Biochemical Synthesis and Unusual Conformational Switching of a Molecular Complex of Polyaniline and DNA
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ABSTRACT Polyaniline (Pani) is one of the most interesting conducting polymers because of its promising electrical properties and unique redox tunability. Recently a new template assisted, enzymatic synthetic approach has been developed which yields a water-soluble and conducting complex of polyaniline and the template used. The mild reaction conditions in this approach allow for the utilization of delicate biological systems as template materials, such as DNA. Here we report the extension of this enzymatic approach to the synthesis of Pani on a DNA matrix. DNA serves as a suitable template for complexation and formation of conducting polyaniline. The redox behavior of Pani induces reversible conformational change in the secondary structure of DNA. Circular dichroism results suggest that the secondary structure of the DNA may be reversibly switched through manipulation of the redox state of the polyaniline. INTRODUCTION Polyaniline (Pani) has received significant interest due to its ease of synthesis, stability and interesting electrical properties. However the commercial application of this polymer has been limited due to the difficulty in obtaining the polymer in a processable form. Current
investigations have been directed towards improving the processability, electrical properties, stability and environmental compatibility of Pani. Several electrochemical and chemical methods have been reported for the synthesis of Pani [ 1,2]. More recently chemical/electrochemical synthesis of Pani in the presence of an anionic polyelectrolyte has been reported [3,4]. The polyelectrolyte helps align the monomer molecules prior to the polymerization, provides the counter-ion for charge compensation in the polyaniline formed after polymerization and maintains water solubility. The low pH conditions involved in this approach however limits the choice of polyelectrolyte template to polymers that are stable in strongly acidic conditions. A biochemical approach towards the synthesis of polyaniline in near neutral pH conditions involves the use of an enzyme, horseradish peroxidase (HRP)[5]. Enzyme catalyzed polymerization of several phenols and anilines using HRP has been reported [6,7]. However in the absence of a polyelectrolyte matrix the polymer formed is of low molecular weight, highly branched and quickly precipitates out of solution. When appropriate template is present a linear, water soluble conducting form of polyaniline is obtained A detailed study of the enzymatic synthesis of Pani in the presence of an anionic polyelectrolyte, poly (sodium 4-styrene sulfonate) (SPS) has also been reported recently [8]. This polyelectrolyte assisted enzyme catalyzed approach provides the benefits of a benign synthetic route and opens new possibilities for the use of biological polyelectrolytes such as DNA as templates. 249 Mat. Res. Soc. Symp. Proc. Vol. 600 0 2000 Materials Research Society
The electrostatic interaction between the phosphate groups (from DNA) and the protonated aniline was used in this study to align the aniline mole
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