Conditional dissociation constant factorization

The choice of the ACE method most suited for a given drug-protein interaction will therefore depend on several factors. Among them are inherent properties of the complexation, such as the estimated dissociation constant, the on/off-rates or multiple binding sites, as well as properties related to the behavior under ACE conditions, such as solubility, detectability, adsorption to the inner capillary wall, and mobility of all species under investigation. 

Environmental pH is the most important factor affecting CP adsorption and mobility (Choi Aomine, 1972, 1974a,b Christodoulatosetal., 1994 Stapleton etal., 1994). Since the dissociation constants (p Ka) of CPs are in the same range as the pH in groundwater, both protonated and deprotonated CPs may exist under natural conditions. Lower chlorinated phenols are more protonated in neutral environments than their polychlorinated congeners. With PCP, for example, the sorption to clay decreases threefold between pH 4 and 8.5 (Stapleton et al., 1994). Low soil pH might also cause CP precipitation, especially from alkaline solution. 

The presence of I on the enzyme changes the dissociation constant for S from Ks to aKs. Note that the dissociation constant of I from ESI must also change by the factor a for the four enzyme species to be at equilibrium, that is, the overall of the reaction between E and ESI must be the same regardless of the path. Thus the path E->ES->ESI has an overall K , of l/KsorKi. The path E->EI >ESI has an overall of l/KiaKs. (See Fig. 1-11 for a similar situation.) ESI is catalytically inactive. The velocity equation for rapid equilibrium conditions is obtained in the usual manner ... 

Charge-transfer (C-T) bands have been located in the spectra of solutions of phenanthridine in 1,2-dimethoxyethane containing bromine solutions containing up to a 2 1 mole ratio of halogen to base were examined, but the structure of the species involved is not clear. Phenanthridine satisfies the conditions necessary for both n and tt- donation and n donation is apparently involved in the charge-transfer interaction with iodine. The equilibrium constant for this reaction has been determined spectrophotometrically, but the claimed correlation (for a series of A -heteroaromatic bases) between values (in 50% ethanol) and these C-T equilibrium constants appears to be an unsatisfactory one and in any case lacks theoretical justification, since it is doubtful whether dissociation constants provide, in general, an accurate measure of w-ionization potentials. In particular, the excellent correlation in the case of phenanthridine is probably fortuitous, since the authors report that w-halogen interactions are markedly sensitive to steric factors which are almost... 

The importance of the equilibrium law is widespread throughout chemistry. We have seen in Chapter 7 that defining the optimum conditions for certain key industrial gas phase reactions is dependent on a thorough understanding of the factors that determine the proportions of reactants and products in an equilibrium mixture, and we will return to a consideration of the Haber process later in this chapter. However, chemists use the equilibrium law to represent the extent to which a weak acid or base ionizes or dissociates, defining terms such as the dissociation constants and in relation to these effects. The behaviour of acid-base indicators is also explained in terms of the equilibria involved and the application of Le Chatelier s principle. 

In Eq. (6.85) K is defined as the ratio between p and s at equifibrium, while F is the same ratio at any conditions, thus F = Ka.t equifibrium. In order to account for the effect of the presence of S on binding of X, an additional factor was introduced. Namely, if the enzyme-modifier complex EX has a dissociation constant Kx, the complex ESX has a constant Kx multiplied by this factor. Finally, is the scaled modifier concentration ( =ar/so.s). 

Several theoretical models, such as the ion-pair model [342,360,361,363,380], the dyneuaic ion-exchange model [342,362,363,375] and the electrostatic model [342,369,381-386] have been proposed to describe retention in reversed-phase IPC. The electrostatic model is the most versatile and enjoys the most support but is mathematically complex euid not very intuitive. The ion-pair model emd dynamic ion-exchange model are easier to manipulate and more instructive but are restricted to a narrow range of experimental conditions for trtilch they might reasonably be applied. The ion-pair model assumes that an ion pair is formed in the mobile phase prior to the sorption of the ion-pair complex into the stationary phase. The solute capacity factor is governed by the equilibrium constants for ion-pair formation in the mobile phase, extraction of the ion-pair complex into the stationary phase, and the dissociation of th p ion-pair complex in the... 

For example, the rate constant for dissociation of hydrated S02, kl2, is 3.4 X 10r s l so that the half-life for dissociation of the hydrated S02 is only 0.2 yu,s. Similarly, the second ionization, reaction (13), occurs on time scales of less than a millisecond (Schwartz and Freiberg, 1981). Thus, regardless of which of the three species, S02 H20, HSO, or SO3-, is the actual reactant in any particular oxidation, the equilibria will be reestablished relatively rapidly under laboratory conditions, and likely under atmospheric conditions as well. The latter is complicated by such factors as the size of the droplet, the efficiency with which gaseous S02 striking a droplet surface is absorbed, the chemical nature of the aerosol surface, and so on for example, the presence of an organic surface film on the droplet could hinder the absorption of S02 from the gas phase. 

A simple diagram depicting the differences between these two complementary theories is shown in Fig. 1, which represents reactions at zero driving force. Thus, the activation energy corresponds to the intrinsic barrier. Marcus theory assumes a harmonic potential for reactants and products and, in its simplest form, assumes that the reactant and product surfaces have the same curvature (Fig. la). In his derivation of the dissociative ET theory, Saveant assumed that the reactants should be described by a Morse potential and that the products should simply be the dissociative part of this potential (Fig. Ib). Some concerns about the latter condition have been raised. " On the other hand, comparison of experimental data pertaining to alkyl halides and peroxides (Section 3) with equations (7) and (8) seems to indicate that the simple model proposed by Saveant for the nuclear factor of the ET rate constant expression satisfactorily describes concerted dissociative reductions in the condensed phase. A similar treatment was used by Wentworth and coworkers to describe dissociative electron attachment to aromatic and alkyl halides in the gas phase. ... 

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