The influence of anionic vesicles on the oligomerization of p-aminodiphenylamine catalyzed by horseradish peroxidase and hydrogen peroxide

• The aniline dimer, PADPA, was oligomerized with HRPC/H2O2 in the presence of AOT vesicles forming a stable oligo(PADPA)-AOT vesicle suspension.
• Besides emeraldine salt-type oligomeric products, other products, containing oxygen atoms and/or phenazine-type units, were also formed.
• Oligo(PADPA)-AOT exhibited redox activity in the range pH = 1.1–6.0, while the product obtained without vesicles was redox active only under very acidic conditions.
• The structure-controlling (template) effects of AOT vesicles were discussed.

The aniline dimer N-phenyl-1,4-phenylenediamine (=p-aminodiphenylamine, PADPA) was oxidized with horseradish peroxidase isoenzyme C (HRPC) and hydrogen peroxide (H2O2) to oligo(PADPA) in an aqueous suspension of 80–100 nm-sized anionic vesicles at pH = 4.3 and at T ≈ 25 °C. The vesicles were formed from AOT (=sodium bis(2-ethylhexyl) sulfosuccinate) and served as templates for obtaining oligo(PADPA) as emeraldine salt form of polyaniline (PANI-ES) in the polaron form. The optimal reaction conditions for obtaining a stable oligo(PADPA)-AOT vesicle suspension with a high conversion and low amounts of HRPC were elaborated by using UV/vis/NIR spectroscopy. The formation of PANI-ES type products was confirmed by in situ UV/vis/NIR, Raman and EPR spectroscopy measurements. However, HPLC–MS analyses indicated that the oligo(PADPA) products obtained are not only of PANI-ES type, but that a number of other products formed as well. Some of these were identified and shown to contain oxygen atoms (as a result of hydrolysis), others most likely containing phenazine (or phenoxazine) moieties. The latter is also supported by Raman and UV/vis/NIR spectroscopy measurements. The as-obtained oligo(PADPA)-AOT vesicle suspension was also analyzed by cyclic voltammetry, which showed redox activity of oligo(PADPA) up to pH = 6.0, a property which is beneficial for various applications. Overall, the work demonstrates the successful use of different complementary and sensitive in situ spectroscopic methods for following the progress of a complex enzyme-triggered reaction. In addition, the good solubility of the deprotonated and reduced reaction products in organic solvents allowed us to use a HPLC–MS analysis for a direct comparison of the product distribution with the previously studied oxidation and oligomerization of PADPA with Trametes versicolor laccase at pH = 3.5. One of the template effects of AOT vesicles takes into account possible interactions between AOT and oligo(PADPA).