Enhancing nucleic acid detection sensitivity of propidium iodide by a three nanometer interaction inside cells and in solutions.

Most interactions inside living cells take place at nano-length scales. The only way to monitor these nano-scale interactions inside living cells is by utilizing the phenomenon of Fluorescence Resonance Energy Transfer (FRET). Thus, the importance of FRET in visualizing intracellular dynamics has provided continuous motivation for discovery/synthesis of biologically useful molecules exhibiting FRET. While working on intracellular uptake of coumarin loaded biodegradable polymeric nanoparticles, we have discovered a remarkable utility of 6-coumarin (Co) forming a FRET pair with propidium iodide (PI). Based on observations on intracellular behavior of Co and PI, we tested and characterized the Co-PI FRET pair in solutions. We report that Co acts as photon donor to PI (photon acceptor) with a Forster's distance of approximately 3 nanometers. We show a FRET based sensitivity enhancement (30-35%) of PI for nucleic acid detection, both inside cells and in solutions. Our results open a variety of applied avenues by utilizing the newly discovered FRET pair.