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S. Lobsiger, S. Blaser, R. K. Sinha, H.-M. Frey, and S. Leutwyler*, Nature Chem. 2014, 6, 989. University of Bern 2-Aminopurine (2AP) iswidely used as a site-specific fluorescent probe of DNA and RNA base-flipping and -folding. The group of S. Leutwyler showed that 2AP is not intrinsically strongly fluorescent, contrary to widespread belief. Gas-phase, jet-cooled 2AP and 9-methyl-2AP have very short fluorescence lifetimes (156 ps and 210 ps, respectively) rendering them essentially nonfluorescent. However, the lifetime of 2AP increases dramatically when it is part of a hydrate cluster, 2AP·(H 2 O) n , where n = 1–3. Selective microhydration of 2AP at its sugar-edge, cis-amino or trans-amino sites increases the fluorescence lifetime by up to 100-fold.