Histidine dissociation curves

The equilibrium dialysis experiment revealed that histidine-substituted salicylamide was selected as an RNA ligand. Subsequent binding analysis by UV titrations and Job plot revealed the histidine-substituted salicylamide Cu + complex bound the target RNA hairpin with an apparent dissociation constant of 150 nM. This binding constant likely reflects more complex binding processes than a simple 1 1 interaction, as the observed binding curve saturates well below the concentration of the histidine-substituted salicylamide, and thus the actual affinity of the complex for targeted RNA is probably lower. Importantly, however, titrations with the... 

Figure 2-6. Titration curves for glycine (A) and histidine (6). The molecular species of glycine present at various pHs are indicated by the molecules above the curve. For histidine, pKa is the dissociation constant of the imidazole (side chain) group.

The conversion of -ammonium groups to guanidinium by such reagents as 0-methylisourea results in a substantial elevation of their pK, and thus has been used for purposes similar to those just described in connection with formaldehyde. Tanford (1950) found that the titration of guani-dinated human serum albumin could be fitted by a curve computed on the assumption that all the e-ammonium groups had been changed to guanidinium, i.e., that below pH 12 only the readily separable dissociations of histidine and tyrosine were involved. 

The dissociation constants of amino acids can be determined, for example, by titration of the acid. Figure 1.2 shows titration curves for glycine, histidine and aspartic acid. Table 1.2 lists the dissociation constants for some amino acids. In amino acids the acidity of the carboxyl group is higher and the basicity of the amino group lower than in the corresponding carboxylic acids and amines (cf. pK values for propionic acid, 2-propylamine and alanine). As illustrated by the comparison of pK values of 2-aminopropionic acid (alanine) and 3-aminopropionic acid ( 3-alanine), the pK is influenced by the distance between the two functional groups. 

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