Microscopic acid dissociation constants of 3,4-dihydroxyphenylpropionic acid and related compounds, and 3,4-dihydroxyphenylalanine (DOPA)

The dissociation constants of 3,4-dihydroxyphenylpropionic acid and related compounds and of DOPA were determined by potentiometric titration and complementary tristimulus colorimetry at 25 degrees and mu = 0.1 (NaClO(4)) in aqueous solution. The thermodynamic parameters were calculated from the values of the dissociation constants at various temperatures. The dissociation constants and corresponding thermodynamic parameters for the first phenol group of the catechols showed almost the same values as those of the phenol derivatives. In the dissociation of the second phenol group of the catechols, formation of an intramolecular hydrogen bond was indicated. The microscopic acid dissociation constants of 3,4-dihydroxyphenylacetic acid and 3,4-dihydroxyphenylpropionic acid were calculated by two different methods. In the physiological pH-region (pH 7.2-7.4), 3,4-hydroxyphenylpropionic acid is present in the carboxylate form, and the two possible monophenolate anions are present to the extent of about 45% and 40%, respectively, at pH 11.0. The twelve micro-constants for the eight chemical species from DOPA were similarly evaluated.