Development of fluorescence oligonucleotide probes based on cytosine- and guanine-rich sequences

Fluorescence Imaging of Extracellular Potassium Ion Using Potassium Sensing Oligonucleotide

Potassium-sensing oligonucleotide, PSO, a conjugate of a quadruplex structure-forming oligonucleotide with a peptide incorporating a Förster Resonance Energy Transfer (FRET) chromophore pair, has been developed for fluorescent detection of potassium ion (K⁺) in aqueous medium. PSO 1 could be introduced into cells for real-time imaging of cytoplasmic K⁺ concentrations. To perform fluorescent imaging of K⁺ on the cell surface, we synthesized twelve PSO derivatives with different types of peptide types and lengths, and oligonucleotide sequences including thrombin-binding aptamer (TBA) sequences with FAM and TAMRA as a FRET chromophore pair, and evaluated their performance. 1 was shown to respond selectively to K⁺, not to most ions present in vivo, and to show reciprocal fluorescence changes in response to K⁺ concentration. For the peptide chains and oligonucleotide sequences examined in this study, the PSO derivatives had K_d values for K⁺ in the range of 5–30 mM. All PSO derivatives showed high K⁺ selectivity even in the presence of excess Na⁺. The PSO derivatives were successfully localized to the cell surface by biotinylated concanavalin A (ConA) or sulfo-NHS-biotin via streptavidin (StAv). Fluorescence imaging of extracellular K⁺ upon addition of apoptosis inducers was successfully achieved by 1 localized to the cell surface.

Fluorescent AgNCs Formed on Bifunctional DNA Template for Potassium Ion Detection

In this study, we examined properties of silver nanoclusters, which are AgNCs stabilized by DNA oligonucleotide scaffold containing G-quadruplex-forming sequences: human telomeric (Tel22) or thrombin-binding aptamer (TBA). Thus, we obtained two fluorescent probes abbreviated as Tel22C12-AgNCs and TBAC12-AgNCs, which were characterized using absorption, circular dichroism and fluorescence spectroscopy. Both probes emit green and red fluorescence. The presence of silver nanoclusters did not destabilize the formed G-quadruplexes. The structural changes of probes upon binding K+ or Na+ ions cause quenching in their red emission. Green emission was slightly quenched only in the case of Tel22C12-AgNCs; on the contrary, for TBAC12-AgNC’s green emission, we observed an increasing fluorescence signal. Moreover, the Tel22C12-AgNCs system shows not only a higher binding preference for K+ over Na+, but it was able to monitor small changes in K+ concentrations in the buffer mimicking extracellular conditions (high content of Na+ ions). These results suggest that Tel22C12-AgNCs exhibit the potential to monitor transmembrane potassium transport.