Molecular assembly systems that have autonomous reproduction and Darwinian evolution abilities can be considered as minimal cell-like systems. Here we demonstrate the reproduction of cell-sized vesicles composed of AOT, i.e., sodium bis-(2-ethylhexyl) sulfosuccinate, coupled with an enzymatic polymerisation reaction occurring on the surface of the vesicles. The particular reaction used is the horseradish peroxidase-catalysed polymerisation of aniline with hydrogen peroxide as oxidant, which yields polyaniline in its emeraldine salt form (PANI-ES). If AOT micelles are added during this polymerisation reaction, the AOT-PANI-ES vesicles interact with the AOT molecules in the external solution and selectively incorporate them in their membrane, which leads to a growth of the vesicles. If the AOT vesicles also contain cholesterol, the vesicles not only show growth, but also reproduction. An important characteristic of this reproduction system is that the AOT-based vesicles encourage the synthesis of PANI-ES and PANI-ES promotes the growth of AOT vesicles.
A laccase-catalysed oligomerisation of p-aminodiphenylamine was investigated in an aqueous medium containing 80–100 nm-sized anionic vesicles formed from AOT, the sodium salt of bis(2-ethylhexyl)sulfosuccinic acid.
Oligoanilines with characteristic properties of the electrically conductive emeraldine salt form of polyaniline (PANI-ES) are promising molecules for various applications. A mixture of such oligoanilines can be obtained, for example, enzymatically under mild conditions from the linear aniline dimer p-aminodiphenylamine (PADPA) with hydrogen peroxide (HO) and low amounts of horseradish peroxidase (HRP) in an aqueous pH = 4.3 suspension of anionic vesicles formed from AOT, the sodium salt of bis(2-ethylhexyl)sulfosuccinate. However, the simultaneous formation of undesired side products containing phenazine-type units or oxygen atoms is unsatisfactory. We have found that this situation can be improved considerably by using a mixture of PADPA and aniline instead of PADPA only but otherwise nearly identical conditions. The PANI-ES-like oligoaniline products that are obtained from the PADPA and aniline mixture were not only found to have much lower contents of phenazine-type units and not contain oxygen atoms but also were shown to be more electroactive in cyclic voltammetry measurements than the PANI-ES-like products obtained from PADPA only. The AOT vesicle suspension remained stable without product precipitation during and after the entire reaction so that it could be analyzed by in situ UV/visible/near-infrared, in situ electron paramagnetic resonance, and in situ Raman spectroscopy measurements. These measurements were complemented with ex situ high-performance liquid chromatography analyses of the deprotonated and reduced products formed from mixtures of PADPA and either fully or partially deuterated aniline. On the basis of the results obtained, a reaction mechanism is proposed for explaining this improved HRP-triggered, vesicle-assisted synthesis of electroactive PANI-ES-like products. The oligomeric products obtained can be further used, without additional special workup, for example, to coat electrodes for their possible application in biosensor devices.
Many previous studies
have shown that (i) the oxidation of aniline
or the aniline dimer
p
-aminodiphenylamine (PADPA)
in a slightly acidic aqueous solution can be catalyzed with heme peroxidases
or multicopper laccases and that (ii) subsequent reactions lead to
oligomeric or polymeric products, which resemble chemically synthesized
polyaniline in its conductive emeraldine salt form (PANI-ES), provided
that (iii) an anionic “template” is present in the reaction
medium. Good templates are anionic polyelectrolytes, micelles, or
vesicles. Under optimal conditions, their presence directs the reactions
in a positive way toward the desired formation of PANI-ES-type products.
The effect of four different types of anionic templates on the formation
of PANI-ES-like products from PADPA was investigated and compared
by using
Trametes versicolor
laccase
(TvL) as a catalyst in an aqueous pH 3.5 solution at room temperature.
All four templates contain sulfonate groups: the sodium salt of the
polyelectrolyte sulfonated polystyrene (SPS), micelles from sodium
dodecylbenzenesulfonate (SDBS), vesicles from a 1:1 molar mixture
of SDBS and decanoic acid, and vesicles from sodium bis(2-ethylhexyl)sulfosuccinate
(AOT). Although with all four templates, stable, inkjet-printable
solutions or suspensions consisting of PANI-ES-type products were
obtained under optimized conditions, considerably higher amounts of
TvL were required with SDBS micelles to achieve comparable monomer
conversion to PANI-ES-like products during the same time period when
compared to those with SPS or the two types of vesicles. This makes
SDBS micelles less attractive as templates for the investigated reaction.
In situ UV/vis/near-infrared, electron paramagnetic resonance (EPR),
and Raman spectroscopy measurements in combination with an high-performance
liquid chromatography analysis of extracted reaction products, which
were deprotonated and chemically reduced, showed seemingly small but
significant differences in the composition of the mixtures obtained
when reaching reaction equilibrium after 24 h. With the two vesicle
systems, the content of unwanted substituted phenazine units was lower
than in the case of SPS polyelectrolyte and SDBS micelles. The EPR
spectra indicate a more localized, narrower distribution of electronic
states of the paramagnetic centers of the PANI-ES-type products synthesized
in the presence of the two vesicle systems when compared to that of
the similar products obtained with the SPS polyelectrolyte and SDBS
micelles as templates. Overall, the data obtained from the different
complementary methods indicate that with the two vesicle systems structurally
more uniform (regular) PANI-ES-type products formed. Among the two
investigated vesicle systems, for the investigated reaction (oxidation
of PADPA with TvL and O
2
), AOT appears a somewhat better
choice as it leads to a higher content of the PANI-ES polaron form.
Different types of templates consisting of sulfonate or sulfate groups were compared for the horseradish peroxidase/H2O2-catalysed synthesis of the emeraldine salt form of polyaniline from aniline at pH = 4.3.
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