Cocrystals of 5-fluorocytosine. II. Coformers with variable hydrogen-bonding sites.
Two flexible molecules, biuret and 6-acetamidouracil, were cocrystallized with 5-fluorocytosine to study their conformational preferences. In the cocrystal with 5-fluorocytosine (I), biuret exhibits the same conformation as in its hydrate. In contrast, 6-acetamidouracil can adopt two main conformations depending on its crystal environment: in crystal (II) the trans form characterized by an intramolecular hydrogen bond is observed, while in the cocrystal with 5-fluorocytosine (III), the complementary binding induces the cis form. Three cocrystals of 6-methylisocytosine demonstrate that complementary binding enables the crystallization of a specific tautomer. In the cocrystals with 5-fluorocytosine, (IVa) and (IVb), only the 3H tautomer of 6-methylisocytosine is present, whereas in the cocrystal with 6-aminoisocytosine, (V), the 1H tautomeric form is adopted. The complexes observed in the cocrystals are stabilized by three hydrogen bonds similar to those constituting the Watson-Crick C·G base pair.
Institut für Organische Chemie und Chemische Biologie, Goethe-Universität Frankfurt, Max-von-Laue-Str. 7, 60438 Frankfurt am Main, Germany.
Maya
M
Tutughamiarso
Ernst
E
Egert
D018360:Crystallography, X-Ray; D005437:Flucytosine / Q000737:chemistry*; D006860:Hydrogen Bonding; D015394:Molecular Structure
D005437:Flucytosine