[(1-Methyl­cytosine)₂H]I, an asymmetric base pair

The X-ray crystal structure of the title compound, 1-methyl­cytosinium iodide 1-methyl­cytosine, [(1-MeC)₂H]I (1-MeC is 1-methyl­cytosine) or C₅H₈N₃O⁺·I^-·C₅H₇N₃O, has been determined at 163 K. In this compound, one protonated (1-MeCH⁺) and one neutral methyl­cytosine (1-MeC) moiety form an asymmetric base pair comprising three hydrogen bonds. A previous structure determination [Krüger, Bruhn & Steinborn (2004). Org. Biomol. Chem. 2, 2513-2516] showed the same cell parameters but a centrosymmetric base pair in space group P2₁/c. At low temperature, however, we discovered the title compound to be a racemic twin showing pseudo-centrosymmetry.