8-Furylimidazolo-2'-deoxycytidine: crystal structure, packing, atropisomerism and fluorescence.

8-Furylimidazolo-2'-deoxycytidine (ImidC), CHNO, is a fluorescent analogue of 2'-deoxycytidine, also displaying the same recognition face. As a constituent of DNA, ImidC forms extraordinarily strong silver-mediated self-pairs. Crystal structure determination revealed that ImidC adopts two types of disordered residues: the sugar unit and the furyl moiety. The disorder of the sugar residue amounts to an 87:13 split. The disorder of the furyl ring results from axial chirality at the C8-C2'' bond connecting the nucleobase to the heterocycle. The two atropisomers are present in unequal proportions [occupancies of 0.69 (2) and 0.31 (2)], and the nucleobase and the furyl moiety are coplanar. Considering the atomic sites with predominant occupancy, an anti conformation with χ = - 147.2 (7)° was found at the glycosylic bond and the 2'-deoxyribosyl moiety shows a C2'-endo (S, T) conformation, with P = 160.0°. A H NMR-based conformational analysis of the furanose puckering revealed that the S conformation predominates also in solution. In the solid state, two neighbouring ImidC molecules are arranged in a head-to-tail fashion, but with a notable tilt of the molecules with respect to each other. Consequently, one N-H...N hydrogen bond is found for neighbouring molecules within one layer, while a second N-H...N hydrogen bond is formed to a molecule of an adjacent layer. In addition, hydrogen bonding is observed between the nucleobase and the sugar residue. A Hirshfeld surface analysis was performed to visualize the intermolecular interactions observed in the X-ray study. In addition, the fluorescence spectra of ImidC were measured in solvents of different polarity and viscosity. ImidC responds to microenvironmental changes (polarity and viscosity), which is explained by a hindered rotation of the furyl residue in solvents of high viscosity.