Laccase/TEMPO-mediated glycerol oxidation Spectroscopic and Electrochemical assessment.

Abstract

Glycerol oxidation has been a subject for several studies all over the world; in general, these studies main purpose is to find an appropriate catalyst to convert, selectively, this building block into value-added products. Laccases immobilized onto solid supports are environment-friendly heterogeneous catalysts, which can be applied for glycerol oxidation in a laccase-mediated system. These enzymes are copper containing proteins that catalyze the oxidation of many phenolic compounds by water reduction to di-oxygen. Despite their specificity to phenolic compounds, when appropriately combined with a mediator, laccase can also act on non-phenolic molecules. The FAPESP doctoral project 2016/24303-0 approaches the synthesis and characterization of nano-sized zeolites, their application as support for different laccases immobilization, and the obtained complexes application for glycerol oxidation mediated by 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO). All zeolites were synthetized according to the literature, in some cases modified by ion exchange with Cu2+. The as-made or copper derivative materials were then amino-functionalized, so the laccases could be covalently immobilized. The supports were characterized by XRD, SEM-EDX, HRTEM, FTIR, and the free enzymes or complexes activities assayed spectroscopically by 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) oxidation. Some complexes have shown considerably ABTS oxidation activities, which allowed selecting these catalysts as potential catalysts for the proposed application. Nevertheless, under several assay conditions, the selected complexes have shown poor catalytic applicability to glycerol oxidation. On the other hand, under the same conditions, the free laccases have shown potential for this reaction, and some products like glyceraldehyde, glyceric acid and oxalic acid yielded. Considering that, an alternative to overcome these drawbacks is to investigate the system spectroscopically and electrochemically. Understanding, in a molecular level, the electron transfer steps involved in the Laccase/TEMPO-mediated glycerol oxidation, would certainly facilitate the system modulation and selectively oxidize glycerol. Detailed information on specific system conditions are difficult to access by means of common bench techniques. In order to know how the support and enzyme are interacting, how these interactions are affecting the enzyme activity, how the complex or the free enzyme act over the substrate, if there are some structural or physicochemical conditions restricting the catalyst action, etc., more advanced techniques are required, such as Nuclear Magnetic Resonance (NMR). In fact, considering the nature of laccases; which are copper-oxidases containing four cooper atoms in their catalytic site, one of them paramagnetic; it can be assessed with the support of the Electronic Paramagnetic Resonance (EPR) combined with Nuclear Magnetic Resonance and Electrochemistry (EC), the latter to monitor the electron transfer of oxidoreduction reactions. This investigation can be executed at Liverpool John Moore University (LJMU), which possess the entire scientific infrastructure for the experiments proposed, under the supervision of Professor Dr. Alistair J. Fielding, an expert in NMR, EPR and EC. By combining these technics, we will be able to assess the system in a molecular level, getting insights on catalyst physicochemical properties and mechanism, which together shall provide the information needed to modulate the system in order to achieve better catalytic results.