Alfonso Blázquez-Castro

Many fluorophores display interesting features that make them useful biological labels and chemosensors, in particular in Cell Biology. Changes in the absorption-emission spectra (ortho- and metachromasia) are accounted among them. Acridine orange (AO) is one such fluorochromes that shows a prototypical orthochromatic vs. metachromatic behavior depending on its concentration and binding mode to different cellular substrates. Here, we revisit the differential AO fluorescence that occurs in selected biological examples, which allows for the identification of single-stranded or double-stranded nucleic acids. Although known for long, the ultimate reason for this phenomenon has not been properly advanced. We provide a potential molecular mechanism that adequately accounts for the different aspects of the phenomenon. This theoretical mechanism implies a difference in the degree of overlap of excited state orbitals whenever AO molecules are interacting with a single-stranded or a double-stranded nucleic acid. In the first case, massive π-electron overlapping between bases and intercalated AO leads to a metachromatic red emission. On the contrary, no excited-state orbital overlapping in AO-intercalated DNA duplexes is possible due to excessive separation between AO molecules and compliancy to the nearest neighbor exclusion principle, which manifests as orthochromatic green fluorescence.