A second form of cytosine
Cytosine is one of the four main bases found in DNA and RNA. A new polymorph of cytosine has just been reported half a century after the report of its first known crystal structure. This is the first time a polymorph has been discovered of a nucleobase [Sridhar et al. (2015). Acta Cryst. C71, 128-135; doi:10.1107/S2053229615000492]. Like many great discoveries, the new form was observed unexpectedly during an attempt to cocrystallise cytosine with catechol. The new form, denoted (Ib), crystallises in the orthorhombic centrosymmetric space group Pccn with two molecules in the asymmetric unit. The previously known form, denoted (Ia), crystallises in the orthorhombic noncentrosymmetric space group P212121. The cystosine molecule is planar in both forms.
