Average Number Of Water Molecules Research Articles

Theoretical study of the absorption and emission spectra of the anionic p-coumaric methyl ester in gas phase and in solution

Solvent effects on the absorption and emission spectra of the p-coumaric methyl ester, a model for the chromophore of the photoactive yellow protein, are analyzed using a QM/MM method known as Average Solvent Electrostatic Potential from Molecular Dynamics (ASEP/MD), which combines state of the art calculations in the description of the solute with a detailed description of the solvent through molecular dynamics simulations. Geometries for the ground and the bright excited state are optimized by using multiconfigurational (CASSCF and CASPT2) and DFT methods. Three different functionals (B3LYP, CAM-B3LYP and PBE0) were used with DFT. The effect of specific interactions and of the interplay between static and dynamic electron correlation on the spectra are analyzed. In absorption there is a flux of charge from the phenolic toward the carboxylic moiety, and the opposite is found in emission. These fluxes are similar in gas phase; however, in water solution, the flux increases with respect to the gas phase value during the absorption but not in the emission. As a consequence of these fluxes the solvent structure around the chromophore changes after the excitation. The average number of water molecules decreases around the phenolic oxygen and increases around the carbonyl oxygen. These changes in the solvent structure together the reduction of the charge on the phenolic oxygen atom during the excitation permit to explain the large Stokes shift, 0.84eV, displayed by this system.

Hydration of the sulfuric acid-methylamine complex and implications for aerosol formation.

The binary H2SO4-H2O nucleation is one of the most important pathways by which aerosols form in the atmosphere, and the presence of ternary species like amines increases aerosol formation rates. In this study, we focus on the hydration of a ternary system of sulfuric acid (H2SO4), methylamine (NH2CH3), and up to six waters to evaluate its implications for aerosol formation. By combining molecular dynamics (MD) sampling with high-level ab initio calculations, we determine the thermodynamics of forming H2SO4(NH2CH3)(H2O)n, where n = 0-6. Because it is a strong acid-base system, H2SO4-NH2CH3 quickly forms a tightly bound HSO4(-)-NH3CH3(+) complex that condenses water more readily than H2SO4 alone. The electronic binding energy of H2SO4-NH2CH3 is -21.8 kcal mol(-1) compared with -16.8 kcal mol(-1) for H2SO4-NH3 and -12.8 kcal mol(-1) for H2SO4-H2O. Adding one to two water molecules to the H2SO4-NH2CH3 complex is more favorable than adding to H2SO4 alone, yet there is no systematic difference for n ≥ 3. However, the average number of water molecules around H2SO4-NH2CH3 is consistently higher than that of H2SO4, and it is fairly independent of temperature and relative humidity.

Concentration dependence of hydration water in a model peptide.

The molecular dynamics of aqueous solutions of a model amphiphilic peptide is studied as a function of concentration by broad-band light scattering experiments. Similarly to protein aqueous solutions, a considerable retardation, of about a factor 6-8, of hydration water dynamics with respect to bulk water is found, showing a slight dependence on solute concentration. Conversely, the average number of water molecules perturbed by the presence of peptide, i.e. the hydration number, appears to be strongly modified by adding solute. Its behaviour, decreasing upon increasing concentration, can be interpreted considering the random close-to-contact condition experienced by solute particles. Overall, the present findings support the view of a "long range" effect of peptides on the surrounding water, extending beyond the first two hydration shells.

Coordination of ions in aqueous solutions of erbium chloride from X-ray diffraction data

The radial distribution functions for aqueous solutions of samarium chloride over a broad concentration range under standard conditions, obtained previously by X-ray diffraction, were used to develop various physically substantiated models for structural organization of the systems. The optimal versions were identified by calculating the theoretical functions for each model and comparing their agreement with experimental functions. The quantitative characteristics of the local environment of Sm3+ and Cl– ions such as coordination numbers, interparticle distances, and sorts of ion pairs were established. The average number of water molecules in the first coordination sphere of the cation was found to decrease from 9 to 6 with increasing concentration. The structure of the systems is determined by solvent-separated ion associates over the whole concentration range.

Hydration of para-aminobenzoic acid (PABA) and its anion—The view from statistical mechanics

The hydration of para-aminobenzoic acid (PABA) and its anion (PABA−) in aqueous solution was investigated by 1D- and 3D-RISM integral equation methods. It was found that the first hydration shell of PABA and PABA− consists of ~28.4 and 27.2 water molecules, respectively. The average number of water molecules close the amino group is ~6.6 for PABA and ~7 for PABA−. The total number of H-bonds formed by the amino group is ~1.8 for PABA and ~1.9 for PABA−. The carboxyl (COOH) group forms on average ~2.9 hydrogen bonds, whereas carboxylate (COO−) has ~5.5. Thus, deprotonation of PABA is facilitated by increased hydration of the carboxylate moiety. For PABA and PABA− ~1.2 π-coordinated water molecules are preferably found above and below the plane of the aromatic ring at a distance of ~0.3nm from the ring center with slightly stronger binding for PABA−.

Water Transport through Carbon Nanotubes with the Radial Breathing Mode

Molecular dynamics simulations are performed to investigate the water permeation across the single-walled carbon nanotube with the radial breathing mode (RBM) vibration. It is found that the RBM can play a significant role in breaking the hydrogen bonds of the water chain, accordingly increasing the net flux dramatically, and reducing drastically the average number of water molecules inside the tube with the frequency ranging from 5000 to 11 000GHz, while far away from this frequency region the transport properties of water molecules are almost unaffected by the RBM. This phenomenon can be understood as the resonant response of the water molecule chain to the RBM. Our findings are expected to be helpful for the design of high-flux nanochannels and the understanding of biological activities, especially the water channelling.

Effects of Sugars on the Formation of Nanometer‐Sized Droplets of Vegetable Oil by an Isothermal Low‐Energy Emulsification Method

Effects of sugars on the formation of nanometer-sized oil droplets induced by the addition of vegetable oil to aqueous dispersions of polyoxyethylene sorbitan monooleate (MOPS, Tween 80) at 25°C without the application of intensive mechanical energy were investigated. Phase diagrams were constructed using polarized light microscopy and small angle X-ray scattering (SAXS) to elucidate the relationship between the type of phases involved in the process of emulsification and the droplet size in the resulting emulsions. Nanometer-sized oil droplets as small as 220 nm in diameter were obtained when the sponge phase (L3 ) was formed at first, followed by the phase transition to coexisting multiple phases including the micellar cubic phase (I1 ) with increasing vegetable oil content. Sugars expanded the area of the sponge phase toward lower MOPS contents, enabling the formation of nano-emulsions from a wider range of the initial composition. The area of the sponge phase increased in the order of d-fructose ≈ d-glucose < sucrose < d-maltose, consistent with the order of the literature value of the mean number of equatorial hydroxyl groups per sugar molecule and that of the hydration number of sugar that represents the average number of water molecules forming a complex with a single molecule of sugar in aqueous solution. The present results confirm that sugars facilitate the formation of nano-emulsions using the isothermal low-energy emulsification method, presumably due to their abilities to shift the effective hydrophile-lipophile balance (HLB) of the surfactant toward the hydrophobic side.

Self-diffusion of water and poly(amidoamine) dendrimers in dilute aqueous solutions

Medium generation PAMAM dendrimers are extensively researched as drug delivery vehicles therefore detailed knowledge of their physico-chemical properties in solution is vital. We have selected ethylenediamine core, generation five poly(amidoamine) (PAMAM_E5) dendrimers (amine terminated PAMAM_E5.NH2 and its succinamic acid derivative PAMAM_E5.SAH, respectively) as model compounds to study their dynamic behavior in water as a function of pH and concentration. Diffusion coefficients of water and the dendrimers were determined in deuterium oxide using pulse-field gradient stimulated echo (PGSE) NMR. The diffusion rate for PAMAM_E5.NH2 increased monotonously with increasing pH values while for PAMAM_E5.SAH a maximum was found at an isoelectric value of pH = 5.7. The apparent diffusion coefficients of dendrimers decreased linearly with increasing concentration measured at their self-pH (pH = 9.4 for PAMAM_E5.NH2 and pH = 5.7 for PAMAM_E5.SAH) in the absence of added salts. The observations could be explained by considering hard sphere interactions between strongly hydrated dendrimer molecules. The average hydrodynamic radii of dendrimers were determined by extrapolating the measured diffusion coefficients to zero dendrimer concentration and applying the Stokes–Einstein equation. The calculated values were RH = 3.05 ± 0.04 nm for PAMAM_E5.NH2 and RH = 3.37 ± 0.08 nm for PAMAM_E5.SAH respectively. Measured diffusion coefficients of water (D2O) also decreased linearly with increasing dendrimer concentration. From the concentration dependence, the average number of water molecules that form one dynamic unit with one macromolecule could be calculated on the basis of three different obstruction models. It has been concluded that hydrated PAMAM dendrimers in aqueous solutions behave as soft colloids against solvent molecules but as hard-sphere colloids against each other. Their equivalent hard-sphere radii were found to be equal to the measured hydrodynamic radii.

The hydration of aniline and benzoic acid: Analysis of radial and spatial distribution functions

Structural aspects of the hydration of aniline and benzoic acid in water were studied with 1D- and 3D-RISM integral equation methods. Analysis of the structural data reveals that the first hydration shell of aniline consists of ~26 water molecules, whereas benzoic acid is hydrated by 28.4 H2O. For aqueous aniline the average number of water molecules surrounding the –NH2 group is 6.7 and on average ~1.8 H-bonds are formed by the amino group. In aqueous benzoic acid the hydration shell of the –COOH group consists of 6.3 water molecules surrounding the carbonyl oxygen and of 6.1 water molecules around the hydroxyl group. The average number of H-bonds formed by the carboxylic group is 2.85. For both systems water molecules are preferably located above and below the plane defined by the phenyl ring at a distance of ~0.3nm from the ring center. The number of the π-coordinated water molecules is 1.16 for aniline and 1.2 for benzoic acid. It was found, that the π-region of aniline is more H-bonded with water than the π-region of benzoic acid.

Structural parameters of glycine zwitterion hydration from the data of the integral equation method in the RISM approach

The structural parameters of glycine zwitterion in water were studied by means of the integral equation method in the framework of the RISM approximation. According to calculations, five water molecules are located in the nearest environment of the -NH3+ group, and two of them are the H-bonded with this group. At the same time, six water molecules are located in the nearest environment of the −COO− group, and three of them are the H-bonded with this group. The average number of water molecules in the first hydration shell of −CH2 group is four. It has been shown that the probability of hydrogen bond formation between water molecules and the hydrogen atom H1 of the −NH3+ group is low, and there is no H-bonding between water molecules and the nitrogen atom the −NH3+ group.

Protocol for classical molecular dynamics simulations of nano-junctions in solution

Modeling of nanoscale electronic devices in water requires the evaluation of the transport properties averaged over the possible configurations of the solvent. They can be obtained from classical molecular dynamics for water confined in the device. A series of classical molecular dynamics simulations is performed to establish a methodology for estimating the average number of water molecules N confined between two static and semi-infinite gold electrodes. Variations in key parameters of the simulations, as well as simulations with non-static infinite gold surfaces of constant area and with anisotropically fluctuating cell dimensions lead to less than 1% discrepancies in the calculated N. Our approach is then applied to a carbon nanotube placed between the gold electrodes. The atomic density profile along the axis separating the slabs shows the typical pattern of confined liquids, irrespective of the presence of the nanotube, while parallel to the slabs the nanotube perturbs the obtained profile.

On the Structure of Aqueous Cesium Fluoride and Cesium Iodide Solutions: Diffraction Experiments, Molecular Dynamics Simulations, and Reverse Monte Carlo Modeling

A detailed study of the microscopic structure of two electrolyte solutions, cesium fluoride (CsF) and cesium iodide (CsI) in water, is presented. For revealing the influence of salt concentration on the structure, CsF solutions at concentrations of 15.1 and 32.3 mol % and CsI solutions at concentrations of 1.0 and 3.9 mol % are investigated. For each concentration, we combine total scattering structure factors from neutron and X-ray diffraction and 10 partial radial distribution functions from molecular dynamics simulations in one single structural model, generated by reverse Monte Carlo modeling. For the present solutions we show that the level of consistency between simulations that use simple pair potentials and experimental structure factors is at least semiquantitative for even the extremely highly concentrated CsF solutions. Remaining inconsistencies seem to be caused primarily by water-water distribution functions, whereas slightly problematic parts appear on the ion-oxygen partials, too. As a final result, we obtained particle configurations from reverse Monte Carlo modeling that were in quantitative agreement with both sets of diffraction data and most of the MD simulated partial radial distribution functions. From the particle coordinates, distributions of the number of first neighbors as well as angular correlation functions were calculated. The average number of water molecules around cations in both materials decreases from about 8.0 to about 5.1 as concentration increases, whereas the same quantity for the anions (X) changes from about 5.3 to about 3.7 in the case of CsF and from about 6.2 to about 4.0 in the case of CsI. The average angle of X···H-O particle arrangements, characteristic of anion-water hydrogen bonds, is closer to 180° than that found for O···H-O arrangements (water-water hydrogen bonds) at higher concentrations.

Hydration of methylamine and methylammonium ion: structural and thermodynamic properties from the data of the integral equation method in the RISM approximation

The structural and thermodynamic properties of hydration of methylamine and methyl-ammonium ion were investigated by the integral equations method in the RISM approximation. According to calculations, the average number of water molecules in the first hydration shell of CH3 group is 14.4 for aqueous methylamine and 12.7 for aqueous methylammonium solution. The first hydration shells of the NH2 group of methylamine and the NH3 + group of methylammonium ion contain 6.9 and 5.6 water molecules, respectively. The average number of H-bonds formed by the NH2 group is 2.4 and that formed by the NH3 + group is 3. The results obtained show no H-bonding between the nitrogen atom of NH3 + group of methylammonium and water molecules. The hydrogen atom of water participating in the hydrogen bonding with the nitrogen atom of methylamine now is a constituent of the NH3 + group of methylammonium ion. The hydration free energies and the ionization constant calculated within the framework of the RISM theory are in good agreement with experimental data.

Amphiphilic Behavior of New Cholesteryl Cyclodextrins: A Molecular Study

Amphiphilic cyclodextrins (CDs) are good candidates to functionalize natural membranes as well as synthetic vesicles. In this paper, we describe the synthesis of the amphiphilic permethylated monocholesteryl α-CD (TASC). Its interfacial behavior is compared with that of the permethylated mono- and dicholesteryl β-CD analogues (TBSC and TBdSC). Langmuir isotherms suggest a reorganization upon compression for all compounds, which is quantified using neutron as well as X-ray reflectivity. The in-plane structure is characterized by atomic force microscopy (AFM) on monolayers deposited on solid substrates. A model involving a reorientation of the CD with respect to the interface to adjust its conformation to the available area per molecule is proposed. Although we observe for TBSC a rearrangement similar to TASC and TBdSC, it is already achieved at lower surface pressures compared with its disubstituted derivative. This specific behavior is explained by an increased structural flexibility and compressibility compared with TBdSC and TASC. The average number of water molecules per CD was determined using the neutron data and validated from X-ray data, which also allows the determination of the CD's molecular volume. The permethylated CD molecules are strongly hydrated in the film, but the α-CD analogue is less hydrated than the β-CD derivatives, and hydration decreases with compression.

Hydration of acetic acid and acetate ion in water studied by 1D-RISM theory

Structural and thermodynamic aspects of the hydration of acetic acid (CH 3COOH) and acetate ion (CH 3COO −) were studied with the 1D-RISM integral equation method. It was found that the average number of water molecules in the hydrophobic hydration shell of the CH 3 group is 8.9 for acetic acid and 10 for acetate. The average number of H-bonds formed by the COOH group is 2.5, whereas that of COO − is 6, indicating that deprotonation of acetic acid leads to increased H-bonding of water molecules with the carboxylate moiety. This step involves significant reorientation of water molecules surrounding the carboxylic group. The hydration free energies and the aqueous ionization constant calculated with the set of semi-empirical corrections are in reasonable agreement with available experimental results.

Hydration structure of pyridine molecule studied by neutron diffraction with isotopic substitution method

Neutron diffraction measurements were carried out at 25 °C for aqueous 10 mol% pyridine heavy water solutions, (C 5 ∗H 5 ∗N) 0.1(D 2O) 0.9, in which the isotopic compositions 14N/ 15N and H Py/D Py (H Py: Pyridine hydrogen atom), were changed. The hydration structure of pyridine molecule was derived from the least squares fitting analysis of observed intermolecular first-order difference functions, Δ N inter( Q) and Δ H inter( Q). It was revealed that a pyridine molecule forms hydrogen bonds of the N⋯D W1 O W D W2 type (D W1, D W2: deuterium atoms in D 2O, O W: oxygen atom in D 2O) with 2.5(2) water molecules ( r N⋯DW1 = 1.95(1) Å, ∠N⋯D W1 O W = 174(11)°). The average number of water molecules around the hydrogen atom of the pyridine molecule was determined to be 1.2(1) with the average intermolecular H Py⋯D 2O distance of 2.82(1) Å.

Rational Design of Unimolecular Star Copolymer Micelles for Drug Delivery: Molecular Dynamics Study of Solvation, Aggregation, and Drug Binding Properties

Multimolecular micelles are excellent delivery vehicles with one major flaw: they spontaneously disassemble and release their cargo when the concentration of unimer falls below critical micelle concentration. One way to circumvent critical-micelle-concentration-based instabilities is to tether the unimers together at the center of the micelle and generate a unimolecular micelle. Star-shaped block copolymers (SCPs) represent a possible material for unimolecular micelles - as long as the molecules can be engineered to avoid self-aggregation. Amphiphilic SCPs, with central hydrophobic blocks surrounded by terminal hydrophilic blocks, can be used for the solubilization of hydrophobic solutes. With the intention of rationally designing a stable unimolecular SCP, we use atomistic molecular dynamics simulations in explicit solvent to systematically evaluate the solution properties of hydrated SCPs successively as unimers, at high concentration, and in the presence of a small molecule drug mimetic. In these studies, the average number of water molecules bound per PEG repeat unit was comparable to experimental results. As well, the water accessible surface area of the PCL core was highly correlated with the molecular weights of PCL and PEG moieties. We postulate that the propensity for aggregation of SCPs is due to hydration of hydrophobic moieties in the unimeric state. SCPs with a PCL core less than 2kDa per arm are predicted to be fully protected from water and may form thermodynamically stable unimolecular micelles at low concentrations when the PEG blocks approach 14.6kDa per arm. Accordingly, simulations of SCPs at high concentration confirm that aggregation reduces exposed hydrophobic surfaces. Finally, simulations of SCPs in the presence of small molecule drug mimetics are performed in an attempt to predict drug loading properties and the impact of drug loading on SCP aggregation.

Dielectric Relaxation and Water Activity in Aqueous Solution of D-Psicose

Hydration behavior of a rare sugar, D-psicose, was investigated based on measurements of the dielectric relaxation and the water activity in its aqueous solutions. Dielectric relaxation spectra in the frequency domain were obtained from time domain reflectometry. The dielectric relaxation time was then determined by fitting the spectra to a Cole-Cole type equation and used to evaluate the average hydration number representing the average number of water molecules forming a complex with a sugar molecule at infinite dilution. The concentration dependence of the dielectric relaxation time in aqueous solutions of all examined sugars generated a single master cur ve. The average hydration number of psicose was found to be identical with that of fructose. Only slight differences were noticeable among the concentration dependences of the water activity of monosaccharides. The equilibrium constant of hydration of sugar was calculated by considering material balances in an aqueous solution of sugar and using experimentally obtained values of the average hydration number and the water activity. The equilibrium constant values increased in the order of glucose < fructose � psicose < trehalose < sucrose < maltose. The present results suggest that both fructose and psicose interact with water in a similar manner.

Average Sequential Water Molecule Binding Enthalpies of M(H2O)19−1242+ (M = Co, Fe, Mn, and Cu) Measured with Ultraviolet Photodissociation at 193 and 248 nm

The average sequential water molecule binding enthalpies to large water clusters (between 19 and 124 water molecules) containing divalent ions were obtained by measuring the average number of water molecules lost upon absorption of an UV photon (193 or 248 nm) and using a statistical model to account for the energy released into translations, rotations, and vibrations of the products. These values agree well with the trend established by more conventional methods for obtaining sequential binding enthalpies to much smaller hydrated divalent ions. The average binding enthalpies decrease to a value of ~10.4 kcal/mol for n > ~40 and are insensitive to the ion identity at large cluster size. This value is close to that of the bulk heat of vaporization of water (10.6 kcal/mol) and indicates that the structure of water in these clusters may more closely resemble that of bulk liquid water than ice, owing either to a freezing point depression or rapid evaporative cooling and kinetic trapping of the initial liquid droplet. A discrete implementation of the Thomson equation using parameters for liquid water at 0 °C generally fits the trend in these data but provides values that are ~0.5 kcal/mol too low.

On the structure of aqueous cesium bromide solutions: Diffraction experiments, molecular dynamics simulations and Reverse Monte Carlo modeling

A detailed study of the microscopic structure of an electrolyte solution, cesium bromide (CsBr) in water, is presented. For revealing the influence of salt concentration on the structure, CsBr solutions at concentrations of 0.9, 4.5 and 7.6 mol% are investigated. For each concentration, we combine total scattering structure factors from neutron and X-ray diffraction and 10 partial radial distribution functions from molecular dynamics simulations in one single structural model, generated by Reverse Monte Carlo modeling. For the present solutions we show that the level of consistency between simulations that use simple pair potentials and experimental structure factors is at least semi-quantitative for even the concentrated solutions. Remaining inconsistencies seem to be caused primarily by water–water distribution functions, whereas slightly problematic parts appear on the ion–oxygen partials, too. As a final result, we obtained particle configurations from Reverse Monte Carlo modeling that were in quantitative agreement with both diffraction data and most of the MD simulated prdf's. From the particle coordinates, distribution of the number of first neighbors, as well as angular correlation functions were calculated. The average number of water molecules around cations decreases from about 7.5 to about 6 as concentration increases from 0.9 mol% to 7.6 mol%, whereas the same quantity for the anions changes from about 6.5 to about 5.5, while not all of these neighboring water molecules are H-bonded to the anion at the higher concentrations. The average angle of Br…H–O particle arrangements, characteristic to anion–water hydrogen bonds, is closer to 180° than that found for O…H–O arrangements (water–water hydrogen bonds) at higher concentrations.