High electrolyte concentration effect on enzymatic oxygen reduction

The nature, the composition and the concentration of electrolytes is essential for electrocatalysis involving redox enzymes. Here, we discuss the effect of various electrolyte compositions with increasing ionic strengths on the stability and activity towards O₂ reduction of the bilirubin oxidase from Myrothecium verrucaria (Mv BOD). Different salts, Na₂SO₄, (NH₄)₂SO₄, NaCl, NaClO₄, added to a phosphate buffer (PB) were evaluated with concentrations ranging from 100 mM up to 1.7 M. On functionalized carbon nanotube-modified electrodes, it was shown that the catalytic current progressively decreased with increasing salt concentrations. The process was reversible suggesting it was not related to enzyme leakage. The enzyme was then immobilized on gold electrodes modified by self-assembling of thiols. When the enzyme was simply adsorbed, the catalytic current decreased in a reversible way, thus behaving similarly as on carbon nanotubes. Enzyme mobility at the interface induced by a modification in the interactions between the protein and the electrode upon salt addition may account for this behavior. When the enzyme was covalently attached, the catalytic current increased. Enzyme compaction is proposed to be at the origin of such catalytic current increase because of shorter distances between the first copper site electron acceptor and the electrode.