propidiumiodid

Propidium iodide, sometimes spelled propidium iodid in other languages, is a fluorescent nucleic acid stain widely used in molecular and cellular biology. It is a water-soluble, cationic dye that binds to DNA and RNA by intercalation and electrostatic interactions. In intact, live cells, propidium iodide cannot cross the cell membrane, so it remains excluded; when membranes are compromised or cells are fixed and permeabilized, PI enters and binds nucleic acids, producing bright red fluorescence.

Propidium iodide is excited by green light (approximately 535 nm) and emits red fluorescence (around 617 nm), making it compatible with common fluorescence microscopes and flow cytometers equipped with appropriate lasers and filters. It is especially valued for measuring DNA content and for viability assays. In flow cytometry, PI is used to distinguish live from dead cells and to analyze cell cycle distribution by staining total DNA content. For DNA-specific staining, RNase treatment is often applied to remove RNA signal, reducing non-DNA fluorescence.

Applications and protocols commonly involve preparing stock solutions in buffer or saline and using working concentrations typically in the microgram per milliliter range, depending on the assay. In viability assays, cells are incubated with PI briefly before analysis, with dead cells appearing PI-positive while live cells remain PI-negative. In fixed-cell imaging, PI can label nuclei for structural visualization. Proper handling includes protection from light, since PI is photoactive, and adherence to safety guidelines due to its hazardous nature.

See also: flow cytometry, fluorescence microscopy, DNA staining, cell viability assays.