Spectroscopic characterization of different protonation/deprotonation states of Barbaloin in aqueous solution

Barbaloin (10-glucopyranosyl-1,8-dihydroxy-3-(hydroxymethyl)-9(10H)-anthraquinone: aloin A), present in Aloe species, is widely used in food, cosmetic and pharmaceutical industries. Here we characterize its optical absorption and emission spectra in aqueous solution at different pH values. Through pH titration, using both absorption and fluorescence spectroscopy, two pK_a values for Barbaloin were determined: pK_a1=9.6±0.6 and pK_a2=12.6±0.8. These acidity constants were found to be higher than those found for Emodin, a similar molecule which lacks the sugar moiety present in Barbaloin. Performing quantum mechanical calculations for non-ionized, singly, doubly, and triply deprotonated forms of Barbaloin in vacuum and in water, we assigned the positions of the site for the first and third deprotonation in the anthraquinone group, and the second deprotonation in the glucose group. The instability of Barbaloin in high pH solutions is discussed here, and the optical absorption and fluorescence spectra due to products resulted from Barbaloin degradation at high pH is well separated from the Barbaloin original spectra. Biological fluids have specific pH values to maintain homeostasis, hence determining the pK_a of Barbaloin is important to evaluate the mechanism of action of this drug in different parts of an organism as well as to predict pharmacological relevant parameters, such as absorption, distribution, metabolism, and excretion.