New investigations on hematoxylin, hematein, and hematein-aluminium complexes. I. Spectroscopic and physico-chemical properties of hematoxylin and hematein.

Analytically pure hematoxylin (Htx), pentaacetylhematoxylin (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 pKa 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.