DNA Binding To Conducting Polymer Films

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DNA BINDING TO CONDUCTING POLYMER FILMS

Jeong-Ok Lim, Daniel S. Minehan, M. Kamath, Kenneth A. Marx, and Sukant K. Tripathy, Department of Chemistry, University of Massachusetts at Lowell, Lowell, MA 01854.

ABSTRACT The polycation conducting polymers, oxidized polypyrrole and polyalkylthiophene, possess the ability to form complexes with polyanionic DNA molecules through largely electrostatic interactions. This study demonstrated the solution uptake and binding of 32p radiolabeled DNA by conducting polymer thick films (50-100ltm). Polypyrrole (PPy) was synthesized by electrochemical methods and poly(3-hexylthiophene) (PHT) and poly(3undecylthiophene) (PUT) were synthesized by chemical methods. The DNA binding rates on PPy films were affected by DNA concentration and the oxidation state (measured as conductivity). The DNA kinetics support a diffusion limited model for binding. We measured DNA binding levels onto all three polymer films; PUT, PHT, and PPy. The binding levels increased in the same order as the conductivities of the polymer films. DNA binding onto oxidized PPy film was diminished upon electrochemical reduction. These observations showed, therefore, the binding may be linked with the positive charge sites responsible for conduction in the polymer films. INTRODUCTION Polythiophene(PT) and polypyrrole(PPy) are two of the most important heteroaromatic conducting polymers, mainly due to their good thermal and environmental stability [ 1,2]. On the other hand, insolubility and infusibility of PPy and PT are their shortcomings. Recently, by substituting long alkyl chains (longer than 4 carbons) at the B-position thiophene based polymers can be made solution and melt processable at the expense of lowered conductivity. Reported values of the electrical conductivity of these polymers lie in a wide range, between 10-3 to 102 S/cm[3,4]. These conducting polymers all contain extended lengths of n- conjugation along their backbones enabling the delocalization of electrons [5,6]. In their most conductive form, electroactive polymers are ionically charged and balanced by the presence of counterions. These counterions polarize the charged topological defects such as bipolaron (shown in Figure 1) and allow electron transport [7]. In other words, the polaron and bipolaron cations form donor-acceptor complexes with electrolyte anions during the synthesis of the conducting polymer. Based upon our experimental data, we feel that these functional groups represent the binding sites for polyanionic DNA, which replaces the dopant counterion upon binding.

Figure 1. The proposed charge carriers in Polypyrrole : Bipolaron.

Mat. Res. Soc. Symp. Proc. Vol. 255. @1992 Materials Research Society

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DNA has been known to bind many classes of molecules, including metals, organics, and bioorganic molecules, such as drugs [8]. The polycationic polymer, PPy, also has affinity for polyanionic DNA largely through electrostatic interaction [9]. In some cases, protein adsorption onto a two dimensional surface may be modeled by the clas