Switching on the fluorescence of 2-aminopurine by site-selective microhydration.

2-Aminopurine (2 AP) 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 2 AP 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, 2 AP · (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.