Effects of some naturally occurring iron ion chelators on in vitro superoxide radical formation

The effects of some naturally occurring iron ion chelators and their derivatives on the electron transfer from ferrous ions to oxygen molecules were examined by measuring oxygen consumption rates. Of the compounds examined, quinolinic acid, fusaric acid, and 2-pyridinecarboxylic acid repressed the oxygen consumption, whereas chlorogenic acid, caffeic acid, gallic acid, catechol l-β-(3,4-dihydroxyphenyl) alanine, and xanthurenic acid accelerated it. Theoretical calculations showed that the energies of the highest occupied molecular orbitals (HOMOs) of [Fe(II)(ligand)₃]⁻ complexes were relatively high when the ligands were caffeic acid and its derivatives such as catechol, gallic acid, and l-β-(3,4-dihydroxyphenyl) alanine. On the other hand, the energies of the HOMOs of [Fe(II)(ligand)₃]⁻ complexes were relatively low when the ligands were quinolinic acid and its derivatives such as 2-pyridinecarboxylic acid and fusaric acid. The energies of the HOMOs appear to be closely related with acceleration or repression of the oxygen consumption; that is to say, when the energy of the HOMO is high, the oxygen consumption is accelerated, and vice versa.