Propidium Iodide (CAS 25535-16-4)

Propidium Iodide (PI) is a fluorescent dye that binds to nucleic acids and is widely used in cell biology to assess cell viability or to analyze cell cycle stages by flow cytometry and fluorescence microscopy. Structurally similar to the nucleic acid base guanine, PI is impermeable to live cells but can penetrate cell membranes of dead or dying cells where it intercalates between the bases of DNA and, to a lesser extent, RNA. Upon binding to nucleic acids, the fluorescence of PI is significantly enhanced, emitting a bright red fluorescence under illumination, making it an excellent tool for identifying and quantifying necrotic and apoptotic cells. The mechanism of action involves the intercalation of PI into double-stranded areas of nucleic acids; this property is exploited in research to assess membrane integrity as a marker of cell death. Additionally, PI is used in conjunction with other DNA-binding dyes in flow cytometry to evaluate the DNA content of cells, thereby facilitating the analysis of cell cycle phases such as G1, S, and G2/M phases. The distinct fluorescence emission of PI at around 617 nm allows for its use in multi-color labeling experiments, where it can be combined with other fluorophores for simultaneous detection of multiple cellular targets, enhancing the depth of cellular analysis in research settings.