Peroxidases phenol polymer formation

The peroxidase-catalyzed oxidative coupling of phenols proceeds rapidly in aqueous solution, giving rise to the formation of oligomeric compounds that, unfortunately, have not well been characterized, as most of them demonstrate a low solubility towards common organic solvents and water. In 1987, the enzymatic synthesis of a new class of phenolic polymer was first reported [15], whereby an oxidative polymerization of p-phenylphenol, using HRP as catalyst, was carried out in a mixture of water and water-miscible solvents such as 1,4-dioxane, acetone. 

Although several peroxidase enzymes obtained from plant, animal, and microbial sources have been investigated for their ability to catalyze the removal of aromatic compounds from wastewaters, the majority of studies have focused on using HRP. In particular, it has been shown HRP can transform phenol, chlorophenols, methoxyphenols, methylphenols, amino-phenols, resorcinols, and various binuclear phenols [7], HRP was also used for the treatment of contaminants including anilines, hydroxyquinoline, and arylamine carcinogens such as benzidines and naphthylamines [7,8]. In addition, it has been shown that HRP has the ability to induce the formation of mixed polymers resulting in the removal of some compounds that are either poorly acted upon or not directly acted upon by peroxidase [7], This phenomenon, termed coprecipitation or copolymerization, has important practical implications for wastewaters that usually contain many different pollutants. This principle was demonstrated when it was observed that polychlorinated biphenyls (PCBs) could be removed from solution through coprecipitation with phenols [20]. However, this particular application of HRP does not appear to have been pursued in any subsequent research. 

As for the oxidation of phenols, the peroxidase reaction with amines usually gives rise to the formation of polymers. This is the case, for instance, of the oxidation of aniline [36] (Fig. 6.2a). 

Typically, peroxidase-catalyzed polymerization of phenol is carried out in the presence of H2O2, which acts as an oxidizing agent. The free radicals of monomers (substrates) formed initially undergo coupling to produce dimers, and successive oxidation and coupling eventually results in the formation of polymers. The peroxidase-catalyzed polymerization of phenols and substituted phenols usually produce the polymer with complicated structures. The main structure was estimated to be of phenylene units or a mixture of phenylene and oxyphenylene units (5). 

This type of reaction is mainly restricted to heme peroxidases and it involves one-electron transfer processes with radical cations and radicals as intermediates (path 2). As a consequence, substrates are usually electron-rich (hetero)aromatics, which upon one-electron oxidation lead to resonance-stabilized radicals, which spontaneously undergo inter- or intramolecular coupling to form dimers or oligomers. This reaction is commonly denoted as the classical peroxidase activity, since it was the first type of peroxidase-reaction discovered. Examples of such reactions are shown in Scheme 2.176. Oxidation of phenols (e.g., guaiacol, resorcin) and anilines (e.g., aniline, < -dianisidine) leads to the formation of oligomers and polymers under mild conditions [1315-1317], In certain cases, dimers (e.g., aldoximes [1318], biaryls [1319]) have been obtained. 

The peroxidase-catalyzed polymerizations of phenols involve the formation of phenoxy radicals as an initial step. These radicals may either condense with each other or may be added to unsaturated compounds forming irregular high molecular polymers (see the biosynthesis of lignin, D 22.2.3). 

The enzymatic reaction kinetics on the HRP-catalyzed oxidation of p-cresol in aqueous 1,4-dioxane or methanol showed that the cataljdic turnover niunber and Michaelis constant were larger than those in water (235). Numerical and Monte Carlo simulations of the peroxidase-catalyzed polymerization of phenols were demonstrated (236). The simulations predicted the monomer reactivity and polymer molecular weight, leading to synthesis of polymers with specific molecular weight and index. In an aqueous 1,4-dioxane, the formation of monomer aggregate was observed (237), which might elucidate the specific polymerization behaviors in such a medium. 

---