Micro-ionization acidity constants for tetracyclines from fluorescence measurements

The pH vs. fluorescence intensity profiles for a number of tetracyclines are investigated. Fluorescence is obtained in alkaline solutions and is associated with the ionized form of the phenolic-β-diketone site of these molecules. The shape of the pH vs. fluorescence intensity profiles is best explained by representing the acid-base equilibria in solution by a micro-ionization scheme. In combination with measurements made on model compounds in which the dimethylammonium site was either blocked or removed, it proved possible to estimate values for the micro-ionization constants in aqueous solution; pk₁₃ values of 7.82, 7.85 and 7.04 were obtained for tetracycline, oxytetracycline and demethylchlortetracyline, respectively.