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

Nature Chemistry volume 6, pages 989–993 (2014)Cite this article

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Abstract

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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Figure 1: Pump/ionization delay measurements of 9H-2AP, 9M-2AP and their mono-, di- and tri-hydrated water clusters.
Figure 2: Comparison of the calculated and experimental adiabatic excitation energies.
Figure 3: Correlation of the experimental 1π π* excited-state lifetime τ of 9H-2AP (9H), 9M-2AP (9M) and their mono-, di- and trihydrate clusters (1A to 3B) with the difference of the 1n π* and 1π π* state minimum energies.

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Acknowledgements

The authors acknowledge financial support from the Schweiz Nationalfonds (SNSF; project numbers 20020-121993 and 200021-132540).

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Authors and Affiliations

  1. Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, Bern, CH-3012, Switzerland

    Simon Lobsiger, Susan Blaser, Rajeev K. Sinha, Hans-Martin Frey & Samuel Leutwyler

  2. Department of Atomic and Molecular Physics, Manipal University, Manipal-576104, Karnataka, India

    Rajeev K. Sinha

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  1. Simon Lobsiger
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  4. Hans-Martin Frey
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Contributions

S.Lo. and R.K.S. performed the nanosecond spectroscopic and pump–probe experiments as well as the DFT calculations, and analysed the spectroscopic and nanosecond kinetic data. S.B. co-designed the picosecond pump–probe experiment and performed the picosecond pump–probe measurements. H.M.F. conceived and designed the picosecond pump–probe experiments and programmed the kinetic fitting analysis. S.Lo. and S.Le. wrote major parts of the manuscript.

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Correspondence to Samuel Leutwyler.

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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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