Showing papers on "Solvation published in 1990"

Semianalytical treatment of solvation for molecular mechanics and dynamics

Abstract: sorption results9 revealed that the iron surface was mostly covered by promoter oxides of AI, Ca, and K. Postreaction XPS results also revealed a C( Is) XPS peak of weak to moderate intensity centered at 284.1-283.7 eV. This binding energy approaches those (ca. 283.5 eV) reported for iron cat bide^.^^*'^ More convincing evidence for carbide formation was obtained from TPHT results collected after reaction studies like those displayed in Figure 1 in which methane was the only product. After reaction at temperatures below 340 OC, only small amounts of reactive carbon could be distinguished with maximum methane desorption rates near 300 OC. However, for higher reaction temperatures, large amounts of methane were produced with a maximum rate just above 400 OC. Since XPS results revealed only small amounts of carbonaceous residue on top of the catalyst surface, this reactive carbon must be associated with carbiding of the catalyst. Consequently, it appears that the active carbon incorporation catalyst is carbided iron. This conclusion is well supported by bulk carbon to iron stoichiometries of 0.1-0.25 estimated from the TPHT peak areas which were adequate to represent 40-60'36 conversion to bulk carbides such as Fe,C or FeSC2. Moreover, preliminary results from studies using bona fide iron carbides have shown similar catalytic b e h a ~ i o r . ~

Relative partition coefficients for organic solutes from fluid simulations

Abstract: A procedure is noted for obtaining the difference in partition coefficients (log P) for two solutes between two solvents. Fluid simulations are required in which one solute is mutated to the other in both solvents, and the changes in free energies of solvation are computed. The method is illustrated for eight pairs of organic solutes partitioning between water and chloroform. Monte Carlo statistical mechanics simulations are used with statistical perturbation theory to calculate the requisite free energy changes

Common features of protein unfolding and dissolution of hydrophobic compounds

Abstract: Protein unfolding and the dissolution of hydrophobic compounds (including solids, liquids, and gases) in water are characterized by a linear relation between entropy change and heat capacity change. The same slope is found for various classes of compounds, whereas the intercept depends on the particular class. The feature common to these processes is exposure of hydrophobic groups to water. These observations make possible the assignment of the heat capacity change to hydrophobic solvation and lead to the description of protein stability in terms of a hydrophobic and a nonhydrophobic contribution. A general representation of protein stability is given by the heat capacity change and the temperature.

Solubility as a function of protein structure and solvent components.

Abstract: This review deals with ways of stabilizing proteins against aggregation and with methods to determine, predict, and increase solubility. Solvent additives (osmolytes) that stabilize proteins are listed with a description of their effects on proteins and on the solvation properties of water. Special attention is given to areas where solubility limitations pose major problems, as in the preparation of highly concentrated solutions of recombinant proteins for structural determination with NMR and X-ray crystallography, refolding of inclusion body proteins, studies of membrane protein dynamics, and in the formulation of proteins for pharmaceutical use. Structural factors relating to solubility and possibilities for protein engineering are analyzed.

Role of nuclear tunneling in aqueous ferrous–ferric electron transfer

Abstract: By computer simulation and also by analytical methods we have computed the nuclear tunneling enhancement of the rate for ferrous–ferric exchange in water. The model we have examined is the one studied earlier where we treated water as a classical liquid [R. A. Kuharski, J. S. Bader, D. Chandler, M. Sprik, M. L. Klein, and R. W. Impey, J. Chem. Phys. 89, 3248 (1988)]. But now we treat water quantum mechanically and find that the tunneling enhancement is a factor of 60 in the rate constant. Further, as observed experimentally, we find that the isotope shift on the rate when changing from D2O to H2O is approximately a factor of 2. The computer simulation aspects of these calculations employ path integral methods and a novel partitioning of the free energy associated with electron transfer. Our results show that it is insufficient to quantize only the atoms composing the ligands. The quantum dynamics of water molecules beyond the first solvation shell prove quite significant.

Cavities in molecular liquids and the theory of hydrophobic solubilities.

Abstract: Thermal configurational data on neat liquids are used to obtain the work of formation of hard spherical cavities of atomic size in six molecular solvents: n-hexane, n-dodecane, n-undecyl alcohol, chloroform, carbon tetrachloride, and water. These results are used to test a recent suggestion that the differences between nonaqueous solvents and liquid water in solvation of inert gases are not principally due to the hydrogen-bonded structure of liquid water but rather to the comparatively small size of the water molecule. The frequencies of occurrence of cavities in liquid water can be meaningfully distinguished from those in the organic solvents. Liquid water has a larger fractional free volume, but that free volume is distributed in smaller packets. With respect to cavity work, water is compared to a solvent of the same molecular density and composed of hard spheres of the same size as the water molecule. That comparison indicates that the hard-sphere liquid finds more ways to configure its free volume in order to accommodate an atomic solute of substantial size and thus, would be more favorable solvent for inert gases. The scaled particle model of inert gas solubility in liquid water predicts cavity works 20% below the numerical data for TIP4P water at 300 K and 1.0 g/cm3 for cavity radii near 2.0 angstroms. It is argued that the sign of this difference is just the sign that ought to be expected and that the magnitude of this difference measures structural differences between water and the directly comparable hard-sphere liquid. In conjunction with previous data, these results indicate that atomic sized cavities should be considered submacroscopic.

Strength of molecular complexation of apolar solutes in water and in organic solvents is predictable by linear free energy relationships: a general model for solvation effects on apolar binding

Abstract: La stabilite d'un complexe d'inclusion forme par un cyclophane macrobicyclique, compose hote et le pyrene est etudiee dans l'eau et dans differents solvants organiques, de differentes polarites. Determination des constantes d'association et des energies libres de formation du complexe dans les solvants utilises

Molecular basis for the Born model of ion solvation

Abstract: Statistical mechanical theory of aqueous solutions is used to provide an interpretation for the Born model of ion hydration at the molecular level. The analysis is based on an asymptotic solution to the extended RISM integral equation in the limit of a large spherical solute. It is shown that a construction of the appropriate direct correlation function yields the Born model for the electrostatic free energy of solvation. In this approximation the free energy of solvation equals half the average solute-solvent interaction energy

Preferential solvation in two- and in three-component systems

Abstract: The concepts of local compositions around a solute and preferential solvaton of a solute are defined in terms of the Kirkwood-Buff integrals. The difference between the local'and the bulk composition is a measure of the preferential solvation of a solute with respect to the various components of the solvent. A statistical mechanical theory is developed that leads to simple relationships between local compositions and experimentally measurable quantities. These are applied to both two- and three-component systems.

Solubility and solvation in mixed solvent systems

Abstract: Au cours de ce congres, des aspects theoriques et experimentaux sur la solubilite ont ete abordes

Surface Tension Measurements Show That Chaotropic Salting-In Denaturants Are Not Just Water-Structure Breakers

Abstract: Since the salting-in agents guanidinium chloride, urea, and lithium perchlorate increase the surface tension of water, the salting-in phenomenon does not reflect easier cavity formation in water. Therefore, these salting-in agents must be directly contributing to the solvation of a solute such as benzene in water, probably by a direct solvation interaction. The increased surface-tension effects do not overbalance these solvation effects since they are smaller than the large surface-tension increases with lithium chloride, a typical salting-out agent. The salting-in agent tetra-n-butylammonium chloride differs in that it lowers the surface tension of water. Thus, it probably contributes both to easier cavity formation and to direct solvation of the substrate. The previous findings that most salting-in agents switch to become salting-out agents in other polar solvents such as ethylene glycol and formamide but that tetra-n-butylammonium chloride does not switch in these solvents can be understood in terms of relative polarities.

Equilibrium and nonequilibrium solvation and solute electronic structure. I. Formulation

Abstract: A theoretical formulation is developed to describe the electronic structure of an immersed solute, electrostatically coupled to a polar and polarizable solvent. The solvent is characterized, in the dielectric continuum approximation, by electronic and orientational polarizations. Starting from a general free‐energy expression for the quantum solute–solvent system, a time‐independent nonlinear Schrodinger equation is derived. The nonlinearity arises from the assumed equilibration of the solvent electronic polarization Peqe, to the solute electronic wave function Ψ and the solvent orientational polarization Por. When Por is arbitrary, there is nonequilibrium solvation. When Por is equilibrated to Peqe and Ψ, equilibrium solvation obtains. The theory is illustrated for a model symmetric electron donor–acceptor solute system in a two state basis set description. Solution of the nonlinear Schrodinger equation in the presence of arbitrary Por yields nonequilibrium solvation stationary states (NSS) for the solut...

Free energies of cation-molecule complex formation and cation-solvent transfers

Abstract: The effects of molecular structure on gas phase basicity toward Lit has been extensive1y.stud ied. It is shown that both the magnitudes and the basicity orders differ markedly from the corresponding gas phase bacisities toward Ht. An analysis of these results is presented together with some preliminary comparisons of basicity orders observed in other adducts having largely e lectrostatic bonds, i.e., for Alt, Kt, Mn', and neutral hydrogen-bond donors. Implications to ionic solvation energies of the differing basicity orders are considered.

Solvation thermodynamics: An approach from analytic temperature derivatives

Abstract: A method is presented that uses integral equation theory to determine analytic temperature derivatives of the radial distribution functions. It is illustrated by studying the solvation thermodynamics of monatomic solutes in aqueous solution. The results agree well with the density derivative method developed previously [Yu and Karplus, J. Chem. Phys. 89, 2366 (1988)]. An expression for the solvation enthalpy is derived which allows direct comparison with experimental and isobaric–isothermal (NPT) ensemble simulation data. Satisfactory agreement with experiment is found for pure water and for the aqueous solvation of monovalent ions. Simple equations that exploit the site–site HNC closures are given for the decomposition of the potential of mean force into its enthalpic (or energetic) and entropic components. Since the extended RISM (HNC‐RISM) theory yields an incorrect (trivial) value of the dielectric constant, two different ways to correct for the asymptotic behavior of the solute–solute potential of me...

Solvent effects on protein association and protein folding

Abstract: Solvent effects on the thermodynamics of two processes--folding of proteins and association between proteins--are examined in detail. A complete inventory of the multitude of solvent effects may be obtained by employing the concept of conditional solvation free energy. This theoretical tool allows for the isolation of specific side-chain effects from the entire protein and for the study of its contribution to the overall free energy change in small model compounds. Some numerical examples are presented, and ways of estimating other cases, for which no relevant experimental data are available, are suggested. Our findings lead to the conclusion that the currently used hydrophobicity scales, based on partition coefficients between water and an organic solvent, are inadequate measures of the contribution of side chains being transferred from water to the interior of the protein. We have also tentatively concluded that correlation between hydrophilic functional groups might be more important than correlations between hydrophobic side chains.

Polarity of AOT micellar interfaces : use of the preferential solvation concepts in the evaluation of the effective dielectric constants

Abstract: Fluorescence lifetimes (τ F ) of 2-[p-(dimethylamino)phenyl]-3,3-dimethyl-3H-indole (1) have been extensively studied in a two-component mixture of p-dioxane/water and also in a three-component mixture of n-heptane/AOT (sodium bis(2-ethylhexyl) sulfosuccinate)/water We have applied the concept of «dielectric enrichment» to solute/mixed solvents systems in order to evaluate the effective dielectric constants at the AOT interfaces in n-heptane We have shown that the effective dielectric constant (D) at the interface increases from 23 (water molar ratio W=O) to approximately 90 before reaching a plateau at Wœ512

Protein model structure evaluation using the solvation free energy of folding.

Abstract: A systematic study of solvation free energy of folding for proteins with known crystallographic structures is presented. There is a linear relationship between the solvation free energy of folding and the protein size. This relationship, which can be rationalized by a simple model of chain folding, allows prediction of the solvation free energy of folding for proteins for which no high resolution structures are available. All misfolded structures analyzed show solvation free energies higher than predicted; however, some of the misfolded structures have values close enough to the predicted values so that one must be very careful when using such a criterion to check the correctness of a protein model.

On the n-pi blue shift accompanying solvation

Abstract: It is well-known that compounds containing lone pairs undergo a blue shift (higher energy) in their n-π* electronic transition upon solvation. The reason generally put forth to explain such a blue shift is that the lone pair, because of its spacial extent, interacts with the solvent, either weakly, as in aprotic solvents, or strongly through the direct formation of hydrogen bonds, as with water or alcohols. It is the purpose of this communication to suggest that this explanation is at best incomplete and, in many cases, incorrect

Solvation structure and the time‐resolved Stokes shift in non‐Debye solvents

Abstract: We develop a microscopic theory of the time‐resolved Stokes shift of a chromophore in a polar solvent which incorporates both non‐Debye dielectric relaxation and solvation shell structure. The present theory depends on the direct correlation function of the pure solvent, the measured frequency‐dependent dielectric constant, and a microscopically derived translational diffusion parameter. We compare the predictions of the theory given here to a variety of experimental results on solvation in protic and aprotic solvents. Good agreement with experiment is found. Our theory compares favorably with the dynamical mean spherical approximation (MSA) theory of time‐dependent solvation.

Picosecond energy transfer of vibrationally hot molecules in solution: Experimental studies and theoretical analysis

Abstract: The cooling of vibrationally hot azulene is studied in different solvents by picosecond spectroscopy. Excitation to the electronic S1 state generates molecules with a vibrationally hot ground state by rapid internal conversion. The subsequent cooling is monitored by the temperature‐dependent change of the S0–S1 absorption edge and occurs via interaction with the solvent on a time scale of several tens of picoseconds. A theoretical model of intermolecular energy transfer in the liquid phase is developed. The vibrational excess energy of azulene is transferred to the solvent molecules by isolated binary collisions, where the multimode vibrational system of the molecules is considered explicitly. The dissipation of energy within the solvent is simulated by the macroscopic conduction of heat. The temporal development of the vibrational temperature of the azulene molecules and the concommitant changes of absorption are calculated taking into account the properties of the specific solvent. The results of the th...

Real-time probing of reactions in clusters

Abstract: In this Communication we report our first study of real-time reaction dynamics in finite size clusters. The reaction is of the type AH + Sn, where the proton transfer (bimolecular) dynamics is examined as the acid AH is solvated with different number of molecules, n = 1,2,... etc. This is in continuation of out effort to study reaction dynamics in real-time [1], but now extending the scope of the previous collisionless (solvent free) condition to a range where condensed phase effects can play a role. Of particular interest to us is the condition at which solvation induces vibrational relaxation and modifies IVR. Real-time studies of reactions in clusters offer great opportunities for obtaining the rates directly [2] and for examining these solvation processes under controlled conditions in molecular beams. Such stepwise solvation by beam methods has been advanced for a variety of systems spanning small molecules [3], large molecules [4], hydrogen-bonded systems [5], and electrons [6].

Gas‐phase methanol solvation of Cs+ : Vibrational spectroscopy and Monte Carlo simulation

Abstract: The solvation of the cesium ion by methanol has been investigated by gas‐phase vibrational spectroscopy and Monte Carlo simulations of small ion clusters: Cs(CH3OH)+N, N=4–25. The solvated ions, generated by thermionic emission and a molecular‐beam source, have considerable amounts of internal energy. After excessive energy is dissipated by evaporation, quasistable cluster ions are mass‐selected for vibrational predissociation spectroscopy using a line‐tunable cw‐CO2 laser. Analysis of the vibrational spectra indicates that the first solvation shell about the cesium ion consists of ten methanol molecules. Larger Cs(CH3OH)+N (N>18) appear to have small clusters of methanol bound to the surface of a solvated ion. Monte Carlo simulations using pairwise interaction potentials at 200, 250, and 300 K have been performed on Cs(CH3OH)+N, N=6–16 and 25. The results from the simulations are consistent with the observed solvent shell size and suggest a significant role for hydrogen bonding in the larger solvated ion...

Solvent effects in molecular recognition

Abstract: Synthetic cyclophane hosts form stable and highly structured inclusion complexes with organic molecules in aqueous solutions. The solution geometries of these complexes are determined in a conformational analysis using Monte Car10 methods. Solvation-desolvation processes are a central factor in determining the stability of apolar inclusion complexes. The tight binding of small aromatic solutes in water is entropically unfavorable and is predominantly enthalpy-driven. A large part of the favorable enthalpy term for strong complexation in water results from its specific contributions. Electron donor-acceptor interactions stabilize complexes between electron-rich cyclophane hosts and electron-deficient aromatic substrates; however, they may be masked by specific solvation effects. Computer liquid phase simulations are undertaken to evaluate at a microscopic level the origin of such solvation effects. The progress in the modeling studies is described. Apolar complexation also occurs in organic solvents. Solvents like 2,2,2-trifluoroethanol and ethylene glycol come close to water in their ability to promote apolar complexation. Binding strength decreases from water to polar protic to dipolar aprotic and to apolar solvents. Complexation strength in solvents of all polarity including water and in binary aqueous solvent mixtures is predictable according to a linear free energy relationship between the complexation free energy and the empirical solvent polarity parameter ET(30).

Molecular dynamics simulation of time-resolved fluorescence and nonequilibrium solvation of formaldehyde in water

Abstract: The results of molecular dynamics simulations of nonequilibrium solvation and time-resolved fluorescence from the first excited singlet state of formaldehyde in water are reported. The laser excitation is simulated by instantaneously switching the solute charges and force constants from ground- to excited-state values during the course of a molecular dynamics simulation of the ground-state species in water. Eighty nonequilibrium trajectories were completed, starting from different configurations obtained from the ground-state simulation. The fluorescence shift and solvation relax nonexponentially on a very fast time scale. The major portion of the relaxation occurs within 100 fs; additional relaxation is observed up to 1 ps. Time-dependent radial and angular distribution functions for solute-solvent interactions have been calculated. The relaxation is dominated by changes in the structure of the first solvation shell. Translational motion of the solvent provides the major mechanism for the relaxation. The relationship of the present computer experiments to theories of nonequilibrium solvation is discussed.

Molecular dynamics simulation of single ions in aqueous solutions: effects of the flexibility of the water molecules

Abstract: We performed molecular dynamics simulations of single Na’ and F ions in aqueous solutions. Two single point charge water models with and without internal degrees of freedom were considered. Structural (radial distribution functions, orientation angles), dynamical (translational, vibrational, and reorientational motions), and other microscopic properties (hydration numbers, residence times) of ions and water molecules of their hydration shell were calculated. Our results are compared both with experimental data and with other simulation results using different interaction models. The influence of the flexibility of water molecules on the different properties is carefully discussed.

Picosecond mass‐selective measurements of phenol‐(NH3)n acid–base chemistry in clusters

Abstract: The rate of proton transfer from the acidic S1 state of phenol to the basis solvent (NH3)n was measured as a function of solvent cluster size n. A distinct reaction threshold was observed for solvent size n=5 for 266 nm picosecond excitation. The proton transfer rate was measured to be ka=(60±10 ps)−1 for n=5–7. A competitive recombination rate of k−a =(350±100 ps)−1 occurs for n=5. Additional solvation stabilizes the product side causing the reaction enthalpy and consequently k−a to decrease. No evidence of proton transfer was observed when phenol was seeded in the less basic solvent clusters (CH3OH)n and (H2O)n.