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

The X‐ray crystal structure of the title compound, 1‐methylcytosinium iodide 1‐methylcytosine, [(1‐MeC) 2 H]I (1‐MeC is 1‐methylcytosine) or C 5 H 8 N 3 O + ·I − ·C 5 H 7 N 3 O, 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 P 2 1 / c . At low temperature, however, we discovered the title compound to be a racemic twin showing pseudo‐centrosymmetry.