Changes In Solvent Structure Research Articles

Molecular Dynamic Insights into the Distinct Solvation Structures of Aromatic and Aliphatic Compounds in Monoethanolamine-Based Deep Eutectic Solvents.

Deep eutectic solvents (DESs) are developing as an alternate medium for aromatic extraction, especially benzene and thiophene from aliphatic hydrocarbon mixtures. In this work, molecular dynamics (MD) simulations were first used to investigate the solvation structure of benzene, thiophene, and n-hexane in monoethanolamine-based DESs. It reveals the liquid structures in the adjacent neighbor shells, which is a function of electron-withdrawing sulfur attached to thiophene and the π-electron cloud of benzene. The intermolecular forces between aromatic, aliphatic, and DES components are analyzed in van der Waals and hydrogen bond interactions. The chloride ions serve as a charge carrier bridge between choline and monoethanolamine precursors. The solvation of benzene, thiophene, and n-hexane in the DESs depends on volume expansion and minor solvent structural changes. Density functional theory results provided information on the mechanism of short-range interactions between organic solutes and studied DES. It aids in understanding the structural orientations of a DES with the addition of solutes, essential to the formation of DES. The solvation shell structure and characteristics were investigated in tandem with the possibility of benzene and thiophene clustering. The 1H NMR and 2D 1H-1H-NOESY were used to investigate the intermolecular interactions between benzene, thiophene, and n-hexane with monoethanolamine-based solvents. It concludes that high-ordered DES1 is more inclined to higher solubility than lower-ordered ones with a higher molar ratio of monoethanolamine. The solvation was reduced because the entropy gain was not maximized in the lower ordered DESs.

Dependence of Quinone Electrochemistry on Solvent Polarity and pH

We studied the polarographic reduction of quinones in aqueous/organic mixtures with dielectric constants (ε) from 78.4 down to 47. Added organic co-solvents were either protic or aprotic. The increase in polarographic half-wave potential, E 1/2, with declining ε was successfully fit to the Born equation down to ε ≈ 55. Cu2+ and ubiquinone0 were reduced in a single two-electron step that was more ε-sensitive when the added organic co-solvent was aprotic. Naphtho- and anthraquinone were reduced in two successive one-electron steps that were influenced identically by protic or aprotic organic co-solvents. The product of the first reduction wave was neutral semiquinone (·QH), which surprisingly, had to be protonated to ·QH2 + before reduction in the second wave. Except for r H+, radii derived from fitting our E 1/2 vs ε results to the Born equation were too small; in other words, non-electrostatic effects destabilized the oxidized species, greatly enhancing the Born electrostatic increase in E 1/2 with declining ε. Additionally, for ε < 55, we observed deviation from the Born equation, which may be due to changes in solvent structure and dynamics, and solvent-solute interactions. Finally, we studied quinones incorporated into phosphatidyl choline sonicated bilayer vesicles: Ubiquinone0 had two distinct irreversible two-electron reduction waves, one due to a population bound at the membrane surface, and another (whose E 1/2 was negatively shifted by 150 mV) due to a population localized in the membrane interior. Ubiquinone10 had a single irreversible two-electron reduction wave that was 250 mV more negative than the UQ0 membrane-interior population.

Elucidation of molecular interactions of aspartic acid with aqueous potassium sorbate and sodium benzoate: Volumetric, viscometric and FTIR spectroscopic investigation

Aspartic acid (Asp), a nonessential amino acid has been taken for the present study. Solutions of different concentrations of aspartic acid were prepared by dissolving it in water and {water + (1.5, 2.0 and 2.5 m) Potassium sorbate (PS)/sodium benzoate (SB)}. Density and viscosity of the solutions were experimentally determined at T = (293.15, 298.15, 303.15, 308.15 and 313.15) K. Volumetric parameters like apparent molar volume (V∅), partial apparent molar volume (V∅0), partial apparent molar expansibility (E∅0) were computed by using standard formulations. Stronger ion-ion, ion-hydrophilic and hydrophilic-hydrophilic interactions between zwitterionic centers (COO- and NH3+) and polar groups of aspartic acid (COOH and NH2) and ions present in aqueous PS/SB are found to dominate the hydrophobic interactions between the nonpolar organic segments of amino acid and PS/SB. Structure building character of aspartic acid in presence of PS/SB are established by the positive values of Hepler’s constant (∂2V∅0/∂T2)P for all compositions of solutions. Positive viscosity B-coefficient (BJ) and negative (∂BJ/∂T)values further establish the structure building character of aspartic acid. Thermodynamic parameters for viscous flow (Δμ20,#,ΔS0,#andΔH0,#) suggest that transition state for viscous flow is less stable than the ground state and formation of transitions state is not thermodynamically favoured. Shifting of IR absorption band for O-H stretching due to H-bonding at 3300–3340 cm-1 confirms changes in solvent structure due to ion-ion and ion–solvent interactions occurring in the investigated systems.

Design of arginine-based therapeutic deep eutectic solvents as drug solubilization vehicles for active pharmaceutical ingredients.

The solvation of lidocaine in three newly designed deep eutectic solvents is studied using combined experimental and theoretical methods that include density functional theory and molecular dynamics methods. The intermolecular forces between lidocaine and the hydrogen bond acceptors and hydrogen bond donors of the deep eutectic solvents were analysed regarding the type and the strength of inter- and intra-molecular bonding. The structure, composition and properties of the lidocaine solvation shells are analysed together with the possible lidocaine-clustering around the studied deep eutectic solvents and their constituent molecules. Furthermore, the changes in the solvent structures upon lidocaine solubilization are also studied. Natural product-based eutectic solvents showed considerably high solvation of lidocaine in all three deep eutectics based on the strong solute-solvent intermolecular interactions accompanied by a slight volume expansion and minor solvent structural changes. These non-toxic and almost null-volatile therapeutic deep eutectic solvents can be considered as suitable solubilization media for developing pharmaceutical applications and they can be considered as effective drug delivery vehicles for active pharmaceutical ingredients.

Observing Solvation Dynamics with Simultaneous Femtosecond X-ray Emission Spectroscopy and X-ray Scattering

In liquid phase chemistry dynamic solute-solvent interactions often govern the path, ultimate outcome, and efficiency of chemical reactions. These steps involve many-body movements on subpicosecond time scales and thus ultrafast structural tools capable of capturing both intramolecular electronic and structural changes, and local solvent structural changes are desired. We have studied the intra- and intermolecular dynamics of a model chromophore, aqueous [Fe(bpy)3](2+), with complementary X-ray tools in a single experiment exploiting intense XFEL radiation as a probe. We monitored the ultrafast structural rearrangement of the solute with X-ray emission spectroscopy, thus establishing time zero for the ensuing X-ray diffuse scattering analysis. The simultaneously recorded X-ray diffuse scattering patterns reveal slower subpicosecond dynamics triggered by the intramolecular structural dynamics of the photoexcited solute. By simultaneous combination of both methods only, we can extract new information about the solvation dynamic processes unfolding during the first picosecond (ps). The measured bulk solvent density increase of 0.2% indicates a dramatic change of the solvation shell around each photoexcited solute, confirming previous ab initio molecular dynamics simulations. Structural changes in the aqueous solvent associated with density and temperature changes occur with ∼1 ps time constants, characteristic for structural dynamics in water. This slower time scale of the solvent response allows us to directly observe the structure of the excited solute molecules well before the solvent contributions become dominant.

Water structure and chaotropicity: their uses, abuses and biological implications.

The concept of "water structure" has been invoked to explain all manner of aqueous phenomena. Here we look at the origins of this tendency to understand solute hydration in terms of structural changes in bulk water, and consider the validity of one particular example: the classification of small solutes as chaotropic or kosmotropic, and the putative relation of this terminology to notions of structure-making and structure-breaking in the solvent. We doubt whether complex phenomena such as Hofmeister and osmolyte effects on macromolecules can be understood simply on the basis of a change in solvent structure. Rather, we argue that chaotropicity, if understood in the original sense, arises from the activities that solutes exert on macromolecular systems, as well as from deviations of solvation water from bulk-like behaviour. If applied judiciously, chaotropicity remains a potent, biologically pertinent parameter useful for classifying and understanding solution phenomena in all types of living system.

How a solute-pump/solvent-probe spectroscopy can reveal structural dynamics: Polarizability response spectra as a two-dimensional solvation spectroscopy

The workhorse spectroscopy for studying liquid-state solvation dynamics, time-dependent fluorescence, provides a powerful, but strictly limited, perspective on the solvation process. It forces the evolution of the solute-solvent interaction energy to act as a proxy for what may be fairly involved changes in solvent structure. We suggest that an alternative, a recently demonstrated solute-pump∕solvent-probe experiment, can serve as a kind of two-dimensional solvation spectroscopy capable of separating out the structural and energetic aspects of solvation. We begin by showing that one can carry out practical, molecular-level, calculations of these spectra by means of a hybrid theory combining instantaneous-normal-mode ideas with molecular dynamics. Applying the resulting formalism to a model system displaying preferential solvation reveals that the solvent composition changes near the solute do indeed display slow dynamics similar to, but measurably different from, that of the solute-solvent interaction--and that this two-dimensional spectroscopy can effectively single out those local structural changes.

Enthalpies of Dilution of N,N′-hexamethylenebisacetamide in Pure Water and Aqueous Solutions of Alkali Halide at 298.15 K

Enthalpies of dilution of N,N′-hexamethylenebisacetamide in water and aqueous alkali halide solutions at the concentration of 0.150 mol⋅kg−1 (approximately the concentration of physiological saline) have been determined by isothermal titration microcalorimetry at 298.15 K. The enthalpic interaction coefficients in the solutions have been calculated according to the excess enthalpy concept based on the calorimetric data. The values of enthalpic pair-wise interaction coefficients (h 2) of the solute in aqueous solutions of different salts were discussed in terms of the different alkali salt ions and weak interactions of the diluted component with coexistent species as well as the change in solvent structure caused by ions.

The Enthalpies of Solution of L-cysteine, L-serine and L-asparagine in Aqueous Solutions of Some Alcohols at 298.15 K

Heats of solution, Δsol H m , of L-cysteine, L-serine and L-asparagine amino acids have been measured at different concentrations of aqueous ethanol, propanol and 2-propanol at 298.15 K using solvation calorimetry. These data are compared with the results reported earlier for L-alanine in ethanol. The enthalpic coefficients, h xy , of the solute-organic cosolvent pair interaction in water have been obtained from the McMillan-Mayer approach and the data have been interpreted in terms of various interactions and changes in solvent structure.

A Universal Approach to Solvation Modeling

Continuum mean-field models that have been carefully designed to address the various electrostatic and nonelectrostatic interactions that develop between a molecule and a surrounding medium are particularly efficient tools for studying the effects of condensed phases on molecular structure, energetics, properties, spectra, interaction potentials, and dynamics. The SM8 model may be combined with density functional theory or Hartree-Fock theory to describe a solute's electronic structure and its self-consistent-field polarization by a solvent. A key feature is the use of class IV charge models to obtain accurate charge distributions (either in the vapor phase or in solution), even when using small basis sets that are affordable for large systems. A second key feature is that nonelectrostatic effects due to cavity formation, dispersion interactions, and changes in solvent structure are included in terms of empirical atomic surface tensions that depend on geometry but do not require atom-type assignments by the user. Use of an analytic surface area algorithm provides very stable energy gradients that allow geometry optimization in solution. The SM8 continuum model, the culmination of a series of SMx models (x = 1-8), permits the modeling of such diverse media as aqueous and organic solvents, soils, lipid bilayers, and air-water interfaces. In addition to predicting accurate transfer free energies between gaseous and condensed phases or between two different condensed phases, SMx models have been useful for predicting the significant influence of condensed phases on processes associated with a change in molecular charge, including acid/base equilibria and oxidation/reduction processes. In this Account, we provide an overview of the algorithms associated with the computation of free energies of solvation in the SM8 model. We also compare the accuracies of the SM8 model with those of other continuum solvation models. Finally, we highlight applications of the SM8 models to compute ionic solvation free energies, oxidation and reduction potentials, and pK(a) values.

Phase Separation Study of a Surface-Active Drug, Promazine Hydrochloride, in Presence of Surfactants and Ureas

Abstract As clouding in drugs causes high concentration of drug at a particular site which may prove harmful to the body, the clouding behaviour of a tricyclic phenothiazine drug promazine hydrochloride (PMZ) in aqueous solutions has been studied in the presence of surfactants and ureas. The cloud point (CP) decreases with increase in pH due to deprotonation of the drug molecules. Addition of non-ionic and cationic (conventional as well as Gemini) surfactants increased the CP whereas anionic surfactants showed a peaked profile. The urea family additives influenced the CP in both ways that depended whether they contain &gt;C=O or &gt;C=S moiety: the former interacts with the micelles and lowers the CP while the latter (due to its electropositive character) may not interact and hence CP increases. Dye solubilization results indicate micellar growth with the addition of surfactants in PMZ solutions. The results are interpreted taking cognizance of mixed micelle formation, hydrophobic interactions, and/or change in solvent structure.

Detection of Specific Solvent Rearrangement Regions of an Enzyme: NMR and ITC Studies with Aminoglycoside Phosphotransferase(3‘)-IIIa

This work describes differential effects of solvent in complexes of the aminoglycoside phosphotransferase(3')-IIIa (APH) with different aminoglycosides and the detection of change in solvent structure at specific sites away from substrates. Binding of kanamycins to APH occurs with a larger negative DeltaH in H2O relative to D2O (DeltaDeltaH(H2O-D2O) < 0), while the reverse is true for neomycins. Unusually large negative DeltaCp values were observed for binding of aminoglycosides to APH. DeltaCp for the APH-neomycin complex was -1.6 kcal x mol(-1) x deg(-1). A break at 30 degrees C was observed in the APH-kanamycin complex yielding DeltaCp values of -0.7 kcal x mol(-1) x deg(-1) and -3.8 kcal x mol(-1) x deg(-1) below and above 30 degrees C, respectively. Neither the change in accessible surface area (DeltaASA) nor contributions from heats of ionization were sufficient to explain the large negative DeltaCp values. Most significantly, 15N-1H HSQC experiments showed that temperature-dependent shifts of the backbone amide protons of Leu 88, Ser 91, Cys 98, and Leu143 revealed a break at 30 degrees C only in the APH-kanamycin complex in spectra collected between 21 degrees C and 38 degrees C. These amino acids represent solvent reorganization sites that experience a change in solvent structure in their immediate environment as structurally different ligands bind to the enzyme. These residues were away from the substrate binding site and distributed in three hydrophobic patches in APH. Overall, our results show that a large number of factors affect DeltaCp and binding of structurally different ligand groups cause different solvent structure in the active site as well as differentially affecting specific sites away from the ligand binding site.

Influence of organic additives on the clouding phenomena of promethazine hydrochloride solutions

This paper reports the influence of organic additives (alcohols, amino acids, sugars) on the micellization and cloud point (CP) of a phenothiazine drug, promethazine hydrochloride (PMT). The critical micelle concentration (CMC) of the drug, determined by surface tension measurements in the presence of a representative of each additive class (i.e., butanol, leucine, arabinose), are used to evaluate the maximum surface excess concentration (Γ max) and the minimum area per surfactant molecule (A min) at the air/water interface. Γ max increases and CMC/A min decreases with increasing concentration of the additives, which indicate mixed micelle formation. The intermicellar interaction coefficients in the mixed micelles (β m and β σ are also calculated, and their negative values imply attractive interactions. Effect of pH revealed CP decrease with increasing pH due to deprotonation of PMT molecules. Effect of amino acids depended upon their nature and polarity, whereas sugars caused a CP decreasing effect. Aliphatic alcohols as well as cycloalkanols and diols decreased the CP. In the presence of arabinose, increase in drug concentration resulted in the CP increase, while increase in pH showed an opposite trend. Results are interpreted on the basis of mixed micelle formation, hydrophobic interactions, and change in solvent structure.

Influence of solvent structure on the aquation of trans-[Ru(3-MePy)4Cl2] complex in mixed aqueous solvents

The kinetics of the solvolytic aquation of trans-[Ru (3-Me Py)4Cl2] was studied spectrophotometrically in water – isopropanol in the range (30–90% v/v), and water acetonitrile in the range (10–70% v/v), and in the temperature range 50–65 °C. Plots of log k versus the reciprocal of the relative permittivity and Grunwald–Winstien gave non-linear plots. This non-linearity is derived from a large differential effect of solvent structure between the initial and transition states. The plot of log k versus water concentration was also non linear; evidence for the presence of a SN1 mechanism. However, extrema in the variation of enthalpy ΔH* and entropy ΔS* of activation correlate well with the extrema in physical properties of the mixtures which are related to changes in solvent structure. Linear plots of ΔH* versus ΔS* were obtained and the iso- kinetic temperature indicates that the reaction is entropy controlled.

The 1.13-Å structure of iron-free cytochrome c peroxidase

The iron-free cytochrome c peroxidase (CCP) crystal structure has been determined to 1.13 A and compared with the 1.2-A ferric-CCP structure. Quite unexpectedly, removal of the iron has no effect on porphyrin geometry and distortion, indicating that protein-porphyrin interactions and not iron coordination or formation of the axial His-Fe bond determines porphyrin conformation. However, there are changes in solvent structure in the distal pocket, which lead to changes in the distal His52 acid-base catalyst. The observed ability of His52 to move in response to small changes in solvent structure is very likely important for its role as a catalyst in assisting in the heterolytic fission of the peroxide O-O bond.

Simulations of the Vibrational Relaxation of a Model Diatomic Molecule in a Nanoconfined Polar Solvent

The vibrational dynamics of a model diatomic anion solute dissolved in a methyl iodide solvent confined in a nanoscale spherical cavity are investigated by molecular dynamics simulations. The effect of confining the solvent on the vibrational energy relaxation time T1, solvent-induced frequency shift 〈δω〉, and pure dephasing time T2* is examined by comparing the results from confined systems of varying size (cavity radius 0.8−2 nm) to those from the bulk system. It is found that T1 increases monotonically toward the bulk solvent value with increasing cavity size, and good agreement is found between equilibrium molecular dynamics simulations based on perturbation theory and classical nonequilibrium dynamics simulations. In contrast to T1, the solvent-induced frequency shift and the dephasing time do not change monotonically with cavity size. The results are discussed in terms of the changes in solvent structure and dynamics due to confinement.

Effects of Solvent Composition and Temperature on K+–18-Crown-6 Complexation in Acetonitrile–Water Mixed Solvents

Complexation of K+ by 18-crown-6 ether (18C6) in pure water and in acetonitrile–water mixed solvents containing 0.1 mol-dm− 3 (C2H5)4NCl has been systematically studied by isothermal titration calorimetry (ITC) at 293, 298, and 303 K. The formation constant K of the 1:1 [K(18C6)]+ complex and the complexation enthalpy Δ rH were simultaneously determined from the titration data. The logK and Δ rH(kJ-mol− 1) values at 298 K are 2.04, −26.2 in pure water and 2.23, −25.0; 2.61, −24.2; 2.95, −23.8; 3.48, −21.0; 3.85, −19.4; 4.36, −18.7; and 5.73, −17.0 in the mixed solvents at xAN (mole fraction of acetonitrile) of 0.043, 0.135, 0.258, 0.448, 0.578, 0.759, and 1.0, respectively. The change in heat capacity for the complex formation, Δ C p °, was also determined by the temperature dependence of Δ rH. Whereas the Δ C p ° is (57 ± 11) and (63 ± 20) J-mol− 1-K− 1 in pure water and in the solvent mixture at xAN = 0.043, respectively, it decreases with increasing xAN. The Δ C p ° values are −(48 ± 11), −(110 ± 25), −(354 ± 40), −(359 ± 24), and −(304 ± 30) J-mol− 1-K− 1 at xAN = 0.135, 0.258, 0.448, 0.578, and 0.759, respectively. The changes in complexation thermodynamics (Δ Δ rG, Δ ΔrH, and Δ Δ rS) are discussed in terms of the corresponding transfer thermodynamics of K+, 18-crown-6, and [K(18C6)]+ upon transferring from water to acetonitrile–water mixed solvents. It was found that hydrophobic solvation of the complex [K(18C6)]+ plays an important role in complex formation occurring in water and in the water-rich mixed solvent. Moreover, changes in solvent structure significantly affect the transfer enthalpy and entropy of each species, i.e., K+, 18-crown-6, and [K(18C6)]+. The observed monotonous changes in the complexation Gibbs energy, enthalpy, and entropy with solvent composition are due to the effective compensation of the Δ trG, Δ trH, and Δ trS for K+ with those for 18-crown-6 and [K(18C6)]+.

Effect of sodium caproate on the volumetric and viscometric properties of glycine, dl-α-alanine, and dl-α-amino-n-butyric acid in aqueous solutions

The apparent molar volumes ( V m,2) and relative viscosities ( η r) at T=(298.15 and 308.15) K have been obtained for glycine, dl-α-alanine, and dl-α-amino-butyric acid in aqueous sodium caproate solutions from measurements of density and the flow time. The standard partial molar volumes ( V ∘ m,2), standard volumes of transfer (Δ t V ∘), the viscosity B-coefficients, and the activation thermodynamic quantities (Δ μ 2 ∘≠ and Δ S 2 ∘≠) of viscous flow have been calculated for the amino acids. It is shown that the standard partial molar volumes, viscosity B-coefficients, and activation free energies for viscous flow increase with increasing number of carbon atoms in the alkyl chain of the amino acids. An increase in V ∘ m,2 and Δ t V ∘ with increasing electrolyte concentrations have been explained due to the interactions of sodium caproate with the charged center of zwitterions for the amino acids. A comparison of the V ∘ m,2 values for glycine, dl-α-alanine, and dl-α-aminon-n-butyric acid in different aqueous salts solutions showed that carboxylate ions have stronger interactions with amino acid than chloride, thiocyanate, and nitrate ions. Results of viscosity are discussed in terms of changes in solvent structure.

Partial molar volumes and viscosities of some amino acids in aqueous electrolyte and non-electrolyte solutions

Apparent molar volumes and viscosities of glycine, DL-α-alanine and L-leucine in water and in 1.0 m (mol kg - 1 ) aqueous solutions of each of potassium chloride, barium chloride, glucose and sucrose have been determined from density and efflux time measurements, using vibrating-tube digital densitymeter and Ubbelohde viscometer, respectively at 298.15 K. Volume data were utilised to calculate partial molar volumes of transfer at infinite dilution (ν 0 2,tr) from water to mixed aqueous solutions and B-coefficients were calculated from viscosity data using Jones-Dole equation. Positive v 0 2,tr values have been rationalized using cosphere overlap model in terms of solute-cosolute interactions. Transfer B-coefficients of the amino acids studied are positive in case of glucose and sucrose whereas these are negative in case of electrolytes (KCI, BaCl 2 ) which have been explained in terms of changes in solvent structure. Contributions of the side chain to v 0 2,tr and to B-coefficient values have also been discussed.

The dominant interaction between peptide and urea is electrostatic in nature: a molecular dynamics simulation study.

The conformational equilibrium of a blocked valine peptide in water and aqueous urea solution is studied using molecular dynamics simulations. Pair correlation functions indicate enhanced concentration of urea near the peptide. Stronger hydrogen bonding of urea-peptide compared to water-peptide is observed with preference for helical conformation. The potential of mean force, computed using umbrella sampling, shows only small differences between urea and water solvation that are difficult to quantify. The changes in solvent structure around the peptide are explained by favorable electrostatic interactions (hydrogen bonds) of urea with the peptide backbone. There is no evidence for significant changes in hydrophobic interactions in the two conformations of the peptide in urea solution. Our simulations suggest that urea denatures proteins by preferentially forming hydrogen bonds to the peptide backbone, reducing the barrier for exposing protein residues to the solvent, and reaching the unfolded state.