Mechanistic aspects of the horseradish peroxidase-catalysed polymerisation of aniline in the presence of AOT vesicles as templates†
The mechanism of the horseradish peroxidase (HRP)–H2O2-catalysed polymerisation of aniline in the presence of AOT vesicles was investigated. AOT (= bis-(2-ethylhexyl)sulfosuccinate) served as vesicle-forming surfactant and dopant for obtaining at pH = 4.3 and room temperature within 24 h under optimal reaction conditions the green emeraldine salt form of polyaniline in 90–95% yield. Based on UV/VIS/NIR and EPR measurements carried out during the polymerisation reaction, and based on changes in aniline and H2O2 concentrations and HRP activity, a mechanism is proposed. According to this “radical cation mechanism” chain growth occurs on the vesicle surface through addition of aniline radical cations to the growing polymer chain. H2O2 plays two essential roles, to oxidise the heme group of HRP, and to oxidise the growing polymer chain for allowing the stepwise addition of new aniline radical cations. The entire reaction can be divided into three kinetically distinct phases. In the first rapid phase (5–10 min), the actual polymer formation takes place to yield the emeraldine salt form of polyaniline in its bipolaron state. In the second and third slower phases (1–2 days) the bipolarons transform into polarons with unpaired electrons. During the reaction, the HRP activity is decreasing until the enzyme becomes inactive after polymer formation. Reactions carried out with partially deuterated anilines were analysed by 2H magic-angle spinning (MAS) NMR spectroscopy to demonstrate the regioselectivity of the chain growth: para-coupling of the aniline units clearly dominates. Association of the formed polyaniline with the vesicle membrane is evident from cryo-TEM and SANS measurements.