Site binding volume change

Strauss, U. P. Leung, Y. P. (1965). Volume changes as a criterion for site binding of counterions by polyelectrolytes. Journal of the American Chemical Society, 87, 1476-80. 

Because plasma protein and tissue binding influences the amount of drug in each body compartment, the volume of distribution of a drug may be dose dependent (nonhnear). A limited number of binding sites may result in capacity-limited binding in plasma or tissues. Transport from the blood may also be capacity limited. Examples of dose-dependent volume changes show that the volume of distribution of recombinant human tumor necrosis factor (TNF) alpha decreases sharply with a fourfold increase in dose and that the volume of distribution of recombinant human DNase,... 

The results may be compared with the observed sequences O ) for reactions involving equilibrium between the alkali metal cations and carboxylated polyanions, which give an order of affinity Li>Na>K. The equilibrium sequence, which is a measure of the free energy change in the reaction, can only be reconciled with the enthalpy data if the endothermic replacement of the condensed sodium ion by a lithium ion is accompanied by an increase in the total entropy. This would accord with the observed order of volume changes, Li>Na>K, for the site binding (condensation) of alkali metal cations to polycarboxylates,... 

The difference between r and ro may be illustrated by way of an example. Assume a cube of dimensions 1x1x1 cm. Its volume, of course, is 1 cm. If it is swelled to 10 cm volume, then each linear dimension in the sample is increased by (10), the new length of the sides, in this instance. Assuming an affine deformation, the end-to-end distance of the chains will also increase by (10) that is, r, = (10) ro. The value of ro does not change, because it is the end-to-end distance of the equivalent free chain. The value of r is determined by the distances between the cross-Unk sites binding the chain. Again, neither r, nor ro represents the end-to-end distance of the whole primary chain but rather the end-to-end distance between cross-link junctions. 

On the other hand t and A r contain the rate constants of the equilibrium under study and, for bimolecular reactions, the concentrations of the species involved in the reaction [4b, 8]. Had counter-ion site binding been a one step process the determination of t would have provided a direct estimation of the lifetime of the bound counter-ions and, therefore, of the exchange rate between bound and free counterions. On the other hand the study of as a function of concentration would have permitted to obtain informations on the distribution of bound counterions between those bound with and without dehydration. Unfortunately, as will be shown in Section 4, site binding is a multistep process involving at least two equilibria. All of the unknown quantities involved in such a process (four rate constants, two volume changes and the concentrations of the species) cannot be obtained from ultrasonic absorption data alone. Independent measurements become necessary. For this purpose we have measured the density d of the polyelectrolyte solutions from which can be obtained the apparent molal volume FcP of the polyelectrolyte CP (C counterion, P polyion) according to ... 

Apparent Molal Volumes of Polyelectrolytes and Their Use for the Determination of the Volume Change Upon Counter-ion Site Binding... 

Equation (6) has been verified to a high degree of accuracy for simple electrolytes. It assumes a complete ionization of the salt at infinite dilution. An examination of Table II reveals that for each of the polyelectrolytes studied in this work the values of (Fp )c depend on the counter-ion. This result means that the additivity law does not hold for polyelectrolyte solutions. A simple explanation for this behaviour is provided by Manning s theory which states that even at infinite dilution part of the counter-ions remain condensed on the polyions. On the other hand, dilatometry [2], refractometry [3] and ultrasonic absorption [9, 10] have shown that part of the condensed counterions and of the sites on which they are bound are dehydrated. Therefore, the value ()c of the apparent molal volume of the polyion P, as obtained from Equation (6) >vill include the volume change dVc associated with the binding of C. This is why the subscript C was placed on the apparent molal volume of the polyion P in Equation (6). If we call (Fp )true> the true apparent molal volume of the polyion P it may be assumed that (Fp )true (Fp)tma since there are numerous evidences [9,10, 18, 4c] that a negligible volume change is associated with the condensation of TMA" ion (this ion... 

Substances that do not target the active site but display inhibition by allosteric mechanisms are associated with a lower risk of unwanted interference with related cellular enzymes. Allosteric inhibition of the viral polymerase is employed in the case of HIV-1 nonnucleosidic RT inhibitors (NNRTl, see chapter by Zimmermann et al., this volume) bind outside the RT active site and act by blocking a conformational change of the enzyme essential for catalysis. A potential disadvantage of targeting regions distant from the active site is that these may be subject to a lower selective pressure for sequence conservation than the active site itself, which can lower the threshold for escape of the virus by mutation. 

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