Add time:07/25/2019         Source:sciencedirect.com

The oxidative polymerization of pyrrole with the enzyme Trametes versicolor laccase (TvL) and dioxygen (O2) in aqueous solution at pH 3.5 was investigated and found to be regulated by anionic vesicles formed from sodium bis-(2-ethylhexyl) sulfosuccinate (AOT). The polymeric products obtained in the presence of the vesicular templates have high absorption at λ ≈ 450 and 1000 nm, which is indicative for polypyrrole (PPy) in its conductive bipolaron state. Absence of unpaired electrons in the bipolaronic PPy product obtained was supported by EPR measurements. Furthermore, the FTIR spectrum of isolated PPy is comparable with the FTIR spectrum reported in literature for chemically or electrochemically synthesized PPy. Under optimal reaction conditions in the presence of the vesicles, i.e., 6.0 mM AOT, 8.0 mM pyrrole, 3.7 μM TvL, pH 3.5 (0.1 M sodium dihydrogenphosphate solution), a pyrrole conversion of about 84% was obtained after 7 days reaction time at T ≈ 25 °C. Most of the dark green PPy products remained dispersed in the aqueous medium thanks to the vesicles, while the products obtained without vesicles or in presence of sodium di-n-butylsulfosuccinate (6.0 mM) had a very different UV/vis/NIR absorption spectrum and almost completely precipitated from the solution. 2H NMR measurements of pyrrole-d5 and MD simulations of an AOT bilayer fragment, solvated into a sodium dihydrogenphosphate solution containing pyrrole, indicate partitioning of pyrrole at pH 3.5 into the region of the AOT membrane before pyrrole oxidation was initiated; however, a preferential binding and specific orientation of pyrrole in the vicinity of the AOT head group could be excluded. Binding of pyrrole to AOT bilayers at pH 3.5 was also compared with the binding of aniline to the same type of bilayers (vesicles) at pH 4.3. This comparison was made since it is known from previous work that aniline can also be polymerized enzymatically in the presence of AOT vesicles. The behavior of the two monomers is, however, quite different. Contrary to pyrrole, in the case of aniline at pH 4.3, anilinium cations were clearly shown by both methodologies – MD simulations and NMR measurements – to bind extensively to the vesicle surface, where the sulfonate group of AOT is localized. Therefore, it can be concluded that a specific pre-orientation of the monomers, before oxidation and polymerization are initiated by the enzymes, is not a common key feature of the two template-assisted reactions and, therefore, cannot be taken as a general explanation for the observed regulating effect of the vesicles. The origin of the vesicle template effect still needs to be elucidated.

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