Changes In Solvent Structure Research Articles

Kinetics of Aquation of Dichloro Tetrapyridine Ruthenium(Ii) Complex in Binary Aqueous Solvents

First-order solvolysis rates of trans-dichloro tetrapyridine ruthenium(II) have been measured UV spectrophoto metrically over a wide range of solvent compositions in temperature ranges(40–55°C) in water–2-propanol and water– t-butanol mixtures. The rate of solvolysis is faster in the former than in the latter. Plots of log (rate constant) versus the reciprocal of relative permitivity of the co-solvent gave a non-linear relation for both co-solvents, this non-linearity is derived from a large differential effect of solvent structure between the initial and transition states. Δ S# of activation correlates well with the extrema in physical properties of the mixtures which are related to changes in solvent structure. Linear plots of Δ H# versus Δ S# were obtained and the isokinetic temperature indicates that the reaction is entropy controlled.

Structural energetics of protein-carbohydrate interactions: Insights derived from the study of lysozyme binding to its natural saccharide inhibitors.

High-sensitivity isothermal titration calorimetry was used to characterize the binding of the glycohydrolitic enzyme hen egg-white lysozyme to its natural saccharide inhibitors, chitobiose and chitrotriose. Measurements were done at a pH of 4.7, in the 15 degrees C -45 degrees C temperature range. Using a structural-energetic parameterization derived previously for lectin-carbohydrate associations, both binding enthalpies and entropies for the present systems and for the complex of chitobiose with turkey egg-white lysozyme from the literature were correctly accounted for. These observations suggest that both lysozymes and lectins follow the same structural-energetic behavior in the binding to their ligands. From the analysis of lysozyme data in conjunction with other binding data reported in the literature, an ad hoc parameterization of DeltaCp for protein-carbohydrate complexes was derived for the first time. The novel parameters for both polar and apolar surface areas differed significantly from correlations obtained previously from model compounds and protein-folding data. As DeltaCp is extremely sensitive to changes in solvent structure, this finding indicates that protein-carbohydrate complexes have distinctive hydration properties. According to our analysis, the dehydration of polar groups is the major cause for the observed decrease in DeltaCp, which implies that these groups behave hydrophobically. The contribution of apolar surface areas was found of the expected sign, but their specific weight is much smaller than those obtained in other correlations. This small contribution to DeltaCp is consistent with Lemieux's hypothesis of a low degree of hydration of apolar surfaces on carbohydrates.

Effect of Ethanol on Micellization and on Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate in Aqueous Cationic Micelles

The effects of ethanol on the critical micellar concentration (cmc) and the rates of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) have been investigated in aqueous cationic surfactants of the cetyltrialkylammonium family with bromide [CT(R)ABr], chloride [CT(R)ACl], and nitrate [CT(R)ANO3] counterions, and methyl (CTAX), n-propyl (CTPAX), and n-butyl (CTBAX) as the head group alkyl moieties. Effects upon cmc and reactivity are similar, featuring a break at the ethanol mole fraction, xEtOH, of ca. 0.055; these effects have been related to changes in solvent structure, with formation of a clathrate at xEtOH=0.055. Rate data in CTBABr were further investigated and fitting of raw kinetic data to the pseudophase model is possible up to xEtOH=0.1, showing an unexpected decrease of rate constant values in the micellar pseudophase, k′M, as ethanol content increases: a significant variation of micellar ionization degree could account for this kinetic effect.

Adsorption of bromide ions on bismuth single crystal planes from solutions in ethanol

The adsorption of Br - ions on the (111), (001) and (011) planes of the bismuth single crystal from solutions in ethanol has been investigated by differential capacity and electrode charge measurement methods. The ionic charge due to the specific adsorption has been obtained using the mixed-electrolyte method for both electrode charge and electrode potential as the independent electrical variables. The Gibbs energy of adsorption of halide ions has been calculated using the simple virial adsorption isotherm. It was found that under comparable conditions the results obtained at constant electrode potential and at constant electrode charge are coincident and the adsorption of Br anions increases in the sequence of Bi single crystal planes (111) < (001) < (011) and solvents methanol < ethanol < 2-propanol. The electrosorption valency and the dipole moment of the dipole formed by an adsorbed halide ion and its image charge in the Bi plane have been calculated. It was found that on the Bi(111) plane, where the chemical bonds are all saturated, the electrosorption valency has a constant value, while on the more active planes it depends on electrode charge. It was concluded that the surface dipole formed is screened significantly by the solvent molecules and the metal electron gas. The surface dipole value diminishes when changing from negative to positive electrode charge and from the (111) plane to more active planes. The complicated dependence of the dipole value on the electrode charge at the Bi(111) plane was attributed to the change in solvent structure on the electrode surface.

Solvent effects on the initial and transition states for the solvolysis of trans-dichlorobis-(N-methylethylenediamine) cobalt(III) complex in water–dimethyl sulfoxide mixtures

Solvent effects on the initial and transition states for the solvolysis of the trans-dichlorobis-(N-methylethylenediamine)cobalt(III), (meen), complex have been investigated in the 25–55 °C range in aqueous DMSO mixtures, of varying solvent composition up to 60% by vol. The log of the first order rate constant, k, varies non-linearly with the reciprocal of the dielectric constant at the same temperature, due to differential solvation of the initial and transition states. The changes in the enthalpy, ΔH‡, and entropy, ΔS‡, of activation with the mole fraction of the co-solvent show extrema at the composition range where the change in solvent structure occurs. The application of a free energy cycle to the process of the initial state going to the transition state suggests that the effect of solvent structure on the complex ion in the transition state dominates the initial state and that this effect increases as the mole fraction of co-solvent increases.

Kinetics of solvolysis of the trans-dichlorobis(R-ethylenediamine)cobalt(III) ion (R = N-methyl or N,N′-dimethyl) in urea–water mixtures

First order solvolysis rates of the trans-dichlorobis(R-ethylenediamine)cobalt(III) ion (R = N-Me or N,N′-Me2) complex have been investigated in mixtures formed by adding urea to H2O, which enhances the dielectric constant and decreases solvent structure. Differential effects of the changes in solvent structure on the initial and transition states are found to be important factors controlling changes in the rate constant with solvent composition. The variation of the enthalpy and the entropy of activation with solvent composition are contrasted with their variations found for the solvolysis of the complexes in mixtures where solvent structure is enhanced by additions of a co-solvent to water. The application of a free energy cycle to the process of the initial state going to the transition state suggests that the cationic cobalt(III) complexes in the transition state are more stable than the cationic cobalt complexes the initial state in the water + urea mixtures.

Calculation of the characteristics of Cl − and Br − ions adsorption on bismuth single-crystal planes from solutions in 2-propanol

The values of the surface excess and the Gibbs energy of adsorption of halide ions on the (111) and (001) Bi single-crystal planes in solutions in 2-propanol have been calculated from the previously obtained experimental data using electrode potential as the independent electrical variable. It was found that under comparable conditions the results obtained at constant electrode potential and at constant electrode charge are coincident. The electrosorption valency and the dipole moment of the dipole formed by an adsorbed halide ion and its charge image in the Bi plane have been calculated. It was found that on the Bi(111) plane, where the chemical bonds are all saturated, the electrosorption valency does not depend on the electrode charge. In contrast, on the more active Bi(001) plane there is a noticeable dependence of the electrosorption valency on the electrode charge. The dipole value formed by an adsorbed halide ion and its charge image in Bi significantly diminishes when changing from negative to positive electrode charge density and at the higher positive charges the covalent bonding between the Bi(001) surface atoms and Br − anions is probable. The more complicated dependence of the dipole value on electrode charge at the Bi(111) planes was attributed to the change in solvent structure on the electrode surface. It was concluded that the dipole formed is significantly screened by the solvent molecules and the metal electron gas.

Induced changes in solvent structure by phospholipid monolayer formation at a liquid–liquid interface

Vibrational sum frequency spectroscopy has been used in conjunction with dynamic surface tension measurements to study formation of a 1,2-dilauroyl- sn-phosphatidylcholine (DLPC) monolayer at a water–carbon tetrachloride interface. Surface tension measurements show that an aqueous solution of liquid crystalline phosphocholine vesicles (4.5 μM DLPC) requires several hours to form a tightly packed, fully equilibrated monolayer of DLPC monomers. Vibrational spectra of the interfacial region at different stages in the monolayer formation process indicate that the solvent structure undergoes dramatic re-organization as the monolayer forms. Initial adsorption of DLPC monomers severely disrupts the interfacial hydrogen bonding. Intensity in the OH stretching region oscillates in a systematic fashion during the first 2 h of monolayer formation before finally settling to a level characteristic of the fully equilibrated monolayer. Frequency shifts of the OH stretching vibration show that water molecules with their C 2 axes aligned parallel to the interface experience a markedly different environment than those water molecules aligned perpendicular to the interface. This difference is attributed to the effect of the adsorbed, zwitterionic DLPC head-groups which, if aligned parallel to the interface, can stabilize in-plane water molecules.

Apparent molal volumes and viscosities of dl-α-alanine in water–alcohol mixtures

A systematic study of the experimental behavior of apparent molal volumes and viscosities of dl-α-alanine in aqueous 1-propanol, 1-butanol and 1,2-propanediol mixtures at 25°C is presented. The concentrations of the alcohol–water mixtures used as solvents were selected taking into account their special thermodynamic behavior and in particular the abrupt change observed in the dependence of partial molal volumes with concentration. The results are discussed in terms of changes in solvent structure.

Anatomy of an antibody molecule: structure, kinetics, thermodynamics and mutational studies of the antilysozyme antibody D1.3.

Using site-directed mutagenesis, x-ray crystallography, microcalorimetric, equilibrium sedimentation and surface plasmon resonance detection techniques, we have examined the structure of an antibody-antigen complex and the structural and thermodynamic consequences of removing specific hydrogen bonds and van der Waals interactions in the antibody-antigen interface. These observations show that the complex is considerably tolerant, both structurally and thermodynamically, to the truncation of antibody and antigen side chains that form contacts. Alterations in interface solvent structure for two of the mutant complexes appear to compensate for the unfavorable enthalpy changes when antibody-antigen interactions are removed. These changes in solvent structure, along with the increased mobility of side chains near the mutation site, probably contribute to the observed entropy compensation. In concert, data from structural studies, reaction rates, calorimetric measurements and site directed mutations are beginning to detail the nature of antibody-protein antigen interactions.

Inhomogeneous Fluid Approach to Solvation Thermodynamics. 1. Theory

The solvation energy and entropy at infinite dilution consist of a solute−solvent term and a solvent reorganization term representing the contributions of changes in solvent structure upon solute insertion. In the standard, homogeneous treatment of solutions, changes in solvent structure are expressed through derivatives of the homogeneous pair correlation function, which are very difficult to obtain by simulation. Tractable expressions for the solvation energy and entropy are here derived by viewing the solution as an inhomogeneous system with the solute fixed at a certain point. The solvent reorganization terms in the inhomogeneous view contain two contributions: the local, “correlation” contributions, which are due to correlations between the solute and the solvent and dominate at high densities, and the nonlocal, “liberation” contributions, which are due to the effective dilution of the solvent caused by the thermal motion of the solute and dominate at low densities. The liberation contributions are independent of the nature or size of the solute and depend only on the properties of the solvent. For common liquid solvents the nonlocal terms are negligible and the solvation properties arise almost entirely from effects localized around the solute. The new expressions are tested by calculations of the solvent reorganization energy and entropy in ideal hard-sphere and Lennard-Jones mixtures (solute identical to solvent). The solvent reorganization energy and entropy make distinct and significant contributions to the solvation free energy. The theory can be applied to truly inhomogeneous systems as well as small solute solvation, thus providing a connection between interfacial phenomena and microscopic solvation.

An investigation of the influence of solute size and insertion conditions on solvation thermodynamics

In this paper we examine the influence of solute size and insertion conditions on solvent structural changes and excess thermodynamic properties in the infinite dilution limit. A general integral equation approach which can be applied under arbitrary conditions is given and isothermal-isochoric and isothermal-isobaric insertions are discussed in detail. Scaling relationships valid in the large solute limit are determined for both structural and thermodynamic properties. This is done by considering macroscopic thermodynamic relationships and explicit evaluation of low solvent density expansions of pair correlation functions. The hypernetted-chain and reference hypernetted-chain closure approximations are used to obtain numerical results for the insertion of hard sphere solutes of varying diameter into hard sphere, dipolar hard sphere and water-like solvents. The results obtained give a good deal of insight into the nature of solvation of inert solutes. It is shown that for all three solvents the excess properties are very well represented by a function obtained by summing terms proportional to the solute volume, surface area and diameter. One would expect such a result for large solutes, but here we show that this expression extrapolates all the way down to solutes comparable in size to the solvent particles. Further, it is shown that both the numerical value, and, more importantly, the physical interpretation of the excess thermodynamic properties strongly depend on the insertion conditions. Under all insertion conditions the chemical potential is a local property in the sense that it is completely determined by solute-solvent correlations which are important only in the immediate vicinity of the solute. However, this is not true of the excess energy, enthalpy and entropy which all contain nonlocal contributions arising essentially from changes in the actual or effective solvent density depending on the insertion conditions. We demonstrate that the nonlocal contributions can be very significant and therefore the excess energies, enthalpies and entropies often cannot provide useful information about solvent structure near solutes. This has significant implications for models which attempt to rationalize excess thermodynamics in terms of local solvent structure in the vicinity of solute particles.

Solvent effects on the kinetics of solvolysis of trans-dichlorobis (N-methylethylenediamine)cobalt(III) ion in water–propan-2-ol and water–acetonitrile mixtures

First order solvolysis rates of the trans-dichlorobis (N-methylethylenediamine) cobalt(III) ion have been measured over a wide range of solvent compositions and temperatures in water–propan-2-ol and water–acetonitrile mixtures. The rate of solvolysis is faster in the former mixtures rather than the latter. Plots of log(rate constant) versus the reciprocal of the dielectric constant of the co-solvent, and also versus the Grunwald–Winstein Y-values are non-linear for both co-solvents; this non-linearity is derived from a large differential effect of solvent structure between the initial and transition states. However, extrema in the variation of enthalpy ΔH‡ and entropy ΔS‡ of activation correlate well with the extrema in physical properties of the mixtures which are related to changes in solvent structure. Linear plots of ΔH‡ versus ΔS‡ were obtained and the isokinetic temperature indicates that the reaction is entropy controlled. The application of a free-energy cycle shows that changes in solvent structure affect the pentacoordinated cobalt(III) ion in the transition state more than the hexacoordinated cobalt(III) ion in the initial state. In addition, the stabilizing influence of changes in solvent structure is greater in propan-2-ol–water mixtures than in acetonitrile–water mixtures, and the difference becomes greater as the mole fraction, x2 of the organic co-solvent increases.

Hydrogen bonding and solvent structure in an antigen-antibody interface. Crystal structures and thermodynamic characterization of three Fv mutants complexed with lysozyme.

Using site-directed mutagenesis, X-ray crystallography, and titration calorimetry, we have examined the structural and thermodynamic consequences of removing specific hydrogen bonds in an antigen-antibody interface. Crystal structures of three antibody FvD1.3 mutants, VLTyr50Ser (VLY50S), VHTyr32Ala (VHY32A), and VHTyr101Phe (VHY101F), bound to hen egg white lysozyme (HEL) have been determined at resolutions ranging from 1.85 to 2.10 A. In the wild-type (WT) FvD1.3-HEL complex, the hydroxyl groups of VLTyr50, VHTyr32, and VHTyr101 each form at least one hydrogen bond with the lysozyme antigen. Thermodynamic parameters for antibody-antigen association have been measured using isothermal titration calorimetry, giving equilibrium binding constants Kb (M-1) of 2.6 x 10(7) (VLY50S), 7.0 x 10(7) (VHY32A), and 4.0 x 10(6) (VHY101F). For the WT complex, Kb is 2.7 x 10(8) M-1; thus, the affinities of the mutant Fv fragments for HEL are 10-, 4-, and 70-fold lower than that of the original antibody, respectively. In all three cases entropy compensation results in an affinity loss that would otherwise be larger. Comparison of the three mutant crystal structures with the WT structure demonstrates that the removal of direct antigen-antibody hydrogen bonds results in minimal shifts in the positions of the remaining protein atoms. These observations show that this complex is considerably tolerant, both structurally and thermodynamically, to the truncation of antibody side chains that form hydrogen bonds with the antigen. Alterations in interface solvent structure for two of the mutant complexes (VLY50S and VHY32A) appear to compensate for the unfavorable enthalpy changes when protein-protein interactions are removed. These changes in solvent structure, along with the increased mobility of side chains near the mutation site, probably contribute to the observed entropy compensation. For the VHY101F complex, the nature of the large entropy compensation is not evident from a structural comparison of the WT and mutant complexes. Differences in the local structure and dynamics of the uncomplexed Fv molecules may account for the entropic discrepancy in this case.

Kinetics of the solvolysis of trans‐dichlorotetra(4‐t‐butylpyridine)‐cobalt(III) ions in mixtures of water with methanol and with ethanol

AbstractFor the solvolysis of Co(4‐t‐Bupy)4Cl2− ions in water + methanol and water + ethanol, log (rate constant) does not vary linearly with the reciprocal of the dielectric constant. The Gibbs free energy, the enthalpy, and the entropy of activation are insensitive to changes in the solvent composition in these mixtures, although a slight broad maximum in ΔH* and ΔS* probably exists at mole fractions of about 0.2 in water + ethanol. This contrasts with the extrema in ΔH* and ΔS* found with more hydrophobic alcohols used as cosolvents. However, the application of a Gibbs energy cycle to the solvolysis in water and in the mixtures shows that there is a differential effect of changes in solvent structure on the emergent solvated CoIII cation in the transition state and on Co(4‐t‐Bupy)4Cl2+ in the initial state. The stability of the former increases relative to that of the latter as the cosolvent content of the mixture rises. © 1995 John Wiley &amp; Sons, Inc.

Kinetics of the solvolysis of [Co(Rpy)4Cl4]+ with R = 4-t-Bu, 3-Me or 3-Et in mixtures of urea with water

The kinetics of the solvolysis of complex ions trans-[Co(Rpy)4Cl2]+, with R = 4-t-Bu, 3-Me and 3-Et, have been investigated in mixtures formed by adding urea to water, which enhances the dielectric constant and decreases solvent structure. Differential effects of the changes in solvent structure on the initial and transition states are found to be important factors controlling changes in the rate constant with solvent composition. The variation of the enthalpy and the entropy of activation with solvent composition are contrasted with their variations found for the solvolysis of [Co(Rpy)4Cl2]+ in mixtures where solvent structure is enhanced by additions of a co-solvent to water. The application of a free energy cycle to the process of the initial state going to the transition state suggests that the Co3+ cation in the transition state is more stable than the Co3+ cation in the initial state in the water + urea mixtures.

Kinetics of the solvolysis of a cationic and of an anionic chloro-cobalt(III) complex in water-propan-1-ol mixtures

The kinetics of the solvolysis of the ions [Co(NH3)5Cl]2+ and [Co(CN)5Cl]3− have been investigated in a range of water-rich mixtures of water with n-PrOH at various temperatures. The variation of the enthalpies and entropies of activation with solvent composition show extrema in composition ranges where the physical properties of the mixtures, influenced by changes in solvent structure, also show extrema. From the application of a free energy cycle to the process of the initial state going to the transition state, it is concluded for the solvolyses of both complexes that the species of CoIII in the transition state is more stable in water + n-PrOH mixtures than the ionic complexes of CoIII in the initial state. The results are compared with those for the solvolysis of these and related complexes in mixtures of water with other alcohols.

Kinetics of the solvolysis of chloropentacyanocobaltate(III) ions in water +t-butanol mixtures

Rates of the solvolysis of [Co(CN)5Cl]3− ions have been determined in mixtures of water with the hydrophobic alcohol, t-butanol over a range of temperatures. No linear correlation of log k with the reciprocal of the dielectric constant is found, suggesting that changes in solvent structure are an important factor influencing these rates. This result is confirmed by the good correlation found for the extrema in the enthalpy and entropy of activation with the extrema in the physical properties of the mixtures influenced by changes in solvent structure. The application of a free energy cycle to the loss of the chloride ion from the Co3+ in the transition state shows that [Co(CN)5]2− in the transition state is stabilised in the mixtures relative to [Co(CN)5Cl]3− in the initial state.

Solvent structural changes in repulsive and attractive supercritical mixtures. a molecular distribution study

By examination of the molecular distribution functions, we are able to determine the long-range solvent structures that lead to “repulsive” or “attractive” supercritical-fluid behavior. Integral equation theories are used to obtain these distribution functions. Depletion of solvent molecules about the solute molecule over 20 molecular diameters is responsible for the positive growth of the partial molar volumes in repulsive mixtures; just as enrichment of solvent molecules over long ranges causes the collapse of the partial molar volumes in attractive mixtures. In fact, there is a competition between the “compressibility” of the solvent near the critical point and the “affinity” of the solvent toward the solute species. The macroscopic properties derive their major contribution from these long-range solvent-solute correlations, although there is evidence of local rarefaction or densification of solvent molecules in the first solvation shell.

Solvent effect on rate for anionic nucleophilic substitution reaction of 4-nitrophenyl acetate in aqueous acetonitrile

Abstract Second order rate constants have been measured for anionic nucleophilic substitution reactions of 4-nitrophenyl acetate in MeCN-H2O mixtures. The results show rate minima near 30–40 mole % MeCN, which are attributed to changes in solvent structure.