IRON COMPLEXES OF GALLOCATECHINS - ANTIOXIDANT ACTION OR IRON REGULATION

Iron(III) complexes of gallocatechins were studied in aqueous Solution s by UV-VIS spectrometry, HPLC, cyclic voltammetry, laser photolysis a nd 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 p henol groups of the gallate moiety play a role in complex formation. T he 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 (epigallocat echin gallate and epicatechin gallate) are lower than those of epigall ocatechin (EGC) and catechin (pK(a1) = 4.9 and pK(a2) = 8.4), very pro bably because of the electron-withdrawing effect of the ester. Apparen t stability constants of iron(III) gallocatechin complexes are high at pH 7, log K approximate to 27, comparable to those of the catechol de rivatives. Photoionization of the iron complexes by the 248 nm laser i s 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 o xidation 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 w ith the gallate ester moiety,namely EGCG and ECG,the high pH cyclic vo ltammograms exhibit a quasi-reversible oxidation-reduction not seen in the other derivatives. The relevance of these findings for the physio logical function and antioxidant and chemopreventive action of galloca techins Is discussed.