Biomolecular hybrid of a conducting polymer with DNA: morphology, structure, and doping behavior

Abstract

A poly(o-methoxyaniline) (POMA)/DNA [weight fraction of DNA (W_DNA) = 0.45] hybrid was prepared by mixing their solutions in sterilized double distilled water. The solution turned green upon aging for a longer time, and the doping of POMA by DNA was complete after about 15 d of aging. The doping was confirmed from the UV-vis spectra where the 599 nm peak of POMA(EB) disappeared and a new peak for a π to localized polaron band-transition appeared. With increasing aging time the new peak gradually shifted from 674 nm at 3 h to 820 nm at 15 d of mixing and thereafter it remained constant. The absence of a free carrier tail in the UV-vis spectra indicated a coiled structure of POMA in the complex. Circular dichroism spectra of the hybrid solution indicated that the DNA conformation (double helical structure) remained unchanged in the hybrid. The SEM micrograph of the freeze-dried hybrid showed a needle-like morphology of the DNA dispersed in a polymer matrix and it was completely different from the fibrillar network morphology of pure DNA in the solid state. The TEM micrograph indicated a homogeneous dispersion of DNA fibrils in the POMA matrix. The melting temperature of the POMA-DNA hybrid showed an increase compared to that of pure DNA by 5 °C, probably caused by an electrostatic interaction between the DNA anion and the POMA radical cation generated in the doping process. WAXS investigations revealed that the DNA crystal structure remained unchanged in the hybrid whereas the POMA crystal structure might be lost. An FT-IR study suggested that interaction occurred between the phosphoric acid group of DNA and a nitrogen atom of POMA through proton transfer from the OH group of the former. A schematic model of the POMA-DNA complex randomly anchoring POMA chains with the DNA molecule was proposed. The dc conductivity of the POMA-DNA complex was found to be ca. 10⁻⁷ S · cm⁻¹. Hence, this work describes a procedure for making a DNA-conducting polymer hybrid without changing the conformation and structure of DNA.