Determination of the stability constants for the binding of sulfonated morin with Fe2+

The complexation of the sodium salt of sulfonated morin (H5SM−) with Fe2+ was studied by potentiometric titration as was its deprotonation. Only four of the five hydroxy groups were deprotonated under the conditions employed. The associated pKa values are 3.80, 7.47, 9.24 and 11.48. Analysis of the titration data suggests formation of (H3SM)Fe−, (H2SM)Fe2−, (HSM)Fe3− and (HSM)2Fe8−. Log β values (based on HSM5− as the ligand species) are 24.8, 16.1, 7.1 and 11.6, respectively. Theoretical calculations predict that the 7-hydroxy group is deprotonated first followed closely by the 3-hydroxy position. Deprotonation of the 2′-hydroxy group results in proton migration from the 3-hydroxy oxygen atom. These calculations along with previous results suggest that chelation of the metal ion likely occurs at the 3-hydroxy-4-keto site.