fluorescence spectroscopy is a technique that is used to assay a sample using a fluorescent probe. dna nucleotides themselves are not good candidates for this technique because they do not readily re-emit light upon light excitation. a fluorescent marker is needed to detect base flipping. 2-aminopurine is a base that is structurally similar to adenine, but is very fluorescent when flipped out from the dna duplex. it is commonly used to detect base flipping and has an excitation at 305‑320 nm and emission at 370 nm so that it well separated from the excitations of proteins and dna. other fluorescent probes used to study dna base flipping are 6map (4‑amino‑6‑methyl‑7(8h)‑pteridone) and pyrrolo‑c (3-[β-d-2-ribofuranosyl]-6-methylpyrrolo[2,3-d]pyrimidin-2(3h)-one). time-resolved fluorescence spectroscopy is also employed to provide a more detailed picture of the extent of base flipping as well as the conformational dynamics occurring during base flipping.