When using fluorescent probes to image inside the body the fluorescence signal must be strong enough and last long enough to distinguish it from the background fluorescence, otherwise the signal is undetectable. Now, by linking two fluorophores, US chemists have created a probe whose properties may make it superior.

Nicholas Turro at Columbia University, New York, and coworkers developed the fluorescent probe, RuEth, which can be used to image RNA-rich regions in cells.

The team made RuEth by attaching a ruthenium(II) isothiocyanate complex to a phenanthridine derivative. They then used the probe to image breast cancer cells using a combination of steady-state and time-resolved spectroscopy. Phenanthridines are known to have stronger fluorescence in regions of mammalian cells rich in nucleic acids, and this part of the probe allows it to highlight these regions rather than the rest of the cell. The ruthenium segment gives the probe long lasting fluorescence, overcoming the issue of background autofluorescence. 

RuEth shows stronger fluorescence (right) in the RNA-rich nucleolus than in the DNA-rich nucleus

The team found that the probe had greater signal strength in cell regions where RNA is known to accumulate. This selectivity is the most impressive characteristic of this probe, says Turro. 'We have developed a probe capable of specifically staining RNA over DNA.' Up to now this has proved difficult to achieve as other probes cannot usually distinguish between the nucleic acids. 

Byeang Hyean Kim, Pohang University of Science and Technology, Korea, an expert in the area of DNA/RNA probing, explains that 'by the simple combination of a phenanthridine moiety and a ruthenium(II) complex, the authors have developed an RNA probe with long-lived emission lifetimes.' RuEth fluoresces four times longer than other phenanthridine derivatives; its fluorescence intensity is also nine times stronger. 

The researchers suggest that given its selectivity, their probe could find potential applications as an in vivo probe for RNA.

Paul Cooper

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