Simple method for the preparation of DNA-poly(o-methoxyaniline) hybrid: structure, morphology, and uncoiling of poly(o-methoxyaniline) on the DNA surface

Abstract

The DNA-poly(o-methoxyaniline) (POMA) hybrid is prepared by mixing aqueous solutions of POMA [emeraldine salt (ES), doping level [Cl]/[N]=0.52] and sodium salt of DNA (Na-DNA) and is then freeze-dried after 4 days. Three different compositions (W_DNA = 0.25, 0.5, and 0.75, W_DNA is the weight fraction of DNA) of the hybrids are prepared. The SEM pictures show a gradation in morphology; for example, for W_DNA = 0.75, fibrils are present but, at lower DNA concentration, a fibrillar network structure of the hybrid is observed. The circular dichroism (CD) spectra of the hybrid solutions indicate unchanged DNA conformation and WAXS patterns indicate intact crystal structure of DNA in the hybrid. The UV-vis spectra suggest no denaturation of DNA during the blending process. The UV-vis spectra of the hybrids in aqueous medium show a gradual red shift of the π band to polaron band transition with time. The plots of these wavelengths with time are sigmoidal, indicating the autocatalytic nature of the process. With an increase in the temperature, the rate of the red shift of the above peak is faster. Arrhenius analysis of the rate (1/t₇₀₀ where t₇₀₀ is the time required to reach the absorption maximum at the wavelength 700 nm) yields straight lines for the three hybrid compositions with activation energy values of 13-15 kcal/mol. These values are almost equal to the activation energy values of conformational transition of other polymers, supporting the red shift in UV-vis spectra occurs from conformational transition. DNA offers a surface where POMA gets adsorbed and the stable conformational transition resulting in the uncoiling of the POMA chain occurs through repulsive interaction among bound radical cations of POMA (ES) on the DNA surface. The conductivity of the hybrids is on the order of 10^-6 S/cm, and the I-V characteristic curves indicate the semiconducting nature of the hybrids.