Coupling Xanthine Dehydrogenase to Electrodes By Means of a Polymer and 3D Structures
Authors: Lisdat Fred 
Published: February 2020

Polymers can provide a suitable matrix for immobilization of active biomolecules on surfaces. Furthermore they can be used to wire redox proteins with electrodes. Here mainly redox polymers with fixed redox centres and conducting polymers with intrinsic electronic conductivity are applied. By focusing on the second approach we have studied the interaction of differently substituted sulfonated polyanilines with different dehydrogenases. Although the enzymes are structurally rather different efficient interaction with the polymers were found. Besides PQQ glucose dehydrogenase1,2 and fructose dehydrogenase3recently xanthine dehydrogenase (XDH) has been studied.
XDH is a complex molybdenum-iron-sulfur flavoprotein, which belongs to the xanthine oxidoreductase family of molybdoenzymes, comprising four redox active cofactors: a mononuclear, five-coordinated molybdenum center, two [2Fe-2S] clusters, and a flavin adenine dinucleotide.
For the study a copolymer PMSA1 (poly(2-methoxyaniline-5-sulfonic acid)-co-aniline has been synthesized and characterized. The polymer-enzyme reaction has been investigated in solution first with UV/Vis and cyclic voltammetry. These measurements indicate that electron transfer from the substrate-reduced enzyme to the polymer is feasible.
The bioelectroctrocatalytic behavior on ITO electrodes has been investigated in dependence on the solution pH. Here two processes could be elucidated: On the one hand polymer supported enzyme electrocatalysis and on the other hand product-mediated enzyme communication with the electrode. Interestingly the polymer supported bioelectrocatalytic process starts already at rather low pH and becomes inefficient at basic pH values – mainly because of decrease in conductivity of the polymer.
On the basis of efficient electron exchange between the catalytic protein and the polymer different enzyme electrodes are constructed. First, enzymes have been fixed on planar ITO electrodes by means of a polymer layer. Such enzyme electrodes exhibit well-defined, substrate-induced bioelectrocatalytic currents starting at rather low potential (-0,05V vs Ag/AgCl). The efficiency can be significantly enhanced when macroporous ITO electrodes are used. They have pore size diameters of ~ 300nm and allow incorporation of higher amounts of polymer and enzyme. A current enhancement of at least one order of magnitude is found compared to flat ITO.
The polymer-enzyme reaction can not only be exploited electrochemically but also spectroscopically since the redox states of the polymer differ in colour. Furthermore enzyme polymer composites can be used on different electrode materials such as gold or carbon. Thus, enzyme polymer interaction can have not only applicability in sensing but also in biofuel cell developments.
1 D. Sarauli, C. Xu, B. Dietzel, B. Schulz, F. Lisdat, Acta Biomaterialia 9 (9) (2013) 8290-8298
2 D. Sarauli, C. Xu, B. Dietzel, B. Schulz, F. Lisdat, Journal of Materials ChemistryB 2 (21) (2014) 151-3404
3 D. Sarauli, C. Wettstein, K. Peters, B. Schulz, D. Fattakhova-Rohlfing, F. Lisdat, ACS Catalysis 5 (2015) 2081-2087