Germanium Dioxide and the Antioxidant Properties of Catechols

Complexes of tetra- and hexa-coordinated germanium (Adr)Cat(2)Ge (1, (Adr)Cat = adrenaline catecholate), (Ald)Cat(2)Ge (2, (Ald)Cat = 4-formylcatecholate), (Ald)Cat(3)Ge(Et3NH)(2) (3) were synthesized by the reaction of germanium dioxide with adrenaline and 3,4-dihydroxybenzaldehyde in aqueous solution. The structures of the complexes were confirmed by H-1 and C-13 NMR and IR spectroscopy, HRMS, and elemental analysis. Cyclic voltammetry in phosphate buffer solutions (pH = 6.86) has shown the oxidation potentials E-p of these complexes to be shifted by ca. 330-360 mV to more positive potentials compared to the parent aromatic diols, which means that their electron-releasing properties were diminished by about 7.6-8.3 kcal mol(-1). A study of the kinetics of the reaction of these complexes with 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical in the presence of 5 % acetonitrile reveals a decrease of approximately one order of magnitude in the radical scavenging activity of adrenaline and model 3,4-dihydroxybenzaldehyde upon their complexation with Ge. The corresponding rate constants drop from (3.23 +/- 0.07) x 10(3) L mol(-1) s(-1) (adrenaline) to (3.80 +/- 0.20) x 10(2) L mol(-1) s(-1) for 1, and from (4.40 +/- 0.20) x 10(3) L mol(-1) s(-1) (3,4-dihydroxybenzaldehyde) to (4.45 +/- 0.25) x 10(2) L mol(-1) s(-1) for 2 and 3 (germanium is equally capable to bind two or three molecules of the diol). Keeping in mind that 1,2-diols are widespread in the living nature and often have important biological functions, these results will contribute to the understanding of role of germanium dioxide in metabolic mechanisms and to alert against unreasonable use of Ge preparations.