Complexation and Catalyzed Oxidative Polymerization of Catechol by Aluminum in Acidic Solution

Abstract

The role of soluble Al in catalyzing the abiotic oxidation of phenolic compounds has been investigated by monitoring the slow oxidation of aerated aqueous solutions of catechol containing Al. Spectroscopic analysis of the various oxidation products has demonstrated that aqueous Al3+ increases the rate of oxidation of catechol by O2, favoring the formation of highly colored polymeric products which may possess charge‐transfer properties. Spectral (UV‐visible, nuclear magnetic resonance, infrared, and electron spin resonance) properties of these products suggest that they have some similarities to polymers generated by alkaline oxidation of polyphenols, but they also have properties which suggest that they are related to melanins generated enzymatically byu fungi and plants. A proposed low‐pH mechanism for the role of Al in promoting oxidative polymerization is based on the concept that Al3+ cations tend to stabilize o‐semiquinone radicals at low pH and direct the manner in which these radicals polymerize. Aluminum may also stabilize charge‐transfer complexes by bridging oxidized and reduced molecules. Preliminary evidence is presented to suggest that the surfaces of Al oxides in soils may, by a similar mechanism, catalyze the oxidation of phenolic compounds by O2.

Aluminum Facilitation of the Iron-Mediated Oxidation of DOPA to Melanin

“…3), characterized by four prominent bands at 3423, 1627, 1491 and 1400 cm -1 , is similar to the spectrum in Ref. 37. The 1627 cm -1 peak is near the position for an aromatic ring, carboxylate and the complex of o-quinone and metal ions.…”

“…[28][29][30][31][32][33] It is known that aluminum ions can bind to catechol and o-semiquinone radical centers within melanin polymers to yield chelate complexes, [34][35][36] but the mechanism of enhancement oxidation of catechol by aluminum ions is unclear. 37 Several enzymes, including glycollate and urate oxidases, liberate H2O2 during their action and hydrogen peroxide may also be actively generated during non-enzymatic reactions, the major source being the dismutation of the superoxide anion radical. [38][39][40][41] The lens of the human eye contains micromolar concentrations of H2O2; H2O2 vapor has been detected in expired human breath.…”

Pyrogallol Transformations as Catalyzed by Nontronite, Bentonite, and Kaolinite

“…Electrons seem to be able to diffuse or tunnel to octahedral sites from layer edges or basal surfaces (Tennakoon et al, 1974). Spectroscopic evidence has demonstrated that A13+ increases the rate of oxidation of a phenolic compound, catechol, by 02 (McBride et al, 1988). Our data indicate that oxidation of pyrogallol resulted in the formation of the semiquinone radical, ring cleavage, polymerization of pyrogallol, polycondensation of its fragments, and the subsequent release of protons.…”

Mn-Catalyzed Oxidation of Multiple-Ringed Aromatics

“…Research addressing pesticides and single-ringed phenolic compounds has identified important environmental fate mechanisms, including enzymatic-and abiotic-catalyzed oxidation that often results in polymerization of pesticide molecules [1][2][3][4]. Qiu and McFarland [5] studied degradation/transformation of [14C]benzo(a)pyrene [B(a)P] in two different soils with and without Phanerochaete chrysosporium inoculation.…”