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Ionization of dimethyluracil dimers leads to facile proton transfer in the absence of hydrogen bonds

Nature Chemistry volume 4, pages 323–329 (2012)Cite this article

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Abstract

Proton transfer is ubiquitous in chemistry and biology, occurring, for example, in proteins, enzyme reactions and across proton channels and pumps. However, it has always been described in the context of hydrogen-bonding networks (‘proton wires’) acting as proton conduits. Here, we report efficient intramolecular ionization-induced proton transfer across a 1,3-dimethyluracil dimer, a model π-stacked system with no hydrogen bonds. Upon photoionization by tunable vacuum ultraviolet synchrotron radiation, the dimethyluracil dimer undergoes proton transfer and dissociates to produce a protonated monomer. Deuterated dimethyluracil experiments confirm that proton transfer occurs from the methyl groups and not from the aromatic C–H sites. Calculations reveal qualitative differences between the proton transfer reaction coordinate in the π-stacked and hydrogen-bonded base pairs, and that proton transfer in methylated dimers involves significant rearrangements of the two fragments, facilitating a relatively low potential energy barrier of only 0.6 eV in the ionized dimer.

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Figure 1: VUV-SPI mass spectra and photoionization efficiency curves.
Figure 2: Relevant computed energetics (eV) of the non-methylated hydrogen-bonded and methylated uracil dimers.
Figure 3: Effect of deuteration on the 1,3-mU mass spectrum and photoionization efficiency curves.
Figure 4: Experimental branching ratios for the proton transfer channel in the uracil, 1,3-mU and d6-1,3-mU beams.
Figure 5: Relevant energetic and structural parameters illustrating the mechanism of ionization-induced proton transfer.

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Acknowledgements

Experiments were carried out at the Advanced Light Source, and the d6-1,3-mU was synthesized at the Molecular Foundry, both at Lawrence Berkeley National Laboratory. Berkeley participants are supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy (contract no. DE-AC02-05CH11231), through the Chemical Sciences Division (A.G., O.K., S.R.L., M.A.) and the Materials Sciences Division (R.K.). R.K. is also supported by the Defense Threat Reduction Agency (IACRO-B0845281). This work was conducted in the framework of the iOpenShell Center (iopenshell.usc.edu), supported by the National Science Foundation through CRIF:CRF (CHE-0625419 + 0624602 + 0625237 and CHE-0951634, to A.I.K.) grants.

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Authors and Affiliations

  1. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, 94720, California, USA

    Amir Golan, Oleg Kostko, Stephen R. Leone & Musahid Ahmed

  2. Department of Chemistry, University of Southern California, Los Angeles, 90089-0482, California, USA

    Ksenia B. Bravaya & Anna I. Krylov

  3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, 94720, California, USA

    Romas Kudirka

  4. Departments of Chemistry and Physics, University of California, Berkeley, 94720, California, USA

    Amir Golan & Stephen R. Leone

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Contributions

M.A. conceived and designed the experiments. A.G. and O.K. conducted the experiments. R.K. synthesized the deuterated compounds. K.B.B. and A.I.K. performed electronic structure calculations. A.G. and K.B.B. contributed equally to this work. A.G., K.B.B., A.I.K., M.A. and S.R.L. co-wrote the paper.

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Correspondence to Anna I. Krylov or Musahid Ahmed.

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Golan, A., Bravaya, K., Kudirka, R. et al. Ionization of dimethyluracil dimers leads to facile proton transfer in the absence of hydrogen bonds. Nature Chem 4, 323–329 (2012). https://doi.org/10.1038/nchem.1298

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