A solid-state photochemical reaction of crystalline thymine hydrate (TH) resulted in a clean topochemical transformation into the cis-syn-dimer (TD), matching the structure as the one responsible for most UV lesions in DNA. Microcrystals of TD grown by drop casting piperidine solutions in a TEM grid made it possible to determine their structure by microelectron diffraction (3D ED) and to confirm expectations that an in situ electron-beam ionization reaction could result in a topotactic dimer splitting that, in this case, retains single-crystal-to-single-crystal character up to ca. 30% conversion. The packing structure of dimer TD and the as formed monomer T displays a novel trimeric hydrogen bonding motif, and the latter represents a new crystal phase. Beyond interesting analogies between single crystals of T and TD, and DNA, such as templated dimer formation and electron-transfer-induced repair, this work is a rare example of an electron beam-induced chemical reaction in the crystalline solid state.

Department of Chemistry and Biochemistry University of California at Los Angeles Los Angeles California 90095 United States

Institute of Physics of the Czech Academy of Sciences Na Slovance 1999 2 18200 Prague Czechia

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