Epitaxially Intergrown Conformational Polymorphs and a Mixed Water/Methanol Solvate of 5′-Deoxy-5′-iodoguanosine

5′-Deoxy-5′-iodoguanosine (I) crystals deposited from mixtures of water and methanol grow as nonsolvated hybrids of conformational polymorphs (Ia, Ib) and as a mixed solvate (Ic). Some solvent-free crystals are purely Ia, while others have varying amounts of Ib epitaxially intergrown with Ia. In Ia and Ib the conformations differ primarily by torsion about the C4′-C5′ bond (guanosine numbering scheme), which dramatically affects the iodine atom position. Powder diffraction and reconstructed reciprocal-lattice-slice images had small peaks incompatible with Ia. Some solvent-free crystals required lattices for both Ia and Ib to index all observable reflections. Unit-cell dimensions for Ia and Ib suggest the potential for epitaxial intergrowth. Hydrogen-bond networks in Ia and Ib are essentially identical and result in double layers of molecules in the ab plane, with layers of iodine at the layer surfaces. The iodine layers of Ia and Ib are incompatible: in Ia adjacent iodine atom layers interdigitate slightly, whereas in Ib they do not. Theoretical calculations support the conclusion that at room temperature Ia is the thermodynamically more stable polymorph and that Ib represents a kinetic product.