Deprotonation processes of ellagic acid in solution and solid states

The electronic absorption and polarized absorption spectra were measured at various pH conditions for ellagic acid. Comparison of these spectral data with MO calculations shows that ellagic acid exists in a 7:3 mixture of ethanol and water as free in the pH region less than 0.5, as a mixture of free and singly deprotonated species in the pH region 0.5-5.6, and as doubly deprotonated form in the pH region larger than 5.6, being deprotonated from hydroxyl group at position 8 (and/or 8'). In the pH region beyond 12.4, one lactone ring of the doubly deprotonated species opens to give a carboxyl derivative. Thermal and single crystal X-ray analyses show that ellagic acid forms a 1:4 complex with pyridine in the solid state. The endothermic energies indicate that the hydrogen bond energy between ellagic acid and pyridine is estimated to be ca. 11.4 kJ mol(-1) per pyridine. Ellagic acid molecules stack almost face-to-face with the intermolecular distance of ca. 4.8 Angstrom, and take column structure along the a-axis, inclining toward the a-axis. The columns of ellagic acid molecules are stabilized through pyridine-pyridine intermolecular interactions.