, same size of confinement, and identical experimental technique used. No indications of true finite size effects are observed, and the pore or droplet size is relevant only indirectly through the relative number of molecules near the surface.

Exploration of the Secondary Structure Specific Differential Solvation Dynamics between the Native and Molten Globule States of the Protein HP-36

Recent experiments have shown that the time dependence of fluorescence Stokes shift of a chromophore is substantially different when the chromophore is located in a molten globule (MG) state and in the native state of the same protein. To understand the origin of this difference, particularly the role of water in the differential solvation of the protein in the native and the MG states, we have carried out fully atomistic molecular dynamics simulations with explicit water of a partially unfolded MG state of the protein HP-36 and compared the results with the solvation dynamics of the protein in the folded native state. It is observed that the polar solvation dynamics of the three helical segments of the protein is influenced in a nonuniform heterogeneous manner in the MG state. While the equilibrium solvation time correlation function for helix-3 has been found to relax faster in the MG state as compared to that in the native state, the decay of the corresponding function for the other two helices slows down in the MG state. A careful analysis shows that the origin of such heterogeneous relative solvation behavior lies in the differential location of the polar probe residues and their exposure to bulk solvent. We find a significant negative cross-correlation between the contribution (to the solvation energy of a tagged amino acid residue) of water and the other groups of the protein, indicating a competing role in solvation. The sensitivity of solvation dynamics to the secondary structure and the immediate environment can be used to discriminate the partially unfolded and folded states. These results therefore should be useful in explaining recent solvation dynamics experiments on native and MG states of proteins.

What Can You Learn from a Molecular Probe? New Insights on the Behavior of C343 in Homogeneous Solutions and AOT Reverse Micelles

The behavior of C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media and in aqueous and nonaqueous reverse micelles (RMs). In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission bands showing that the solvatochromic behavior of the dye depends not only on the polarity of the medium but also on the hydrogen-bonding properties of the solvent. Specifically, in the ground state the molecule displays a bathochromic shift with the polarity polarizability (pi) and the H-bond acceptor (beta) ability of the solvents and a hypsochromic shift with the hydrogen donor ability (alpha) of the media. The carboxylic acid group causes C343 to display greater sensitivity to the beta than to the pi polarity parameter; this sensitivity increases in the excited state, while the dependence on alpha vanishes. This demonstrates that C343 forms a stable H-bond complex with solvents with high H-bond acceptor ability (high beta) and low H-bond donor character (low alpha). Spectroscopy in nonpolar solvents reveals J-aggregate formation. With information from the Kamlet-Taft analysis, C343 was used to explore RMs composed of water or polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/isooctane using absorption, emission, and time-resolved spectroscopies. Sequestered polar solvents included ethylene glycol (EG), formamide (FA), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). Dissolved in the AOT RM systems at low concentration, C343 exists as a monomer, and when introduced to the RM samples in its protonated form, C343 remains protonated driving it to reside in the interface rather than the water pool. The solvathochromic behavior of the dye depends the specific polar solvent encapsulated in the RMs, revealing different types of interactions between the solvents and the surfactant. EG and water H-bond with the AOT sulfonate group destroying their bulk H-bonded structures. While water remains well segregated from the nonpolar regions, EG appears to penetrate into the oil side of the interface. In aqueous AOT RMs, C343 interacts with neither the sulfonate group nor the water, perhaps because of intramolecular H-bonding in the dye. DMF and DMA interact primarily through dipole-dipole forces, and the strong interactions with AOT sodium counterions destroy their bulk structure. FA also interacts with the Na+ counterions but retains its H-bond network present in bulk solvent. Surprisingly, FA appears to be the only polar solvent other than water forming a "polar-solvent pool" with macroscopic properties similar to the bulk.

Sensitivity of Polar Solvation Dynamics to the Secondary Structures of Aqueous Proteins and the Role of Surface Exposure of the Probe

The structure and dynamics of water around a protein is expected to be sensitive to the details of the adjacent secondary structure of the protein. In this article, we explore this sensitivity by calculating both the orientational dynamics of the surface water molecules and the equilibrium solvation time correlation function of the polar amino acid residues in each of the three helical segments of the protein HP-36, using atomistic molecular dynamics simulations. The solvation dynamics of polar amino acid residues in helix-2 is found to be faster than that of the other two helices (the average time constant is smaller by a factor of 2), although the interfacial water molecules around helix-2 exhibit much slower orientational dynamics than that around the other two helices. A careful analysis shows that the origin of such a counterintuitive behavior lies in the dependence of the solvation time correlation function on the surface exposure of the probe-the more exposed is the probe, the faster the solvation dynamics. We discuss that these results are useful in explaining recent solvation dynamics experiments.

Solvation dynamics of molecular glass-forming liquids in confinement

Triplet state solvation dynamics experiments on geometrically confinedglass-forming liquids are performed in order to study the effects of confinement onlength scales of several nanometres. Variations of the surface chemistry of porousglasses and the spatially selective observation of the interfacial liquid layerindicate that the surface interactions determine the difference between thedynamics of the bulk and confined situations. For 3-methylpentane in pores of 7.5nm diameter, the timescale of interfacial dynamics is slowed down by three ordersof magnitude, while structural relaxation already remains bulk-like just a fewnanometres away from the silica surface. The time-resolved optical linewidthsindicate dynamical heterogeneity in confined liquids, but the signature differsfrom the bulk case.

Heterogeneous solvent dynamics and time-resolved optical linewidths

The time resolved inhomogeneous optical linewidth observed in solvation dynamics experiments is sensitive to the nature of the solvent dynamics, homogeneous or heterogeneous. Linewidth data are assessed with regard to intrinsic non-exponentiality, the length scale of heterogeneity, and the time scale of rate exchange. For the glass-forming solvent under study, heterogeneity is found for average relaxation times in the range 3 ms to 54 s and in the regime of nanosecond dynamics.

Triplet state solvation dynamics: Basics and applications

Applying solvation dynamics experiments to viscous liquids or glassy materials near their glass transition involves long lived triplet probes, whose time dependent phosphorescence signals depend upon the local dipolar orientational dynamics, mechanical responses, and polarities. The current understanding of experimental results regarding steady state and time dependent optical line shapes and positions is reviewed with emphasis on the relation to the macroscopic dielectric properties. Several applications are discussed in detail, where advantage is taken of the spatially local instead of ensemble averaging character of this technique. These examples include studies of dynamical heterogeneity, rotational solute/solvent coupling, secondary relaxations in the glassy state, as well as confinement and interfacial effects.

Influence of Morphology on Polar Solvation Dynamics in Lecithin Reverse Micelles

The continuous nonpolar phase can change the morphology of lecithin reverse micelles dramatically. In alkane solvents, increasing the hydration leads to gelation as isolated water droplets transform into an entangled tube network. In contrast, increasing the hydration for lecithin in benzene maintains isolated, largely spherical traditional reverse micelles. Solvation dynamics experiments have been carried out in the two different micellar environments. These measurements show that solvation dynamics are considerably more restricted in the tubular micelles than in the spherical droplets. This is interpreted as evidence for water pool formation and disorder at the interface in the spherical micelles and supports the hypothesis of substantial water incorporation into the tubular micelles. To perform solvation dynamics experiments in the benzene/lecithin/water reverse micelles, we have synthesized a new headgroup-labeled probe molecule. This synthesis, coupling the standard solvation dynamics probe, coumarin 343, to phosphatidylethanolamine through an amide bond, is presented.

Ultrafast Optical Kerr Effect and Solvation Dynamics of Liquid Aniline

Femtosecond optically heterodyne detected optical Kerr effect (OHD-OKE) measurements have been made on liquid aniline. These are contrasted with solvation dynamics measurements made earlier. The OHD-OKE data in the frequency domain can be analyzed in terms of three components: a low-frequency diffusional part; a collisional part; an underdamped intermolecular mode. The latter is quite intense, suggesting strong intermolecular interactions and a relatively high degree of structure in liquid aniline. When the solvation dynamics calculated from the pure liquid data are compared with those measured directly, the agreement beyond 1 ps is good. This result suggests that it is possible to use experimental data obtained on the pure solvent dynamics to model the solvation dynamics response. However, the agreement is not as good at very early times. While this may simply reflect the lower time resolution of solvation dynamics experiments compared to OHD-OKE experiments, we speculate that the differences could also result from either fundamental differences in the measured solvation dynamics and those calculated from OHD-OKE data or a perturbation of the intermolecular interactions in liquid aniline by the solute.