2-Aminopurine (2AP) is a fluorescent isomer of adenine and has a fluorescence lifetime of ~11 ns in water. It is widely used in biochemical settings as a site-specific fluorescent probe of DNA and RNA structure and base-flipping and -folding. These assays assume that 2AP is intrinsically strongly fluorescent. Here, we show this not to be the case, observing that gas-phase, jet-cooled 2-aminopurine and 9-methyl-2-aminopurine have very short fluorescence lifetimes (156 ps and 210 ps, respectively); they are, to all intents and purposes, non-fluorescent. We find that the lifetime of 2-aminopurine increases dramatically when it is part of a hydrate cluster, 2AP·(H2O)n, where n = 1–3. Not only does it depend on the presence of water molecules, it also depends on the specific hydrogen-bonding site to which they attach and on the number of H2O molecules at that site. We selectively microhydrate 2-aminopurine at its sugar-edge, cis-amino or trans-amino sites and see that its fluorescence lifetime increases by 4, 50 and 95 times (to 14.5 ns), respectively.
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The authors acknowledge financial support from the Schweiz Nationalfonds (SNSF; project numbers 20020-121993 and 200021-132540).
The authors declare no competing financial interests.
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Lobsiger, S., Blaser, S., Sinha, R. et al. Switching on the fluorescence of 2-aminopurine by site-selective microhydration. Nature Chem 6, 989–993 (2014). https://doi.org/10.1038/nchem.2086
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