Determination of pKa constants of hypericin in aqueous solution of the anti-allergic hydrotropic drug Cromolyn disodium salt

Abstract

In this work we established three from altogether six proton dissociation constants (pKa) of hydroxyl groups of hypericin in its monomeric form. The monomeric state of hypericin (5.0×10−6mol·L−1) in aqueous solution was stabilised by the presence of hydrotropic drug Cromolyn disodium salt (6.0×10−2mol·L−1). Data show that one acid-base transition occurs with the pKa of 7.8 and the other two are characterised by the apparent single pKa of 11.5. The spectral changes of hypericin above pH 13 indicate that the last two hydroxyls are deporotonized at this high pH values.