A Colorimetric and Fluorometric Chemosensor Based on Quinizarin-Metal Complexes for Alkaline Phosphatase Activity

Hydroxyanthraquinones are well known colorimetric indicators for pH and metal ions. In high pH media, these substances exist in phenolate anionic forms, which have absorption maxima in the visible region. In addition, hydroxyanthraquinone phenolates readily bind metal ions, such as magnesium and aluminum, to produce highly fluorescent metal-anthraquinone complexes. For example, a greater than 30 fold increase of fluorescence intensity is observed when 1,4-dihydroxyanthraquinone (quinizarin, QNZ, 1) complexes with aluminum ion. Tight binding by these quinizarins is due to metal ion interactions with the ionized hydroxyl groups and neighboring carbonyl moieties, as depicted in 2. If the hydroxyl groups of these substances are protected, metal-QNZ complex formation does not take place. Moreover, significant differences in absorption maxima and fluorescence intensities are expected for protected and metal complexed QNZ. Thus, appropriately protected QNZs should serve as dual signal (colorimetric and fluorometric) chemosensors for processes (e.g., enzymatic) that promote cleavage of the hydroxyl protecting groups. In this communication, we report, the results of initial studies leading to the development of a hydroxyquinone-based, enzyme-cleavable colorimetric and fluorometric chemosensor, quinizarin diphosphate (QDP, 3), and its application to a method for assaying alkaline phosphatase activity.