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

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·(H 2 O) 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 H 2 O 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. The adenine analogue 2-aminopurine has been considered as intrinsically fluorescent and is widely used in biochemical assays to probe DNA and RNA structure. It is now shown that the molecule alone is nearly non-fluorescent, however, its fluorescence is increased by up to 95 times through hydrogen bonding to a single water molecule.