Topochemical Synthesis, <i>micro</i>-Electron Diffraction (3D ED), and Electron Beam-Initiated Topotactic Dimer Splitting of cis,syn-Thymine Dimer Crystals

A solid-state photochemical reaction of crystalline thymine hydrate (<b>TH</b>) resulted in a clean topochemical transformation into the cis-syn-dimer (<b>TD</b>), matching the structure as the one responsible for most UV lesions in DNA. Microcrystals of <b>TD</b> 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 <b>TD</b> and the as formed monomer <b>T</b> displays a novel trimeric hydrogen bonding motif, and the latter represents a new crystal phase. Beyond interesting analogies between single crystals of <b>T</b> and <b>TD</b>, 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.