Visualizing the quadruplex: from fluorescent ligands to light-up probes.

Detection of quadruplex structures by visual methods is a major challenge of the quadruplex nucleic acid research area. Consequently, considerable efforts are under way for the discovery of quadruplex specific agents endowed with fluorescence properties. In this review chapter we propose a comprehensive and critical overview of the diverse molecular design and strategies that have been described to identify quadruplex-selective fluorescent probes. Innovative compounds as well as classical DNA dyes are reviewed. The compounds have been divided into three classes: (1) "light-up" probes that display a strong enhancement upon G4 binding, (2) "light-off" probes that display a decreased fluorescence upon binding, and (3) permanent probes ("tagged" G4-binders) that exhibit no variation of fluorescence but display quadruplex binding specificity. The labeling performances of probes in various analytical contexts (in solution, in gel, at the level of chromosomes, and in fixed cells) are also reported and commented on when available. Finally we address the strengths and weaknesses of each probe class and highlight the critical features that must be addressed in developing a practicable quadruplex-specific labeling agent.