Thermodynamics of dissociation

The thermodynamics of dissociative electron transfer reactions were first characterized by Hush12 from thermochemical data in the case of the electrochemical reactions... 

The use of differential scanning microcalorimetry for measuring the thermal denaturation of proteins is described in Chapter 17, section Ale. Typically, 0.5-1 mg of protein in 1 mL of buffer, or 0.1-0.2 mg in 0.5 mL with the most sensitive apparatus, is required for an accurate determination of the enthalpy of denaturation. The thermodynamics of dissociation of a reversibly bound ligand may be calculated from its effects on the denaturation curve of a protein.14 The binding of ligands always raises the apparent Tm (temperature at 50% denaturation) of a protein because of the law of mass action the ligand does not bind to the denatured state of the protein, and so binding displaces the denaturation equilibrium toward the native state. 

A. Briggs, J. Sawbridge, P. Tickle and J. Wilson, Thermodynamics of dissociation of some barbituric acids in aqueous solution, /. Chem. Soc. (B), 1969,802-805. 

Some colloids where the details of the charged groups are known, as in proteins, not only allow an accurate estimation of charge but also provide an inventory of which groups are charged by what amount from the absorption of light in the ultraviolet range. This can be done as a fimction of pH. The thermodynamics of dissociation in the presence of electrostatic double layers allows one... 

Thakur P, Mathur JN, Moore RC, Choppin GP (2007) Thermodynamics of dissociation constants of carboxylic acids at high ionic strength and temperature. Inorg Chim Acta 360 3671 3780... 

It seems safer to anchor the scale to a theoretical value. CCSD(T) calculations [139] on the thermodynamics of dissociation of the CpNiNH3+ adduct yield CpNiCBCNHj) = 172.1 kJ moF and CpNiCA(NH3) = 212.3 kJ mol . The latter value compares well with the value of 219.3 kJ mol estimated fromZ o(CpNi+—NO) = 192.5 kJ mol (assumed to be equal to CpNiCA) and the difference in basicity (26.8 kJ mol ) between NH3 and NO (assumed to be equal to the affinity difference). The CpNiCB scale anchored to the ab initio value for NH3 is given in Table 6.12. 

Internal and External Phases. When dyeing hydrated fibers, for example, hydrophUic fibers in aqueous dyebaths, two distinct solvent phases exist, the external and the internal. The external solvent phase consists of the mobile molecules that are in the external dyebath so far away from the fiber that they are not influenced by it. The internal phase comprises the water that is within the fiber infrastmcture in a bound or static state and is an integral part of the internal stmcture in terms of defining the physical chemistry and thermodynamics of the system. Thus dye molecules have different chemical potentials when in the internal solvent phase than when in the external phase. Further, the effects of hydrogen ions (H" ) or hydroxyl ions (OH ) have a different impact. In the external phase acids or bases are completely dissociated and give an external or dyebath pH. In the internal phase these ions can interact with the fiber polymer chain and cause ionization of functional groups. This results in the pH of the internal phase being different from the external phase and the theoretical concept of internal pH (6). 

Although the thermodynamic aspects of acylotropy are well documented, there have been few kinetic studies of the process. The activation barrier is much higher than for prototropy and only Castells et al. (72CC709) have succeeded in observing a coalescence phenomenon in H NMR spectra. At 215 °C in 1-chloronaphthalene the methyl groups of N-phenyl-3,5-dimethylpyrazole-l-carboxamide coalesce. The mechanism of dissociation-combination explains the reversible evolution of the spectra (Scheme 9). 

D is the chemical energy of dissociation which cair be obtained from thermodynamic data, aird is the reduced mass of the diatomic molecule... 

The basic thermodynamic data for the design of such reactions can be used to assess the dissociation energies for various degrees of dissociation, and to calculate, approximately, tire relevant equilibrium constants. One important source of dissociation is by heating molecules to elevated temperamres. The data below show the general trend in the thermal dissociation energies of a number of important gaseous molecules. 

In the region of pure CH4, the equilibrium is governed by Equation 4. For this reaction, the equilibrium constant increases with temperature so that at high enough temperatures there will be appreciable dissociation of CH4 to H2 and graphite. In the lower temperature range considered here, the thermodynamic equilibrium indicates only a very small amount of dissociation so the intersection of the graphite deposition curve with the H2-CH4 line occurs at almost pure CH4. As the temperature increases, the point of intersection will move toward pure H2 on the H2-CH4 line. 

Real-time spectroscopic methods can be used to measure the binding, dissociation, and internalization of fluorescent ligands with cell-surface receptors on cells and membranes. The time resolution available in these methods is sufficient to permit a detailed analysis of complex processes involved in cell activation, particularly receptor-G protein dynamics. A description of the kinetics and thermodynamics of these processes will contribute to our understanding of the basis of stimulus potency and efficacy. 

In a number of cases, the temperature of the filament and thermodynamic parameters allow one to calculate [9] the flux intensity of free atoms produced in dissociation of molecules. Specifically, in the case of dissociation of hydrogen, oxygen, and nitrogen molecules on hot metal filaments under pressures of molecular gases higher than lO" Torr, the flux intensity of atoms A originating from A2 molecules is given by... 

In this chapter we described the thermodynamics of enzyme-inhibitor interactions and defined three potential modes of reversible binding of inhibitors to enzyme molecules. Competitive inhibitors bind to the free enzyme form in direct competition with substrate molecules. Noncompetitive inhibitors bind to both the free enzyme and to the ES complex or subsequent enzyme forms that are populated during catalysis. Uncompetitive inhibitors bind exclusively to the ES complex or to subsequent enzyme forms. We saw that one can distinguish among these inhibition modes by their effects on the apparent values of the steady state kinetic parameters Umax, Km, and VmdX/KM. We further saw that for bisubstrate reactions, the inhibition modality depends on the reaction mechanism used by the enzyme. Finally, we described how one may use the dissociation constant for inhibition (Kh o.K or both) to best evaluate the relative affinity of different inhibitors for ones target enzyme, and thus drive compound optimization through medicinal chemistry efforts. 

As we described in Chapter 3, the binding of reversible inhibitors to enzymes is an equilibrium process that can be defined in terms of the common thermodynamic parameters of dissociation constant and free energy of binding. As with any binding reaction, the dissociation constant can only be measured accurately after equilibrium has been established fully measurements made prior to the full establishment of equilibrium will not reflect the true affinity of the complex. In Appendix 1 we review the basic principles and equations of biochemical kinetics. For reversible binding equilibrium the amount of complex formed over time is given by the equation... 

From thermodynamic considerations it is evident that bulk nickel cannot be oxidized by CO2. However, it is not justified to conclude from this that dissociative chemisorption of CO2 will not occur. Consider, for example, the chemisorption of oxygen or hydrogen which on several metals takes place under conditions where bulk oxides or hydrides are not at all thermodynamically stable. Dissociative adsorption of CO2 has indeed been observed by Eischens and Pliskin (35). 

Streukens, G., B. Bogdanovic, M. Felderhoff, F. Schuth, Dependence of dissociation pressure upon doping level of Ti-doped sodium alanate—a possibility for "thermodynamic tailoring" of the system, Phys. Chem. Chem. Phys., 8(24), 2889-2892, 2006. 

The objective of the preceding equilibrium calculation has been to determine the state of a molecule such as an amino acid in the conditions that prevailed on the early Earth. The pH, degree of dissociation and the extent of the reaction all have a direct effect on the population of the species present. Temperature and cooperative effects have not been considered but serve to complicate the problem. Any prebiotic reaction scheme must take account of that troublesome restriction to chemistry - the second law of thermodynamics. 

In the case of hydrogen, for example, at a temperature of 2500 K, the equilibrium constant for dissociation has the value, calculated from the thermodynamic relation between the Gibbs energy of formation and the equilibrium constant of 6.356 x 10 4 and hence at a total pressure of 10 2 atmos, the degree of dissociation is 0.126 at 2500 K, which drops to 8.32 x 10 3 at 2000 K. 

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