Solution structures of purine base analogues 6-chloroguanine, 8-azaguanine and allopurinol.

Analogues of purine bases are highly relevant in the biological context and have been implicated as drug molecules for therapy against a number of diseases. Additionally, these molecules have been implicated to have a role in the prebiotic RNA world. However, experimental data on the structures of these molecules in aqueous solution is lacking. In this work, we report the ultraviolet resonance Raman spectra of 6-chloroguanine, 8-azaguanine and allopurinol, obtained with 260 nm excitation. The reported spectra have been assigned to normal modes computed from density functional theory (B3LYP/6-31G (d,p)) calculations. This work has been useful in identifying the solution-state structures of these molecules at neutral pH. We find that the guanine analogues 6-chloroguanine and 8-azaguanine exist as keto-N9H and keto-N7H tautomers in solution, respectively. On the other hand, the hypoxanthine analogue allopurinol exists as a mixture of keto-N9H and keto-N8H tautomers in solution. We predict that this work would be particularly useful in future vibrational studies where these molecules are present in complexes with their target proteins.