Carbon Nanotubes as Activating Tyrosinase Supports for the Selective Synthesis of Catechols

Abstract

A series of redox catalysts based on the immobilization of tyrosinase on multiwalled carbon nanotubes has been prepared by applying the layer-by-layer principle. The oxidized nanotubes (ox-MWCNTs) were treated with poly­(diallyl dimethylammonium chloride) (PDDA) and tyrosinase to yield ox-MWCNTs/PDDA/tyrosinase <b>I</b>. Catalysts <b>II</b> and <b>III</b> have been prepared by increasing the number of layers of PDDA and enzyme, while <b>IV</b> was obtained by co-immobilization of tyrosinase with bovine serum albumin (ox-MWCNTs/PDDA/BSA-tyrosinase). Attempts to covalently bind tyrosinase provided weakly active systems. The coating of the enzyme based on the simple layer-by-layer principle has afforded catalysts <b>I–III</b>, with a range of activity from 21 units/mg (multilayer, <b>II</b>) to 66 units/mg (monolayer, <b>I</b>), the best system being catalyst <b>IV</b> (80 units/mg). The novel catalysts were fully characterized by scanning electron microscopy and atomic force microscopy, showing increased activity with respect to that of the native enzyme. These catalysts were used in the selective synthesis of catechols by oxidation of <i>meta</i>- and <i>para</i>-substituted phenols in an organic solvent (CH₂Cl₂) as the reaction medium. It is worth noting that immobilized tyrosinase was able to catalyze the oxidation of very hindered phenol derivatives that are slightly reactive with the native enzyme. The increased reactivity can be ascribed to a stabilization of the immobilized tyrosinase. The novel catalysts <b>I</b> and <b>IV</b> retained their activity for five subsequent reactions, showing a higher stability in organic solvent than under traditional buffer conditions