propidiumjodid

Propidium iodide is a fluorescent dye used in molecular and cellular biology to stain nucleic acids. It intercalates between base pairs of DNA or RNA and emits red fluorescence when excited, with an excitation maximum near 535 nm and an emission maximum around 617 nm. The dye is a cationic compound that is commonly supplied as a salt.

A central property of propidium iodide is its membrane impermeability to viable cells. Because intact cell membranes prevent PI entry, only cells with compromised membranes—such as dead or dying cells—take up the dye. This makes PI a standard tool for viability assays in flow cytometry and fluorescence microscopy. In combination with other dyes, such as calcein-AM for live cells, PI helps distinguish live, apoptotic, and necrotic populations. In apoptosis assays, PI is frequently used alongside Annexin V to identify late apoptotic or necrotic cells.

In fixed or permeabilized cells, propidium iodide can stain total cellular nucleic acids, which allows researchers to assess DNA content and perform cell cycle analysis. When used for such measurements, RNase treatment is often applied to remove RNA and prevent it from contributing to the signal.

Practical use typically involves working concentrations in the low microgram per milliliter range for flow cytometry, with variations depending on the protocol and the instrument. PI is photolabile and can be quenched by high salt or heavy staining, so samples are usually protected from light during handling.

Safety considerations include handling as a hazardous chemical with appropriate protective equipment and disposal procedures. PI is a potent intercalating agent and can be mutagenic; it should be used in well-ventilated areas with proper training.