Optical Probes for Cellular Imaging of G-quadruplexes: Beyond Fluorescence Intensity Probes

The study of G-quadruplex (G4) structures that form in DNA and RNA is a rapidly growing field, which has evolved from in vitro studies of isolated G4 sequences to genome-wide detection of G4s in a cellular context. This work has revealed the tangible and significant effects that G4s may have on biological regulation. This minireview describes recent progress in the design of photoluminescent intensity-independent optical probes for G4s. We discuss the design and use of probes based on fluorescence or phosphorescence lifetime, rather than intensity-based detection; spectral ratiometric probes; and fluorescent probes for single-molecule G4-detection. We argue that each of these modalities improve unbiased G4 detection in cellular experiments, overcoming problems associated with unknown cellular uptake of probes or their organelle concentration. We discuss the improvements offered by these types of probes, as well as limitations and future research directions needed to facilitate more robust research into G4 biology.