Measured Equilibrium Constants Research Articles

Calculation of Standard Transformed Formation Properties of Biochemical Reactants and Standard Apparent Reduction Potentials of Half Reactions

The standard Gibbs energies of formation and standard enthalpies of formation of species involved in biochemical reactions are used to calculate standard transformed Gibbs energies of formation and standard transformed enthalpies of formation of 62 biochemical reactants (sums of species) at 298.15 K, pH 7, and ionic strengths of 0, 0.10, and 0.25 M. It has been possible to put the oxidized and reduced forms of some reactants in this table because their standard apparent reduction potentials are known at pH 7. This paper emphasizes redox reactions. Two applications have been made of these 62 new values of standard transformed Gibbs energies of formation at pH 7: (1) They have been used to calculate standard transformed Gibbs energies of formation of 16 more biochemical reactants from measurements of apparent equilibrium constants of redox reactions.(2) They have been used to calculate standard apparent reduction potentials at pH 7 for half reactions involving reactants discussed in this article and the previous one. This table of standard apparent reduction potentials can be extended considerably from known apparent equilibrium constants for enzyme-catalyzed redox reactions. This brings the total number of reactants for which the standard transformed Gibbs energy of formation at 298K, pH 7, and ionic strengths of 0, 0.10, and 0.25 M have been calculated to 142.

Roles of Individual Disulfide Bonds in the Stability and Folding of an ω-Conotoxin

Although it contains only 25 amino acid residues, omega-conotoxin MVIIA folds into a well-defined three-dimensional structure that is stabilized by 3 disulfide bonds. To assess the contributions of the disulfides to folding and stability, three analogues, each with one pair of disulfide-bonded Cys residues replaced with Ala, were prepared and characterized. The analogues also contained a C-terminal Gly residue that is believed to be present when the peptide folds in vivo and has been shown previously to stabilize the native structure. Circular dichroism spectra and biological assays of the analogues indicated that removing any one of the disulfides greatly destabilized the native conformation. The two disulfides in each analogue were also reduced much more rapidly than in the native form with three disulfides. When the analogues were fully reduced and allowed to form disulfides in the presence of oxidized and reduced glutathione, the native disulfides were not formed in preference to non-native disulfides, further indicating that the forms with two-native disulfides are not significantly stabilized by noncovalent interactions. However, the measured equilibrium constants for disulfide formation indicate that forming any two of the three native disulfides leads to an effective concentration of approximately 25-50 M for the two remaining thiols. The two-disulfide analogues thus appear to represent a stage of folding in which the polypeptide is constrained to a distribution of relatively compact conformations that greatly favor formation of the third disulfide and the final folded structure.

RELATIVE STABILITIES OF XSiH2B5H8 (X = H, SiH3, Cl) SILYLPENTABORANE(9)S: SYNTHESIS OF CHLOROSILYL- AND DISILANYL-PENTABORANES

Reaction of LiB5H8 with Si2H5Cl in Et2O at −45°C yields μ-Si2H5B5H8 (10), which in Et2O at 25°C isomerizes to 2-Si2H5B5H8 (11). At 120°C in the gas phase, 11 undergoes conversion to an 11/1-Si2H5B5H8 (12) mixture. The three disilanylpentaborane(9) isomers have been separated by high-vacuum fractional distillation. Reaction of 1- SiH3B5H8 with BCI3 yields 1-(ClSiH2)B5H8 (9). The new silylpentaboranes have been characterized by spectral data. Thermal equilibration of the μ-, 2- and 1-isomers of SiH3B5H8, (ClSiH2)B5H8 and Si2H5B5H8 in a static reactor at 120°C and in a flow reactor at 350°C and 425°C results in 1- and 2-isomer mixtures. The 1-isomer for all compounds studied is favored at all temperatures. The measured equilibrium constants (K) range from; 8.8 ± 0.4 (SiH3B5H8) and 7.3 ± 0.4 (Si2H5B5H8) at 120°C to 1.6 ± 0.4 (SiH3B5H8), 1.3 ± 0.2 (Si2H5B5H8) and 2.2 ± 0.6 [(ClSiH2)B5H8] at 425°C. These are discussed in terms of substituent conjugative effects.

Experimental and theoretical study on thermodynamic stabilities of silyl cations. Substituent effect on the stability of dimethylphenylsilyl cation in the gas phase †

Thermodynamic stabilities of aryldimethylsilyl cations were determined by measuring equilibrium constants of hydride-transfer reaction in the gas phase using a FT ion cyclotron resonance spectrometer. An application of the Yukawa–Tsuno equation to the substituent effect gave a ρ of –5.87 and an r+ of 0.29. Both r+ and ρ values are significantly smaller than those for a carbon analog, α-cumyl cation. The same trend was observed for the results of ab initio calculations. In addition, the r+ value of 0.3 for gaseous silyl cations agrees with that observed for the kinetics of the hydride-transfer reaction from aryldimethylsilanes to diarylcarbenium ions in dichloromethane. It is concluded that the dimethylphenylsilyl cation is characterized by no significant π-delocalization of the positive charge into the benzene ring in addition to a small ρ value compared to that of the α-cumyl cation.

Thermodynamic Equilibrium of Water and Ice in Hydrated Gliadin and Hemoglobin

Crystallization of water and the water−ice equilibrium in the hydrated states (1 g of H2O/g of dry protein) of gliadin and hemoglobin have been studied by differential scanning calorimetry. Water and ice coexist at a thermodynamic equilibrium at all temperatures in the 230−272 K range. Their relative amounts have been determined from 260 to 273 K, and a formalism based on equilibrium thermodynamics has been developed. The temperature dependence of the equilibrium constant of the water ↔ ice interconversion does not obey the Gibbs−Helmholtz equation, and this indicates a strong interaction of proteins with water. By using the measured equilibrium constant at different temperatures and the difference between the Cp of the solutions in equilibrium with ice and the ice itself in the hydrated proteins, the DSC scans obtained during cooling have been simulated. The kinetics of crystallization is not determined entirely by the grain-growth process. A double crystallization peak has been observed on cooling hydra...

Conductance of Dilute LiCl, NaCl, NaBr, and CsBr Solutions in Supercritical Water Using a Flow Conductance Cell

Conductivities of LiCl(aq), NaCl(aq), NaBr(aq), and CsBr(aq) solutions have been measured in the range of concentrations from 0.013 to 4 × 10-8 mol·dm-3 using a flow conductance cell at temperatures between 603 K and 674 K and pressures between 15 MPa and 28 MPa (water densities from 650 to 200 kg·m-3). Limiting equivalent conductances and ionization equilibrium constants calculated from the concentration dependence of the equivalent conductance are reported. Previous measurements of equilibrium constants in the low-density supercritical region, although of lesser accuracy, agree well with the present results. Even at the critical density and 2.5 K above the critical temperature there is no evidence for the critical effects which make the Debye−Huckel limiting law invalid at the critical isotherm−isobar. No critical scaling of Λ0 is observed. Walden's rule is not obeyed and the Stokes radius increases by 70% near the critical region as the density goes from 700 to 200 kg·m-3, suggesting an increase in the...

Complexation of Photochromic Crown Ether Styryl Dyes with Mg2+ As Probed by Surface-Enhanced Raman Scattering Spectroscopy

Complexation with magnesium cations of 2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin (1), as well as derivatives of crown ether styryl dyes perchlorate 2-[2-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-16-yl)ethenyl]-3-ethylbenzothiazolium (2), betaine 2-[2-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-16-yl)ethenyl]-3-(3-sulfopropyl)benzothiazolium (3), betaine 2-[2-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-16-yl)ethenyl]-3-(3-sulfoethyl)benzothiazolium (4) was studied by surface-enhanced Raman scattering (SERS) spectroscopy. Comparative studies of isomerization and complexation of photochromic ionophores 2−4 with Mg2+ in solution and when adsorbed on the silver surface were performed. The aim was to characterize parameters of these processes depending on the nature (alkyl versus sulfo-alkyl) as well as the length (sulfoethyl versus sulfopropyl) of the N substituent in the benzothiazolium residue. The data obtained indicate stabilization of the trans conformation and prevention of the trans−cis isomerization for the adsorbed molecules as well as for their complexes with Mg2+ due to fast nonradiative energy transfer from the dye excited state to metal surface. Adsorption of a dye having initially cis conformation in solution is accompanied by surface-induced cis−trans relaxation. Adsorption constants calculated from the Langmuir isotherms revealed slightly preferential adsorption of 2−4 complexed with Mg2+ as compared to the free dyes. Desorption of the dyes was found to occur on the hours time scale, whereas reversible capture/release of metal ions by adsorbed molecules is quite fast and may serve as an indicator of a cation level and/or complexation processes in bulk solution. The SERS titration procedure was developed permitting the complexation of the ionophores with metal cations to be followed and equilibrium constants of these processes to be measured in a wide range (down to 10-7 M) of ligand concentrations. By the measurement of the equilibrium constants, adsorption on the silver surface was concluded not to affect the complexation of 1 and trans-2 with Mg2+. Association of the Mg2+ cation with the sulfonate group was found to be more favorable than binding with the crown ether moiety for the adsorbed (trans-3)Mg2+ and (trans-4)Mg2+ complexes. Appearance of the (trans-3)(Mg2+)2 and (trans-4)(Mg2+)2 species was detected in the solution at increased metal/ligand molar ratios. These complexes were found to be unperturbed if absorbed on the silver surface. Since the measurement of equilibrium constants are intractable by other techniques, measuring equilibrium constants for (trans-3)(Mg2+)2 and (trans-4)(Mg2+)2 complexe formation is readily acquired by SERS spectroscopy.

Effect of mobile phase composition on the binding kinetics of chiral solutes on a protein-based high-performance liquid chromatography column:: Interactions of d- and l-tryptophan with immobilized human serum albumin

This work examined the kinetic interactions of chiral solutes on immobilized protein columns, using the binding of d- and l-tryptophan to human serum albumin as a model. Based on band-broadening studies and previous measurements of the association equilibrium constants ( K a) for this system, estimates were obtained for the dissociation and association rate constants ( k d and k a) for d- and l-tryptophan under a variety of operating conditions. The relative importance of k a versus k d in creating changes in the overall binding affinity was then considered. For example, an increase in temperature from 4 to 45°C gave a large change in k a for l-tryptophan that was due both to an increase in k d and to a decrease in k a, while k a and k d for d-tryptophan showed a parallel increase that led to a much smaller temperature dependence for K a. Similar comparisons between k a, k d and K a were performed over a range of pH values, ionic strengths and solvent polarities. It was also possible from these studies to examine the changes in enthalpy and entropy that accompanied the formation of the activated complex between human serum albumin and each solute. The results from this work were then used to illustrate the importance of kinetics and band-broadening in protein-based chiral separations, and an example was provided showing how this type of kinetic data might be used to help optimize such separations.

Targeting E2F1-DNA complexes with microgonotropen DNA binding agents.

Microgonotropen (MGT) DNA binding drugs, which consist of an A+T-selective DNA minor groove binding tripyrrole peptide and polyamine chains attached to a central pyrrole that extend drug contact into the DNA major groove, were found to be extraordinarily effective inhibitors of E2 factor 1 (E2F1) association with its DNA promoter element (5'-TTTCGCGCCAAA). The most active of these drugs, MGT-6a, was three orders of magnitude more effective than distamycin and inhibited complexes between E2F1 and the dihydrofolate reductase promoter by 50% at 0.00085 microM. A relationship was found between the measured equilibrium constants for binding of MGTs to the A+T region of d(GGCGA3T3GGC)/d(CCGCT3A3CCG) and their inhibition of complex formation between E2F1 and the DNA promoter element. A representative of the potent MGT inhibitors was significantly more active on inhibition of E2F1-DNA complex formation compared with disruption of a preexisting complex.

Comment on the accuracy of thermodynamic data derived from studies of gas phase ion-molecule equilibria by the third law and van't Hoff methods

Much of the thermodynamic data in the literature, such as proton affinities, derived from studies of gas phase ion-molecule equilibria, is based on the “third law” method. The van't Hoff approach is more rigorous, but it can carry greater uncertainty (S.W. Benson, Thermochemical Kinetics, Wiley, 1976). Comparison is made here between the two methods when there is a probability of interfering reactions, which becomes greater for more complex systems at both low and high temperatures. The extrapolation procedure used in the third law method has the effect of significantly demagnifying the error introduced by an inaccurate measurement of the equilibrium constant, caused by competitive loss of ions or neutrals in other reactions. The point is illustrated by the results from a study of the proton transfer reactions of isomers of protonated fluorobenzaldehyde with both benzaldehyde and mesitylene. Measured equilibrium constants can be in error by a factor of five at 300 K but still give answers accurate to within 4 kJ mol −1 (less than 1 kcal mol −)! Good thermodynamic data can therefore be generated even in systems in which there is significant interference and by techniques in which even the temperature is not well defined.

Hydration of Gas Phase Proteins: Folded +5 and Unfolded +7 Charge States of Cytochrome c

Free energy changes for the first steps in the hydration of the +5 and +7 charge states of gas-phase cytochrome c have been determined from equilibrium constant measurements at 271 K. Previous ion mobility studies (Shelimov, K. B.; Jarrold, M. F. J. Am. Chem. Soc. 1996, 118, 10313) indicate that the +5 charge state is tightly folded in the gas phase, while the +7 charge state is partially unfolded under the conditions employed. Free energy changes have been determined for the adsorption of the first nine water molecules on the folded +5 charge state and the first five water molecules on the unfolded +7 charge state. The free energy changes for transfer from bulk water to the hydration shell of the protein are remarkably small, between −2 and −7 kJ mol-1, indicating that the charge is very effectively shielded in both the unfolded and folded conformations. The free energy changes for initial hydration of the folded +5 charge state are significantly more negative than for the unfolded +7 state. This may ind...

Determination of keto–enol equilibrium constants and the kinetic study of the nitrosation reaction of β-dicarbonyl compounds

E. Iglesias, J. Chem. Soc., Perkin Trans. 2, 1997, 431 DOI: 10.1039/A607039F

The Role of Chemical Species in the Passivation of 〈100〉 Silicon Surfaces by HF in Water‐Ethanol Solutions

The role played by the chemical species present in HF/water‐ethanol solutions as last cleaning steps of the 〈100〉 silicon surface has been analyzed. The concentrations of these species as a function of the ethanol content have been calculated from measured equilibrium constants for the dissociation and homoconjugation reactions of HF in water‐ethanol solutions. The correlation of these data with the measured etching rates of silicon oxide in these solutions suggests that etching in HF/pure ethanol is controlled almost completely by the ion . The contribution of the dimers to the etching process diminishes as ethanol content increases. This indicates an increasing activation energy of the dimer etching reaction when the ethanol content is raised. This characteristic behavior is associated with the slower and more homogeneous etching action of HF/ethanol solutions as well as with the absence of faceting and lower microroughness generation in 〈100〉 silicon surfaces. The effects of overetching are not significant, and moreover, the characteristic surfactant properties of alcohols contribute to the effects of these cleaning solutions.

Relative Gas-phase Acidities of Fluoro- and Chlorobenzene

The gas-phase equilibrium reaction between fluorobenzene and chlorobenzene and their conjugate bases was studied by Fourier transform ion cyclotron resonance spectrometry in order to reconcile conflicting experimental results obtained using different techniques. The results of a large number of measurements yield equilibrium constants which show chlorobenzene to be more acidic than fluorobenzene by 0.55 ± 0.15 kcal mol -1 (ΔG° at 330 K). Different control experiments were carried out to ensure that the measured equilibrium constants reflect adequately the thermal distribution of the different isomeric halophenide ions.

Gibbs energies of gas-phase electron transfer reactions involving the larger fullerene anions

Accurate data on the Gibbs energies of gas-phase fullerene anion/fullerene molecule exchange reactions have been obtained from equilibrium constant measurements using the Knudsen cell mass spectrometry method. The electron affinities (EA) for the series of [70 + 2 n]fullerenes ( n = 0, 1,…, 18) were determined. The EA values show an increase with the size of the molecule over the whole range, although disruption from the monotonic change was observed in the series of [70]-, [72]-,[74]- and [76]fullerenes. The correlation of the EA change in the larger fullerene series with their electronic structures is discussed.

Thermokinetic Determination of Gas-Phase Basicities. Application to Ketene, Methylketene, and Formaldimine

Rate constants have been determined for proton transfer reactions of the type [MH]+ + B ⇆ M + [BH]+, where M is ketene, methylketene, and formaldimine and B is a reference base. A quantitative relationship between the rate constant and the free energy (or enthalpy) of the reaction allows the determination of the gas-phase basicity, GB, (or proton affinity, PA) of M. This thermokinetic method gives results comparable to that obtained from equilibrium constant measurements. The values derived for ketene, methylketene, and formaldimine follow: GB(ketene) = 788 ± 3 kJ/mol, PA(ketene) = 817 ± 3 kJ/mol; GB(methylketene) = 809 ± 3 kJ/mol, PA(methylketene) = 842 ± 3 kJ/mol; GB(formaldimine) = 830 ± 3 kJ/mol and PA(formaldimine) = 860 ± 5 kJ/mol. Heats of formation of methylketene and of formaldimine that may be deduced from a combination of these results with literature data are as follows: ΔHf°300 (CH3CHCO) = −97 ± 5 kJ/mol and ΔHf°300 (CH2NH) = 75 ± 5 kJ/mol.

Comparative Total Syntheses of Turkey Ovomucoid Third Domain by Both Stepwise Solid Phase Peptide Synthesis and Native Chemical Ligation

Turkey ovomucoid third domain, OMTKY3, is a potent protein inhibitor of most serine proteinases that prefer a neutral residue at the P1 position. It has been a target of increasingly intense scrutiny by site-directed mutagenesis in an attempt to elucidate the sequence-to-reactivity algorithm of the inhibitor. Here we report the total chemical synthesis of (6−56)OMTKY3 using both the stepwise solid phase peptide synthesis (SPPS) technique and the native chemical ligation approach. After refolding and affinity purification, the resultant products were characterized by electrospray ionization mass spectrometry, analytical ion exchange chromatography, measurement of association equilibrium constants with six different serine proteinases, and thermal denaturation studies. The two synthetic proteins were found to be functionally as well as structurally identical to their recombinant counterpart. Moreover, the native chemical ligation and stepwise SPPS techniques gave rise to comparable yields and similar product quality. The native chemical ligation strategy used here presents highly efficient synthetic access to novel analogs of OMTKY3 that can be used for understanding the molecular basis of enzyme−inhibitor recognition.

Measurement of Equilibrium Constants for Complex Formation between Phenol and Hydrogen-Bond Acceptors by Kinetic Laser Flash Photolysis1

Measurement of Equilibrium Constants for Complex Formation between Phenol and Hydrogen-Bond Acceptors by Kinetic Laser Flash Photolysis¹

Measurement of Equilibrium Constants for Complex Formation between Phenol and Hydrogen-Bond Acceptors by Kinetic Laser Flash Photolysis J. Am. Chem. Soc. 1996, 118, 6790−6791

Measurement of Equilibrium Constants for Complex Formation between Phenol and Hydrogen-Bond Acceptors by Kinetic Laser Flash Photolysis <i>J</i>. <i>Am</i>. <i>Chem</i>. <i>Soc</i>. <b>1996</b>, <i>118</i>, 6790−6791

Determination of the Rate Constant for the Reaction of Hydroxyl and Oxide Radicals with Cysteine in Aqueous Solution

The techniques of pulse radiolysis and absorption spectroscopy have been used to determine absolute rate constants for the reaction of hydroxyl and oxide radicals with cysteine (CysS) species in aqueous solution. Using measured equilibrium constants for radical disulfide anion formation, over the pH range 6.0-13.0, observed reaction rate constants were resolved into their constituent reactions .OH + CysSH-->H2O + CysS. and .O(-) + CysS-( + H2O)-->CysS. + 2OH- with calculated limiting rate constants of (5.35 +/- 0.82) x 10(9) and (3.28 +/- 0.49) x 10(8) dm3 mol-1 s-1, respectively. The rate constant for hydroxyl radical reaction with CysS- was found to be the same as that for CysSH.