Iron complexes of gallocatechins. Antioxidant action or iron regulation?

Iron(III) complexes of gallocatechins were studied in aqueous Solutions by UV-VIS spectrometry, HPLC, cyclic voltammetry, laser photolysis and pulse radiolysis techniques. The blue-violet colored complexes are readily formed in water. The Job plots indicate 1:1 stoichiometry for the reaction of iron(III) with gallocatechins and methyl gallate, and 1:3 for that of iron(III) and catechin. This suggests that the three phenol groups of the gallate moiety play a role in complex formation. The formation constants of the complexes are found to be pH dependent, as expected for polyhydroxybenzene derivatives. pK(a1) = 4.3 and pK(a2) = 7.4 for the polyphenols with the gallate ester moiety (epigallocatechin gallate and epicatechin gallate) are lower than those of epigallocatechin (EGC) and catechin (pK(a1) = 4.9 and pK(a2) = 8.4), very probably because of the electron-withdrawing effect of the ester. Apparent stability constants of iron(III) gallocatechin complexes are high at pH 7, log K approximate to 27, comparable to those of the catechol derivatives. Photoionization of the iron complexes by the 248 nm laser is more efficient at higher pH, phi = 0.13 at pH 7 vs. phi = 0.26 at pH 11.5. The absorption spectra, which resemble those of ligand phenoxyl radicals, indicate that photoionization yields unstable phenoxyls, t(1/2) similar to 1 ms. Similar spectra are recorded when one-electron oxidation by the azide radical, N-3(.), is used to generate the ligand radicals. The reduction potential of Fe(III)gallocatechins is -0.325 V vs. NHE, which is similar to 0.45 V less negative than the reduction potential of the Fe(II)/Fe(III) couple. In the case of the catechins with the gallate ester moiety,namely EGCG and ECG,the high pH cyclic voltammograms exhibit a quasi-reversible oxidation-reduction not seen in the other derivatives. The relevance of these findings for the physiological function and antioxidant and chemopreventive action of gallocatechins Is discussed.