Alkaloid acts as luminescent DNA probe in living cells

An alkaloid isolated from a Himalayan herb emits red fluorescence in the presence of double-stranded DNA but blue emission in its absence and can be used as a dye to measure DNA in living cells. The quaternary benzo[c]phenanthridine alkaloid known as chelirubine was isolated from the Himalayan horned poppy (Dicranostigma lactucoides) and found to have unique spectral properties when examined by scientists from Masaryk University in Brno, Czech Republic, as they described in Talanta.

Upon irradiation with light at 360 nm, two emission bands were observed at 420 nm (violet‐blue fluorescence) and 620 nm (red luminescence). The former emission was dominant under neutral aqueous or acidic conditions. However, when dsDNA was added to the system, the blue emission was suppressed and the red emission enhanced as the alkaloid became bound to the DNA. Similar results were obtained with irradiation at about 500 nm.

The behaviour was conserved when the alkaloid was added to living HeLa cells, when chelirubine passed through the cell membrane and attached to the DNA within. Under irradiation by green or blue light, only red fluorescence from the cell nuclei was observed. The blue irradiation corresponds to an argon laser operating at 488 nm so any microscope fitted with this light source could be used. It is better than irradiation at 360 nm because it is less harmful to living cells.

The intensity of the emitted light was proportional to the amount of DNA present, so the alkaloid can be used as a probe to measure DNA in living cells, as an alternative to conventional dyes like cyanines and ethidium bromide, over which it has the advantage of good cell permeability.