Tautomerization microscopic dissociation constants

To calculate microscopic constants from stepwise constants and tautomeric ratios, consider Eq. 6-76 in which [HP]a and [HP]B are the concentrations of the two tautomers and kj is the first stoichiometric or macroscopic dissociation constant for the diprotonated species H2P. 

Table 2 Microscopic Acid Dissociation Constants and Tautomeric Constants of Tyrosine Peptides...

Figure 7.1 shows the pK s of some dicarboxylic acids along with the pK s of some related monocarboxylic acids. As noted earlier, complications can arise when a compound contains groups that have dissociations with close pK values [93,173,434]. In such a case one must consider the equilibria shown in Figure 7.2 for dissociation of an original acid RH Ht [173], In this scheme K, Kg, Kc, and are so-called microscopic equilibrium constants and is the equilibrium constant for tautomerization between RH and RH. . The experimentally measured (macroscopic) dissociation constants Kj and K2 pertain to the following equilibria ... 

The two monoprotonated forms of pyridoxine are the tautomeric pair shown in Eq. 6-75 and whose concentrations are related by the tautomeric ratio, R = [neutral form]/[dipolar ion], a pH-independent equilibrium constant with a value of 0.204/0.796 = 0.26 at 25°C.75 Evaluation of microscopic constants for dissociation of protons from compounds containing non-identical groups depends upon measurement of the tautomeric ratio, or ratios if more than two binding sites are present. In the case of pyridoxine, a spectrophotometric method was used to estimate R. 

---