Microscopic dissociation constant

If the microscopic dissociation constants are not equal, then the Adair equation predicts a nonhyperbolic (i.e., cooperative) curve. Values for and 24 can be obtained by studying ligand binding at very low and very high ligand concentrations. Values for / 2 and 23 can be ob-... 

The dissociation constant is about 1.7 x 10 5 and pKa = 4.8. Since there is only one proton, the observed dissociation constant is also the microscopic dissociation constant. Now consider the cation of pyridoxine which has two dissociable protons bound to distinctly different sites, the phenolic oxygen and the ring nitrogen. 

Either of the two protons might dissociate first as the pH is raised (Eq. 6-75). However, the two microscopic dissociation constants pK x and pKB are distinctly different. The result is that at 25°C in the neutral (monoprotonated) form 80% of the molecules carry a proton on the N, while the other 20% are protonated on the less basic - O. Notice that the subscripts a and b used in this discussion do not refer to acidic and basic but to the individual dissociation steps shown in Eq. 6-75. Microscopic constants will always be indicated with asterisks in this discussion. 

The four microscopic dissociation constants were determined using 0.1 ionic strength carbonate/ bicarbonate buffers with the values pki = 9 74, pk2= 10.56, pk 2 = 11 17 and pk2i = 10.35. From these the macroscopic dissociation constants were determined from the relations ... 

The effect of pH on the fluorescence of ciprofloxacin and norfloxacin has been studied, and the ground-state microscopic dissociation constants were determined [14]. After their acidic solutions (pH 2-4) were allowed to stand for 5 min at 25°C, the optimum wavelength for fluorescence determinations was found to be 445 nm (excitation at 280 or 331 nm) of these compounds. 

By measurement of the dissociation constants of the amino acids, and of suitable esters and other derivatives. The data have been analyzed by Edsall and co-workers (Edsall and Wyman, 1958 Martin et od., 1958) so as to yield, after suitable assumptions, the twelve microscopic dissociation constants. The ones applicable here are those which refer to dissociation of the group in question from an otherwise uncharged molecule. 

Either of fhe two protons might dissociate first as the pH is raised (Eq. 6-75). However, the two microscopic dissociation constants pR dis-... 

Many glycosidases exhibit such a pH-rate profile and the temptation to equate Ka with the dissociation constant of the acid-catalytic group and A b with the dissociation constant of the base or nucleophile frequently proves irresistible. Even in an Alberty-Bloomfield system, however, the equation should be not made unthinkingly. and A b are macroscopic dissociation constants, which are related to the four microscopic dissociation constants of the two-carboxylate system (Figure 5.13) by eqns (5.38) and (5.39) (since Figure 5.13... 

Van de Graaf B, Hoefnagel AJ and Wepster BM, Substituent effects. 7. Microscopic dissociation constants of 4ramino- and 4-(dimethylamino)benzoic acid, /OC, 46(4), 653-7 (1981). NB The method gave a pJC value for benzoic acid of 4.21 (comparable to the best in the literature) and hence file mefiiod was well validated. Results at low ionic strengfiis (0.04 to 0.06) were corrected to zero ionic strength. 

Microscopic dissociation constants of 3-hydroxy-a-(methylamino) methylbenzene-methanol have been calculated from the titration spectrophotometer data (c = 3.8 x 10" M. Ionic strengdi = 0.16 buffer system H3BO3/KOH) by application of a spectral deconvolution method. The results found (pKa = 9.48 pJCb = 9.71 pJCc = 10.12 pi d = 9.88) are in good concordance with those obtained from the conventional regression linear method (pJCa = 9.45 pXb = 9.77 pK, = 10.14 p d = 9.81)."... 

Potentiometric titration of phenylephrine vs NaOH (Figure 4-8) yields the macroscopic dissociation constants while the microscopic dissociation constants are accessible from simultaneous observation of potentiometric and C H -NMR-controlled titrations (Figure 4-9 and Figure 4-10). TTie microscopic dissociation species defined in Table 4-1 are involved in the microscopic dissociation equilibrium shown in Figure 4-6. 

Results from C H -NMR-controlled titration of phenylephrine hydrochloride + 1 ITNO3 vs NaOH shown below in Figure 4-10 - Figure 4-12 were used to evaluate the microscopic dissociation constants. 

Table 4-3 lists the microscopic dissociation constants as obtained from carbon atoms Cl to C3 and C7 to C9. Suitable calculations showed, that Cl and C9 are good indicators holding the restrictions as required above. 

The undissociated RSH group and the RS group have widely different uv spectra (see Section II,D). Assuming that the extinction values are not influenced by the protonation state of the amino group, the microscopic dissociation constants can be determined by uv measurements at various pH values... 

The problem of interpreting macroscopic dissociation constants in terms of which groups in the molecules actually dissociate does not only occur when the pX values are close to each other. Thus, for 2,3-diaminopropionic acid with pXa and pXg values 6.78 and 9.53 the following set of pX values corresponding to microscopic dissociation constants have been determined pXg, 7.07 pXg, 7.10 pX, 9.25 pX a, 9.21. Thus, the ratio between (XXIII) and (XXIV) is 0.9, indicating equal amounts of the two species (Sayer and Rabenstein, 1976). 

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