How experimental details matter. The case of a laccase-catalysed oligomerisation reaction

Abstract

The Trametes versicolor laccase (TvL)-catalysed oligomerisation of the aniline dimer p-aminodiphenylamine (PADPA) was investigated in an aqueous medium of pH = 3.5, containing 80-100 nm-sized anionic vesicles formed from AOT, the sodium salt of bis(2-ethylhexyl)sulfosuccinic acid. If run under optimal conditions, the reaction yields oligomeric products which resemble the emeraldine salt form of polyaniline (PANI-ES) in its polaron state, known to be the only oxidation state of linear PANI which is electrically conductive. The vesicles serve as “templates” for obtaining products with the desired PANI-ES-like features. For this complex, heterogeneous, vesicle-assisted, and enzyme-mediated reaction, in which dissolved dioxygen also takes part as a re-oxidant for TvL, small changes in the composition of the reaction mixture can have significant effects. Initial conditions may not only affect the kinetics of the reaction, but also the outcome, i.e., the product distribution once the reaction reaches its equilibrium state. While a change in the reaction temperature from T ≈ 25 to 5 °C mainly influenced the rate of reaction, increase in enzyme concentration and the presence of millimolar concentrations of chloride ions were found to have significant undesired effects on the outcome of the reaction. Chloride ions, which may originate from the preparation of the pH = 3.5 solution, inhibit TvL, such that higher TvL concentrations are required than without chloride to yield the same product distribution for the same reaction runtime as in the absence of chloride. With TvL concentrations much higher than the elaborated value, the products obtained clearly were different and over-oxidised. Thus, a change in the activity of the enzyme was found to have influence not only on kinetics but also led to a change in the final product distribution, molecular structure and electrical properties, which was a surprising find. The complementary analytical methods which we used in this work were in situ UV/vis/NIR, EPR, and Raman spectroscopy measurements, in combination with a detailed ex situ HPLC analysis and molecular dynamics simulations. With the results obtained, we would like to recall the often neglected or ignored fact that it is important to describe and pay attention to the experimental details, since this matters for being able to perform experiments in a reproducible way.