New investigations on hematoxylin,hematein, and hematein-aluminium complexes

Summary Analytically pure hematoxylin (Htx), penta-acetylhematoxylin (PAHtx), and hematein (Hm) were isolated and characterized by ¹H-NMR spectroscopy. The VIS/UV spectra of Htx and Hm were recorded in MeOH and in H₂O at various pH values. The molar extinction coefficients of the long wavelength absorption bands are reported. The pK_a value for the 1st acidic dissociation step of Hm has been determined from the pH dependency of the absorption spectra of Hm in aqueous buffer solutions. Finally, the absorption spectra are qualitatively discussed.