Detection of the Enzymatic Cleavage of DNA through Supramolecular Chiral Induction to a Cationic Polythiophene.

Abstract

Water-soluble π-conjugated polymers are increasingly envisioned in biosensors, in which their unique optical and electronic properties permit a highly sensitive detection of biomolecular targets. In particular, cationic π-conjugated polymers are attractive for DNA sensing technologies, through the use of the fluorescence signals either in physiological solutions or in thin films. However, in the context of enzymatic activity assays, fluorescence-based methods require covalently labeling DNA with a dye or an antibody and are limited to short time scale due to dye photobleaching. In this frame, we report here a novel possible approach to probe the cleavage of DNA by a restriction enzyme, in continuous and without covalently labeled DNA substrate. This is achieved by exploiting unique chiroptical signals arising from the chiral induction of DNA to a poly[3-(6'-(trimethylphosphonium)hexyl)thiophene-2,5-diyl] upon interaction. The cleavage of DNA by HpaI, an endonuclease enzyme, is monitored through circular dichroism (CD) signals in the spectral range where the polymer absorbs light, i.e., far away from the spectral ranges of both DNA and the enzyme. We compare the results to a conventional noncontinuous assay by polyacrylamide gel electrophoresis, and we demonstrate that induced CD signals are effective in probing the enzymatic activity. By means of molecular dynamics simulations and calculations of CD spectra, we bring molecular insights into the structure of DNA/polymer supramolecular complexes before and after the cleavage of DNA. We show that the cleavage of DNA modifies the dynamics and the organization of the polymer backbone induced by the DNA helix. Altogether, our results provide detailed spectroscopic and structural insights into the enzymatic cleavage of DNA in interaction with a π-conjugated polymer, which could be helpful for developing chiroptical detection tools to monitor the catalytic activity in real time.