In order to assess a possible role for A13+ in dementias, its stability constants for binding to four catecholamines have been determined by potentiometric titration at 25 °C and 0.2 M ionic strength. Because A13+ binds to the catecholate locus with the ammonium group still protonated, conventional treatments are not applicable and the concentration of the chelating microform requires estimation. Owing to the high catecholate basicity, uncertainty in the pKa value for the last phenolic ionization, and the low concentration of the chelating microform, conditional stability constants applicable to pH 7.0 have also been calculated. For the addition of one through three ligands to Al3+, the successive average pH 7.0 conditional stability constant logarithms are as follows: L-Dopa and dopamine, 8.05, 5.32, and 1.25; norepinephrine and epinephrine, 8.27, 5.69, and 1.94. The corresponding values for tiron (l,2-dihydroxybenzene-3,5-disulfonate) are significantly enhanced to 10.4, 7.9, and 4.6, making tiron the strongest catecholate Al3+ binder in neutral solutions. The conditional stability constants indicate that about a 1000-fold excess of catecholamine is required for its Al3+ binding to compete with that of citrate and a 100-fold excess for catecholamine-Al3+ binding to compete with that of ATP.
ASJC Scopus subject areas
- Colloid and Surface Chemistry