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The structure of the film formed by oleylpropanediamine (OLDA) on the copper (Cu) metal surface in water at 150 °C was investigated by combining quantitative NMR and surface characterization methods. We succeeded in quantifying the amount of film formation by precisely determining all mass balances in the systems examined. 2D IR microscopic mapping showed that the film thickness is uneven in the horizontal direction with a length scale of ~100 μm and hundreds of OLDA layers. This film thickness was also confirmed by AFM. The analysis of the C–H stretching vibrational frequency disclosed that the alkyl chains are highly ordered in the layers close to the Cu surface and are conformationally disordered in the layers distant from the Cu surface in the thicker portion of the film. Combining XPS measurements using argon gas cluster ion beam etching with the ICP-AES analysis, we revealed that the key to multiple layering is the formation of a coordination complex of the unprotonated amino groups of OLDA with Cu that presumably results in polymer chain-like network structures. Contact angle measurements at different OLDA concentrations and treatment times showed that the water repellency of the film originated from the thick layering of OLDA molecules with disordered hydrophobic chains.

A 14N and 1H NMR spectroscopic study 1 was carried out to shed light on the microscopic aspects of the reaction of model alkylamines at a supercritical temperature of 400 °C. It is disclosed that NH3 and ROH (R = CH3CH2 and CH3 (CH2)3) are initially produced from the hydrolysis of ethylamine and butylamine, respectively. When the water density is doubled from 0.2 g cm−3, the pseudo-first-order reaction rate is markedly enhanced beyond the linear response. It suggests that the transition state of C-N bond cleavage is in a dipolar (ionic) state that can be more stabilized due to the many-body solvation by highly polar water molecules at a higher density.

Decarbonization of the energy system is a key goal of the Paris Agreements, in order to limit temperature rises to under 2° Celsius. Hydrogen has the potential to play a key role through its versatile production methods, end uses and as a storage medium for renewable energy, engendering the future low-carbon energy system. This research uses a global model cognizant of energy policy, technology learning curves and international carbon reduction targets to optimize the future energy system in terms of cost and carbon emissions to the year 2050. Exploring combinations of four exploratory scenarios incorporating hydrogen city gas blend levels, nuclear restrictions, regional emission reduction obligations and carbon capture and storage deployment timelines, it was identified that hydrogen has the potential to supply approximately two percent of global energy needs by 2050. Irrespective of the quantity of hydrogen produced, the transport sector and passenger fuel cell vehicles are consistently a preferential end use for future hydrogen across regions and modeled scenarios. In addition to the potential contribution of hydrogen, a shift toward renewable energy and a significant role for carbon capture and storage is identified to underpin carbon target achievement by 2050.

The effect of rotations on the line shape of the bending vibrational spectrum for supercritical water was analyzed using classical molecular dynamics simulation for the flexible point-charge SPC/Fw model. The experimental infrared spectrum of the bending mode at the low densities of 0.01–0.04 g·cm−3 and at 400 °C was essentially reproduced without any other assumptions. The spectrum line shape at low densities consists of two broad rotational bands due to the rotational couplings, as in the case of the O–H stretch mode. This is due to the time-scale separation breakdown but is not due to the presence of any definite clusters. The rotational couplings become more significant at higher temperatures. The separations between the bending band center and the rotational broad side-bands are found to be linearly correlated with the inverse of the total moment of inertia of the water isotopic species, which is clear molecular-level evidence for the rotational couplings.

The self-diffusion coefficients of water (Dw) and cyclohexane (Dch) in their binary mixtures were determined using the proton pulsed field gradient spin-echo method from medium to low densities in subcritical and supercritical conditions. The density (ρ), temperature (T), and water mole fraction (xw) are studied in the ranges 0.62–6.35 M (M = mol dm−3), 250–400 °C, and 0.109–0.994, respectively. A polynomial fitting function was developed for a scaled value of Ξ = ρDT−1/2 with ρ, T, and xw as variables in combination with a comprehensive molecular dynamics (MD) simulation. The NMR and MD results agree within 5% for water and 6% for cyclohexane, on average. The differences between Dw and Dch in the dependence on ρ, T, and xw are characterized by the activation energy Ea and the activation volume ΔVΞ‡ expressed by the scaled fitting function. The decrease in the ratio Dw/Dch and the increase in the Ea of water with increasing xw are related to the increase in the number of hydrogen bonds (HBs). The Dw value for a solitary water molecule at a low xw is controlled by the solvation shell, most of which is occupied by nonpolar cyclohexane molecules that provide less friction as a result of weaker interactions with water. A microscopic diffusion mechanism is discussed based on an analysis of the HB number as well as the first-peak height of the radial distribution functions that are taken as measures of the potential of the mean field controlling self-diffusion.The self-diffusion coefficients of water (Dw) and cyclohexane (Dch) in their binary mixtures were determined using the proton pulsed field gradient spin-echo method from medium to low densities in subcritical and supercritical conditions. The density (ρ), temperature (T), and water mole fraction (xw) are studied in the ranges 0.62–6.35 M (M = mol dm−3), 250–400 °C, and 0.109–0.994, respectively. A polynomial fitting function was developed for a scaled value of Ξ = ρDT−1/2 with ρ, T, and xw as variables in combination with a comprehensive molecular dynamics (MD) simulation. The NMR and MD results agree within 5% for water and 6% for cyclohexane, on average. The differences between Dw and Dch in the dependence on ρ, T, and xw are characterized by the activation energy Ea and the activation volume ΔVΞ‡ expressed by the scaled fitting function. The decrease in the ratio Dw/Dch and the increase in the Ea of water with increasing xw are related to the increase in the number of hydrogen bonds (HBs). The Dw value...

We have investigated the assembly of a two-dimensional coordination polymer, Nd(2)(C(6)H(2)N(2)O(4))(2)(C(2)O(4))(H(2)O)(2), that has been prepared from the hydrothermal reaction of Nd(NO(3))(3)·6H(2)O and 2,3-pyrazinedicarboxylic acid (H(2)pzdc). In situ oxalate formation as observed in this system has been been investigated using (1)H and (13)C nuclear magnetic resonance spectroscopy, and a pathway for C(2)O(4)(2-) anion formation under hydrothermal conditions has been elucidated. The oxalate ligands found in Nd(2)(C(6)H(2)N(2)O(4))(2)(C(2)O(4))(H(2)O)(2) result from the oxidation of H(2)pzdc, which proceeds through intermediates, such as 2-pyrazinecarboxylic acid (2-pzca), 2-hydroxyacetamide, 3-amino-2-hydroxy-3-oxopropanoic acid, 2-hydroxymalonic acid, 2-oxoacetic acid (glyoxylic acid), and glycolic acid. The species are generated through a ring-opening that occurs via cleavage of the C-N bond of the pyrazine ring, followed by hydrolysis/oxidation of the resulting species.

By applying 13C and 1H NMR spectroscopy the pyrolysis of site-selectively 13C-enriched (H313CO12C6H5) and normal anisole compounds was studied in the dark at 0.001–1.0 M (M, mol dm−3) and at 400–60...

Self-diffusion coefficients D for water, benzene, and cyclohexane were determined in high-temperature conditions along the liquid branch of the coexistence curve and in supercritical conditions including an extremely low density region. The diffusion data available in literature were compared and evaluated. A fifth-order polynomial for ln D with the single variable T−1 (ln(D/10−9 m2·s−1) = a0+ a1x + a2x2 + a3x3 + a4x4 + a5x5 with x = 1000/(T/K)) was found to provide good fitting along the liquid branch of the coexistence curve. A single polynomial function for the scaled quantity ρD/T1/2 with the two variables, density ρ and temperature T−1 (third-order polynomial of ρ and T−1 and the cross terms), can universally represent the diffusion data over a wider range including both the gas−liquid coexistence and the extremely low density conditions. The function gives a reliable and reasonable behavior of D in the medium-density supercritical states in which the experimental uncertainty is rather large due to t...

The equilibrium for the reversible decomposition of formic acid into carbon dioxide and hydrogen is studied in the ionic liquid (IL) 1,3-dipropyl-2-methylimidazolium formate. The equilibrium is strongly favored to the formic acid side because of the strong solvation of formic acid in the IL through the strong Coulombic solute-solvent interactions. The comparison of the equilibrium constants in the IL and water has shown that the pressures required to transform hydrogen and carbon dioxide into formic acid can be reduced by a factor of approximately 100 by using the IL instead of water. The hydrogen transformation in such mild conditions can be a chemical basis for the hydrogen storage and transportation using formic acid.

Chemical reactions in super- and subcritical water have been studied in our group over the decades, and recent advances are reviewed. The reaction mechanism in hydrothermal conditions is disclosed for ether and aldehyde in general form, and a new type of C-C bond formation is discovered in connection to the chemical evolution on primitive earth. Toward a new-generation hydrogen-fuel technology, it is proposed on the basis of physico-chemical reaction properties of formic acid that formic acid acts as a chemical tank for hydrogen storage and transportation. The hot-water chemistry is further discussed in the contexts of energy and environmental concerns, and its role in establishing green chemistry is stressed.

Self-diffusion coefficients D for benzene and cyclohexane are determined by using the proton pulsed-field-gradient spin echo (PGSE) method at high temperatures of 150–400 °C and in the low-density (0.007–0.21 and 0.005–0.15 g cm − 3 for benzene and cyclohexane, respectively), one-phase region. For benzene, the density-diffusivity product in the zero-density limit divided by the square root of the absolute temperature, ( ρD ) 0 / T is smaller than that of the hard sphere model at 150–400 °C and decreases with lowering temperature as a reflection of the attractive interaction effect. The ( ρD ) 0 / T for cyclohexane is comparable to the hard-sphere value and its temperature dependence is smaller than that for benzene, showing that the effect of the attractive interaction is stronger for benzene than for cyclohexane. The dynamic effect of the hydrogen bonding is discussed through comparison to our previous results on water [J. Chem. Phys., 125, 074307 (2006); J. Chem. Phys. 126, 089901 (2007)].

A sensitive in situ NMR spectroscopic method for detecting acids contaminating ionic liquids (ILs) has been developed. The chemical shift and the spectral width of water added to ILs were used as indicators to measure the impurity acid level. Owing to the high resolution power of NMR, the detection limit is below the level of 10(-3) mol kg(-1). A new method is applicable to a number of commonly used ILs such as the imidazolium- and ammonium-based ILs except for those composed of acidic cations or anions. The method was utilized to monitor the purification efficiency in the recrystallization of a typical hydrophilic IL, 1-butyl-3-methylimidazolium methanesulfonate from acetone. It was demonstrated that impurity acids can be almost perfectly removed by single or double recrystallization.

Reaction pathways and kinetics of C2 (carbon-two) aldehyde, acetaldehyde (CH3CHO), and formic acid HCOOH or HOCHO, are studied in neutral and acidic subcritical water at 200-250 degrees C. Acetaldehyde is found to exhibit (i) the acid-catalyzed C-C bond formation between acetaldehyde and formic acid, which generates lactic acid (CH3CH(OH)COOH), (ii) the cross-disproportionation, where formic acid reduces acetaldehyde into ethanol, and (iii) the aldol condensation. The lactic acid formation is a green C-C bond formation, proceeding without any organic solvents or metal catalysts. The new C-C bond formation takes place between formic acid and aldehydes irrespective of the presence of alpha-hydrogens. The hydrothermal cross-disproportionation produces ethanol without base catalysts and proceeds even in acidic condition, in sharp contrast to the classical base-catalyzed Cannizzaro reaction. The rate constants of the reactions (i)-(iii) and the equilibrium constant of the lactic acid formation are determined in the temperature range of 200-250 degrees C and at HCl concentrations of 0.2-0.6 M (mol/dm(3)). The reaction pathways are controlled so that the lactic acid or ethanol yield may be maximized by tuning the reactant concentrations and the temperature. A high lactic acid yield of 68% is achieved when acetaldehyde and formic acid are mixed in hot water, respectively, at 0.01 and 2.0 M in the presence of 0.6 M HCl at 225 degrees C. The ethanol yield attained 75% by the disproportionation of acetaldehyde (0.3 M) and formic acid (2.0 M) at 225 degrees C in the absence of added HCl.

Neutron diffraction measurements were carried out on H/ D isotopically substituted water in the low-density supercritical condition (T = 673 K, P = 26.3 MPa, and rho = 0.0068 molecules.A-3) in order to obtain direct information on the intermolecular partial structure functions, gHH inter(r), gOH inter(r), and gOO inter(r). In correspondence to the high-density supercritical water previously reported, the intermolecular nearest neighbor peaks in gHH inter(r), gOH inter(r), and gOO inter(r) are diffuse compared with the ambient ones. The nearest neighbor O...O distance (3.3 A) and its coordination number (2.6) were determined from the observed gOO inter(r). These results indicate that the orientational correlation between neighboring water molecules is considerably lost in low-density supercritical water. Small clusters involving a few water molecules are preferentially formed in low-density supercritical water, which is consistent with results obtained by previous IR and NMR studies.

High-sensitivity Raman vibrational spectroscopic equipment was developed to study the hydrogen-bonding structure in supercritical fluids over a wide density range. Supercritical water was investiga...

The chemical equilibrium between formaldehyde (HCHO) and its hydrated form, methanediol (CH2(OH)2), is computationally investigated in solvent water. Using the method of energy representation, the solvation free energy of methanediol is evaluated as a function of (solvent) density and temperature, and the solvent effect is discussed over a wide range of thermodynamic conditions. It is shown, in agreement with the previously reported tendency, that the solvation free energy of methanediol depends strongly on the temperature and is not monotonic against the density variation at a fixed supercritical temperature. In the hydration reaction of formaldehyde, the solvent water favors methanediol more at a lower temperature. The formation of methanediol is unfavored in the absence of solvent, and formaldehyde in hot water above ∼ 200 °C is predominantly present in the unhydrated form.

Self-diffusion coefficients (D) are measured for normal (nondeuterated) and deuterated cholesterol-d(6) (C26 and C27 methyl groups deuterated) in 1-octanol, chloroform, and cyclohexane at concentrations of 1-700 mM by varying the impurity water concentration (2 mM) and temperature (30-50 degrees C). The pulsed field gradient spin-echo (PGSE) (1)H and (2)H NMR were used, respectively, at 600 and 92 MHz. At 30 degrees C, the hydrodynamic radius (R) obtained at 20 mM from the D value and solvent viscosity is 5.09, 7.07, and 6.17 A, respectively, in 1-octanol, chloroform, and cyclohexane when the impurity water is negligible. The R value in 1-octanol is the smallest and comparable with the average length of the molecular axes for the cholesterol molecule. In 1-octanol, R is invariant against the concentration variation, whereas in chloroform, R is larger and increases almost linearly with cholesterol concentration. At the highest concentration, 700 mM, the R in chloroform is 13.5 and 16.7 A, respectively, when the impurity water is at negligible and saturated concentrations. The R value larger than that in hydrogen-bonding 1-octanol indicates that cholesterol forms an aggregate through hydrogen bonding. The aggregate structure is confirmed by comparing NOESY spectra in chloroform and 1-octanol. The NOESY analysis reveals the presence of one extra cross peak (C4-C19) in chloroform compared to 1-octanol. Because the carbon atoms related to the cross peak are close to the hydroxyl group (C3-OH), cholesterol molecules are considered to be not piled but are found to be OH-centered in the aggregate. This is supported also by larger rotational hydrodynamic radii measured on cholesterol deuterated at positions C2, C3, C4, and C6. This shows that the aggregate formation is driven by the hydrogen-bonding between cholesterol molecules.

Extremely hindered bis(germacyclopropa)benzenes (3a,b; the IUPAC name is 4,8-digermatricyclo[5.1.0.03,5]octa-1,3(5),6-triene) were synthesized as stable crystalline compounds by the reaction of the corresponding dilithiogermane Tbt(Dip)GeLi2 (8; Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Dip = 2,6-diisopropylphenyl) with 1,2,4,5-tetrabromobenzene. The structures of the two stereoisomeric bis(germacyclopropa)benzenes (3a, cis isomer; 3b, trans isomer) were definitively determined by X-ray crystallographic analysis. The central benzene ring of 3a was found to be folded, in contrast to the planar benzene ring of 3b. The X-ray crystallographic analyses of 3a and 3b and the theoretical calculations for some model molecules revealed that the annelated benzene rings have no distinct bond alternation.

Natural-abundance 13C and high-field-gradient NMR at 600 MHz were applied to shed light on the binding site and the mobility of anesthetics in lipid bilayer membranes. A high-power pulsed-field-gradient probe was specially designed for the reliable monitoring of molecular motions in a viscous membrane environment. A newly developed sensitive probe for the 10 mm diameter tube was also used for the natural abundance 13C NMR measurement. Large unilamellar vesicles, rather than micelles, were adopted as the most appropriate model for cell membranes in view of the dynamical structure. The binding of ethanol, chloroform, and sevoflurane in membranes was not deep but shallow at the membrane interface whose polarity is intermediate between water and the hydrophobic core. The anesthetics moved much faster than the lipid matrices in membrane. The movement was in sharp contrast to the slow motion of endocrine disruptors, the mobility comparable with the membrane lipid diffusion.

The energy representation of the molecular configuration in a dilute solution is introduced to express the solvent distribution around the solute over a one-dimensional coordinate specifying the solute–solvent interaction energy. On the basis of the energy representation, an approximate functional for the solvation free energy of a solute in solution is constructed by adopting the Percus-Yevick-type approximation in the unfavorable region of the solute–solvent interaction and the hypernetted-chain-type approximation in the favorable region. The solvation free energy is then given exactly to second order with respect to the solvent density and to the solute–solvent interaction. It is demonstrated that the solvation free energies of nonpolar, polar, and ionic solutes in water are evaluated accurately and efficiently from the single functional over a wide range of thermodynamic conditions. The extension to a flexible solute molecule is straightforward. The applicability of the method is illustrated for solute molecules with a stretching or torsional degree of freedom.

The effect of solvent density on the single-molecular reorientational relaxation of deuterated methanol and benzene in supercritical methanol are investigated both by the nuclear magnetic resonance (NMR) experiment and the molecular dynamics (MD) simulation. In the NMR experiment, the rank-2 reorientational relaxation time is determined from the spin-lattice relaxation time of deuterons. The relaxation time of the O – D vector of methanol is an increasing function of density above 0.3 ρ c , where ρ c stands for the critical density, while that of the C – D vector of benzene shows a shallow minimum around ρ c . To clarify the origin of the difference, MD simulations of both neat methanol and benzene in methanol are performed. The MD simulation revealed that the reorientational relaxation of the O – D vector of methanol is diffusive above 0.3 ρ c , whereas the effect of the inertial motion is strong on the reorientational relaxation of benzene.

Noncatalytic reaction pathways and rates of diethyl ether in supercritical water are determined in a quartz capillary by observing the liquid- and gas-phase 1H and 13C NMR spectra. The reaction is investigated at two concentrations (0.1 and 0.5 M) in supercritical water at 400 degrees C and over a water-density range of 0.2-0.6 g/cm3, and in subcritical water at 300 and 350 degrees C. The neat reaction (in the absence of solvent) is also studied for comparison at 0.1 M and 400 degrees C. The ether is found to decompose through (i) the proton-transferred fragmentation to ethane and acetaldehyde and (ii) the hydrolysis to ethanol. Acetaldehyde from reaction (i) is consecutively subjected to the unimolecular and bimolecular redox reactions: (iii) the unimolecular proton-transferred decarbonylation forming methane and carbon monoxide, (iv) the bimolecular self-disproportionation producing ethanol and acetic acid, and (v) the bimolecular cross-disproportionation yielding ethanol and carbonic acid. Reactions (ii), (iv), and (v) proceed only in the presence of hot water. Ethanol is produced through the two types of disproportionations and the hydrolysis. The proton-transferred fragmentation is the characteristic reaction at high temperatures and is much more important than the hydrolysis at densities below 0.5 g/cm3. The proton-transferred fragmentation of ether and the decarbonylation of aldehyde are slightly suppressed by the presence of water. The hydrolysis is markedly accelerated by increasing the water density: the rate constant at 400 degrees C is 2.5 x 10(-7) s(-1) at 0.2 g/cm3 and 1.7 x 10(-5) s(-1) at 0.6 g/cm3. The hydrolysis becomes more important in the ether reaction than the proton-transferred fragmentation at 0.6 g/cm3. In subcritical water, the hydrolysis path is dominant at 300 degrees C (0.71 g/cm3), whereas it becomes less important at 350 degrees C (0.57 g/cm3). Acetic acid generated by the self-disproportionation autocatalyzes the hydrolysis at a higher concentration. Thus, the pathway preference can be controlled by the water density, reaction temperature, and initial concentration of diethyl ether.

Here is reviewed our recent research on the title subject that has led to the Award of The Japan Society of High Pressure Science and Technology, 2004. In particular, we focus on the explanation of motivations and backgrounds for selecting the research subject as well as the scientific contents. First we show what has been thought to carry out experimental and theoretical investigations of the dynamic behaviors of electrolytes in water at high pressures. Second it is described how the high-pressure solution NMR probe has been developed to attain high resolution and sensitivity required for the study of very dilute aqueous solutions at high pressures. The newly developed high-pressure probe has enabled us to examine the dynamics of water in benzene and benzene in water; in solubility they hate each other. Third it is illustrated how our recent studies have been performed on the structure, dynamics, and reactions of supercritical water. Issues discussed here are related to the environmental and energy problems to be solved to make our life and earth sustainable in the 21st century.

Noncatalytic reaction pathways and their rates of acetaldehyde were determined in the neat system and in supercritical water at 400 °C and the density of 0.1−0.6 g/cm3. In supercritical water, acetaldehyde undergoes five types of reactions: (i) decarbonylation into methane and carbon monoxide, (ii) self-disproportionation producing ethanol and acetic acid, (iii) cross-disproportionation generating ethanol and carbonic acid, (iv) condensation forming crotonaldehyde, and (v) the subsequent polymerization of crotonaldehyde or its decarbonylated monomer. Reactions i and iv proceed irrespective of the presence of water, while water suppresses reaction i. Reactions ii and iii are characteristic of aldehyde under hydrothermal conditions. Although reaction ii produces the same products as the classical Cannizzaro reaction, it does not require any added catalysts. Reaction iii manifests the role of formic acid as a reducing aldehyde. Actually, it is shown that reaction iii involves a larger weight than reaction i...

A methodology has been proposed to compute the solvation free energy of a molecule described quantum chemically by means of quantum mechanical/molecular mechanical method combined with the theory of energy representation (QM/MM-ER). The present approximate approach is quite simple to implement and requires much less computational cost as compared with the free energy perturbation or thermodynamic integration. Furthermore, the electron distribution can be treated faithfully as a quantum chemical object, and it is no longer needed to employ the artificial interaction site model, a reduced form of the realistic electron distribution, which is commonly used in the conventional solution theory. The point of the present approach is to employ the QM solute with electron density fixed at its average distribution in order to make the solute-solvent interaction pairwise. Then, the solvation free energy can be computed within the standard framework of the energy representation. The remaining minor contribution originating from the many-body effect inherent in the quantum mechanical description can be evaluated separately within a similar framework if necessary. As a test calculation, the method has been applied to a QM water solute solvated by MM water solvent in ambient and supercritical states. The results of the QM/MM-ER simulations have been in excellent agreement with the experimental values.

The water−gas−shift (WGS) reaction (CO + H2O ⇄ CO2 + H2) is investigated in connection to formic acid. Using NMR spectroscopy, the reversible decomposition pathways of formic acid to both sides of the WGS reaction are studied in hot water at 240−260 °C. This reversibility strongly suggests that formic acid exists as an intermediate in the WGS reaction, and it is indeed demonstrated that carbon monoxide is treated in hot water to produce formic acid. The present result enables us to generate and store hydrogen in the liquid form of formic acid and to transform formic acid to hydrogen in water by tuning the thermodynamic conditions.

The site-selective H/D exchange reaction of phenol in sub- and supercritical water is studied without added catalysts. In subcritical water in equilibrium with steam at 210-240 degrees C, the H/D exchange proceeds both at the ortho and para sites in the phenyl ring, with no exchange observed at the meta site. The pseudo-first-order rate constants are of the order of 10(-4) s(-1); 50% larger for the ortho than for the para site. In supercritical water, the exchange is observed also at the meta site with the rate constant in the range of 10(-6)-10(-4) s(-1). As the bulk density decreases, the exchange slows down and the site selectivity toward the ortho is enhanced. The enhancement is due to the phenol-water interaction preference at the atomic resolution. The site selectivity toward the ortho is further enhanced when the reaction is carried out in benzene/water solution. Using such selectivity control and the reversible nature of the hydrothermal deuteration/protonation process, it is feasible to synthesize phenyl compounds that are deuterated at any topological combination of ortho, meta, and para sites.

Molecular mechanism of the binding of neuropeptide achatin-I (Gly-d-Phe-Ala-Asp) to large unilamellar vesicles of zwitterionic egg-yolk phosphatidylcholine (EPC) was investigated by means of natural-abundance 13C and high-resolution (of 0.01 Hz order) 1H NMR spectroscopy. The binding equilibrium was found to be sensitive to the ionization state of the N-terminal \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{NH}}_{3}^{+}\end{equation*}\end{document} group in achatin-I; the de-ionization of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{NH}}_{3}^{+}\end{equation*}\end{document} decreases the bound fraction of the peptide from ∼15% to nearly none. The electrostatic attraction between the N-terminal positive \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{NH}}_{3}^{+}\end{equation*}\end{document} group and the negative \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{PO}}_{4}^{-}\end{equation*}\end{document} group in the EPC headgroup plays an important role in controlling the equilibrium. Analysis of the 13C chemical shifts (δ) of EPC showed that the binding location of the peptide within the bilayer is the polar region between the glycerol and ester groups. The binding caused upfield changes Δδ of the 13C resonance for almost all the carbon sites in achatin-I. The changes Δδ for the ionic Asp at the C-terminus are more than five times as large as those for the other residues. The drastic changes for Asp result from the dehydration of the ionic \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{CO}}_{2}^{-}\end{equation*}\end{document} groups, which are strongly hydrated by electrostatic interactions in bulk water. The side-chain conformational equilibria of the aromatic d-Phe and ionic Asp residues were both affected by the binding, and the induced changes in the equilibria appear to reflect the peptide-lipid hydrophobic interactions.

The single-molecular reorientational relaxation of supercritical deuterated methanol and ethanol is studied by the 2 H nuclear magnetic resonance spin-lattice relaxation measurement, and the results are compared with the corresponding study on supercritical water (Matubayasi, N.; Nakao, N.; Nakahara, M. J. Chem. Phys. 2001, 114, 4107). The relaxation times of the OD deuterons of both alcohols show significant increase when they are transferred from liquid to supercritical phases. The reorientational relaxation of the OD vector is thus largely enhanced in the supercritical phase, as is the case of water. The relative increase in the reorientational relaxation rates of alcohols with density is larger than that of water. It indicates that the reorientational motion of supercritical alcohols is more diffusive than that of water. The molecular dynamics simulation of the supercritical methanol is also performed, and the results are consistent with experimental ones. The reorientational relaxation of two deuterated solute molecules, benzene (C 6 D 6 ) and pyrazine (C 4 D 4 N 2 ), is studied in supercritical methanol in addition to the neat fluid. The density dependence of the 2 H spin-lattice relaxation rates of methanol, benzene, and pyrazine in supercritical methanol show a minimum. The density of the minimum relaxation rate decreases with increasing solute-solvent interaction, which can be explained in terms of the angular-momentum relaxation time.

The method of energy representation for evaluating the solvation free energy is extended to a solute molecule with structural flexibility. When the intramolecular structure of the solute molecule exhibits a strong response to the solute–solvent interaction, the approximate functional for the solvation free energy needs to be modified from the original form presented previously [J. Chem. Phys. 117, 3605 (2002); 118, 2446 (2003)]. In the modification of the functional, the solvation-induced change in the distribution function of the solute structure is taken into account with respect to the intramolecular energy of the solute. It is then demonstrated over a wide range of thermodynamic conditions that the modified form of functional provides an accurate and efficient route to the solvation free energy of a flexible solute molecule even when the structural distribution function of the solute in solution overlaps barely with that of the solute at isolation.

The 13C chemical shifts δ of carbonyl (13CO) and nitrile (13C⋮N) compounds in aqueous solutions were measured as functions of temperature to study the hydration structure around dipolar solutes. The δ values showed an anomalous maximum in the temperature dependence; at −9 °C for urea, at 23 °C for diethyl ketone, at 24 °C for cyanamide, at 32 °C for acetone, at 63 °C for acetaldehyde, and at 96 °C for acetonitrile. No such anomaly was observed in any other organic solvents. The presence of the maximum is due to the competition of two factors, solute−solvent and solvent−solvent interactions. One is the electrostatic hydrogen bond between the solute and water, and the other is the hydrogen bond network characteristic of solvent water. When the network structure of water is collectively stabilized with decreasing temperature, the effective water−water interaction becomes stronger than the solute−water interaction, giving the maximum in the temperature dependence of δ. The temperature of the maximum chemical ...

A high-temperature and high-pressure NMR method to investigate the structure and dynamics of supercritical water is reviewed. In this method, a high-temperature condition is realized by introducing hot air directly into the sample portion of a specially designed probe, and a high pressure is achieved by sealing the material of interest into a quartz capillary. The method allows a high-resolution measurement up to 400°C and 0.6 g/cm3 (corresponding to 55 MPa) of water. It is found from the proton chemical shift measurement that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one in the supercritical densities. The measurement of the spin-lattice relaxation time also shows that while the reorientational relaxation proceeds on the order of picosecond in ambient water, it does on the order of several tens of femtoseconds in supercritical water.

Trans and gauche conformational equilibria in the side chains of aspartic acid (Asp) and asparagine (Asn) were investigated by measuring the vicinal spin−spin coupling constants of 1H NMR in acidic, neutral, and basic aqueous solutions over a wide range of temperature (5−90 °C). The standard free-energy changes ΔG0 were obtained for the trans to gauche conformational variations on the Cα−Cβ bond with respect to the α-carboxyl group and the β-carboxyl group in Asp (β-amide in Asn) and were decomposed into enthalpic ΔH0 and entropic −TΔS0 components. The hydration of ionic and polar groups in Asp competes against the large intramolecular electrostatic repulsion energy and stabilizes the gauche more than the trans conformer in correspondence to a larger degree of separation of positive and negative partial charges. In the neutral solutions, where both the carboxyl groups are negatively ionized, the hydration part even overwhelms the intramolecular repulsion and leads to a negative ΔH0. The fact that the hydr...

An approximate functional for the chemical potential of a solute in solution is presented in the energy representation. This functional is constructed by adopting the Percus–Yevick-like approximation in the unfavorable region of the solute–solvent interaction and the hypernetted-chain-like approximation in the favorable region. The chemical potential is then expressed in terms of energy distribution functions in the solution and pure solvent systems of interest, and is given exactly to second order with respect to the solvent density and to the solute–solvent interaction. In the practical implementation, computer simulations of the solution and pure solvent systems are performed to provide the energy distribution functions constituting the approximate functional for the chemical potential. It is demonstrated that the chemical potentials of nonpolar, polar, and ionic solutes in water are evaluated accurately and efficiently from the single functional over a wide range of thermodynamic conditions.

The nonpolar solvation dynamics in Lennard−Jones fluids is discussed in terms of the relationship with the dynamic structure factor of neat solvent, using the theoretical expression that describes the solvation correlation function as the superposition of solvent dynamic structure factors at various wavenumbers. Several expressions for the coupling strength between the state transition of the solute and the solvent density modes are examined with respect to their abilities to predict the static fluctuation. In the present theoretical model, it is found that the difference between the ground- and excited-state solute−solvent interactions can be adequately taken as the coupling strength with the solvent density mode. Employing this expression for the coupling, the solvent fluctuation around k ≅ σ-1 (σ stands for the diameter of the solvent) contributes dominantly to the static fluctuation in all the densities investigated. This corresponds to the feature of the solvation dynamics in mixed solvents, in which...

The validity of the Hubbard-Onsager (HO) continuum dielectric friction theory for the limiting ionic conductance is examined by its application to LiCl, NaCl, and KCl in sub- and supercritical water. At medium and higher densities (above 1.4 ϱ c = 0.45 g cm −3 ; ϱ c , the water critical density) and lower temperatures (below 0.9 T c = 300°C; T c , the critical temperature), the HO theory predicts an increase in the limiting conductance with increasing temperature and decreasing density. The HO theory explains the general trends of the experimental results along the liquid-vapor coexistence curve, the isochors, and the isotherms. The agreement between the theory and experiment becomes more quantitative at higher temperatures. This indicates that in these conditions the translational friction on a moving ion is dominated by the dielectric friction because the HO radius, the solvent scaling length, is larger than the ionic radius due to the small dielectric constant. At medium densities (slightly above 1.4 ϱ c ) and higher temperatures (above 0.9 T c ), the HO theory can reproduce the experimental observation that the limiting conductance increases with decreasing density with rather small temperature dependence. At lower densities (below 1.4 ϱ c ) and supercritical temperatures, however, the HO theory fails to reproduce the density dependence of the limiting electrolyte conductance. With respect to the density dependence, the experimental limiting conductance shows a maximum or plateau at ∼ 1.4 ϱ c , whereas the theoretical limiting conductance increases monotonously with decreasing density as in the higher density regions. Thus the continuum model becomes invalid at densities around and below the critical density of water ( ϱ c ).

Chloride ion solvation in supercritical water is investigated by computer simulations, including water polarizability explicitly. Comparisons are made between the TIP4P fluctuating charge and the original TIP4P models. Particular attention is paid to the density dependence of the equilibrium structural and transport properties. The chloride ion hydration number slowly decreases with density reduction in a similar way for both the fluctuating charge and the fixed-charge water models. The diffusion coefficients for the two models also exhibit a similar density dependence, except at the lowest density examined (0.05 g/cm3), where the chloride ion diffusion rate in polarizable water is significantly larger than that in nonpolarizable water because of a more rapid loss of the hydration shell in polarizable water at the lowest density. The remarkable similarity between the two models is related to the insensitivity of the local polarization in the first hydration shell to the bulk conditions. The results also s...

The adsorption of a cationic polyelectrolyte, poly(2-vinyl-1-methyl-pyridinium bromide), P2VP, on colloidal silica (0.15 μm radius) and the subsequent counterion binding of perfluorinated anionic surfactants, CF 3 CF 2 COONa, CF 3 (CF 2 ) 2 COONa, CF 3 (CF 2 ) 6 COONa, CF 3 (CF 2 ) 7 SO 3 Li and CF 3 (CF 2 ) 9 COOLi, were studied by electrophoresis. The zeta potential ( ζ -potential) of silica changed its sign from negative to positive with an increase in P2VP concentration at pH 4.0, 6.6, and 9.2. The fractional surface coverage of P2VP on silica ( θ ) was estimated from the zeta potentials of bare silica ( ζ 1 ) and silica fully covered with P2VP ( ζ 2 ), and the Langmuir adsorption model as a function of P2VP concentration and pH. The negative ζ 1 increased with increasing pH, whereas the positive ζ 2 was constant at all pH. The higher θ values at high pH suggested that the dominant interaction of the P2VP adsorption on silica was electrostatic. Surfactant anions did not adsorb onto the bare silica surface when pH>isoelectric point, but did adsorb onto a silica surface modified to saturation with P2VP. Electrokinetic measurements in the presence of 0.1 mM lower alkyl chain-length surfactants ( n ≤3) indicated that they behaved as indifferent electrolytes, with no surfactant adsorption detectable. The adsorption increased with the chain-length for 0.1 mM surfactants with >6 carbon chains, indicating specific binding. An increase in the concentration of CF 3 (CF 2 ) 6 COONa or CF 3 (CF 2 ) 7 SO 3 Li changed the sign of the ζ -potential of the P2VP modified silica surface from positive to negative. The mechanism of charge reversal was discussed in terms of the excess adsorption of the surfactant anions.

The natural abundance 13C NMR method was applied to directly determine the binding site of peptides and proteins in lipid bilayers and emulsions on the atomic level. Reliable NMR criteria for the location and depth of peptides and proteins in membranes were shown by the chemical shift and line width analyses, which reproduced not only the deep penetration of a transmembrane channel peptide gramicidin A but also the superficial binding of Ac-18A-NH2 (Ac-DWLKAFYDKVAEKLKEAF-NH2), a synthetic model peptide of amphipathic helices of plasma apolipoprotein A-I (apoA-I). The reliability was ensured by the NMR information, which was consistent with the recent X-ray diffraction study of Ac-18A-NH2 in oriented lipid bilayers (Hristova et al. J. Mol. Biol. 1999, 290, 99). Our method first provided the atomic-level evidence for native apoA-I binding in egg phosphatidylcholine (EPC) vesicles and triolein (TO)-EPC emulsions as spherical model lipoproteins. Membrane perturbation was most significant at EPC glycerol and e...

The effect of the solute–solvent and the solvent–solvent attractive interactions on the diffusion coefficient of the solute is studied by the mode-coupling theory. The mode-coupling part of the friction coefficient in our calculation is compared with that of molecular dynamics simulation. The theory qualitatively reproduces the increase in the friction coefficient with the solute–solvent attractive interaction, although the absolute value is larger than the simulation. A theoretical scheme is further provided to divide the memory function into attractive and repulsive components, and a mode-coupling approximation is introduced to both components separately. It is found that the solute–solvent interaction affects the attractive friction mainly, in agreement with the simulation. Further analyses reveal the presence of the large cross term of the attractive and repulsive random forces. When the solvent–solvent attractive interaction is weakened while keeping the solute–solvent interaction fixed, the mode-coupling friction is increased, which is opposite to the simulation results.

The binding maximum of apoA-1 (N) in triolein (TO)−egg yolk phosphatidylcholine (PC) emulsions was 10-fold larger than that in PC large unilamellar vesicles (LUV) of similar size (100 nm) with no significant difference in the affinity. Replacement of the long-chain triglyceride, TO, by medium-chain triglycerides or cholesteryl oleate in emulsion cores significantly decreased the N value. The 13C NMR chemical shifts of the PC carbonyl carbon at the surface layers indicated that PC polar headgroups are more separated and exposed to water molecules in emulsions than in vesicles. The N values were satisfactorily correlated with the chemical shift, that is, the degree of separation between the carbonyl groups at the surface. Although apoA-1 binding to the PC monolayers of emulsions brings about bending of the surface layers and creates local defects in the hydrocarbon regions in a similar manner as PC LUV, the surface−core interaction seems to fill the defects with the core neutral lipids, compensates for the ...

The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time T1 of heavy water (D2O). The experimentally determined T1 is shown to be governed by the quadrupolar relaxation mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time τ2R of the O–D axis of a single water molecule. It is then found that while τ2R decreases rapidly with the temperature on the liquid branch of the saturation curve, it remains on the order of several tens of femtoseconds when the density is varied up to twice the critical at a fixed supercritical temperature of 400 °C. The comparison of τ2R with the angular momentum correlation time shows that the rotational dynamics is not diffusive in supercritical water. The dependence of τ2R on the hydrogen bonding state is also examined in combination with molecular dynamics simulations, and the effect of the hydrogen bonding on the rotational dynamics in supercritical water is found to be weaker than but to be on the same order of magnitude as that in ambient water on the relative scale. Actually, although τ2R is divergent in the limit of zero density, it is observed to increase with the density when the density is above ∼1/3 of the critical.

Our recent works on supercritical water are reviewed. In order to elucidate the hydrogen bonding state of supercritical water, the proton chemical shift of the water proton is measured at temperatures up to 400 °C and densities of 0.19, 0.29, 0.41, 0.49, and 0.60 g/cm3. The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. The chemical shift is then related to the average number of hydrogen bonds in which a water molecule is involved. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one for a water molecule at the densities larger than the critical. The density dependence of the chemical shift at supercritical temperatures is analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities. The dipole moment of water at supercritical states is also estimated from the number of hydrogen bonds. The dynamical counterpart of our structural study of supercritical water has been performed by NMR relaxation measurements. Using D2O, we measured the spin-lattice relaxation time and determined the reorientational relaxation time as a function of the water density and temperature. It is then found that while the reorientational relaxation time decreases rapidly with the temperature in the subcritical condition, it is a weak function of the density in the supercritical conditions.

Silica surfaces were premodified by the saturated adsorption of the cationic polyelectrolyte of poly(2-vinyl-1-methyl-pyridinium bromide), P2VP, in water. The interaction forces between and the zeta potential of the silica surfaces were then measured in water and in ethanol solutions of perfluorinated anionic surfactants as a function of their chain length and concentration using the AFM surface force and electrophoresis methods. In water, the electrostatic repulsive forces between P2VP-modified silica surfaces in CF3CF2COONa, CF3(CF2)6COONa, and CF3(CF2)7SO3Li solutions of 0.1 mM were identical to the force curve in 0.1 mM NaNO3 but greatly decreased in 0.1 mM CF3(CF2)9COOLi (critical micelle concentration (cmc), 0.39 mM). The concentration increase of CF3(CF2)7SO3Li (cmc, 6.3 mM) from 0.1 to 1.0 mM caused the repulsive force curves to weaken and then to strengthen after passing through a zero repulsive force. The surface potentials obtained by the best curve fitting of the force curves agreed well with ...