Proton Transfer and Tautomerism in 2‑Aminopurine–Thymine and Pyrrolocytosine–Guanine Base Pairs

Pyrrolocytosine (PC) and 2-aminopurine (2AP) are fluorescent nucleobase analogues of the DNA nucleobases cytosine and adenine, respectively, and form base pairs with guanine and thymine. Both fluorescent nucleobases are used extensively as probes for local structure in nucleic acids as the fluorescence properties of PC and 2AP are very sensitive to changes such as helix formation, although the reasons for this sensitivity are not clear. To address this question, ab initio calculations have been used to calculate energies, at the MP2 and CIS level, of three different tautomer pairings of PC–G, and two of 2AP–T, which can potentially be interconverted by double proton transfer between the bases. Potential energy curves linking the different tautomer pairs have been calculated. For both PC–G and 2AP–T, the most stable tautomer pair in the electronic ground state is that analogous to the natural C–G and A–T base pair. In the case of 2AP–T, an alternative, stable, tautomer base pair was located in the first electronically excited state; however, it lies higher in energy than the tautomer pair analogous to A–T, making conversion to the alternative form unlikely. In contrast, in the case of PC–G, an alternative tautomer base pair is found to be the most stable form in the first electronically excited state, and this form is accessible following initial excitation from the ground state tautomer pair, thus suggesting an alternative deactivation route via double proton transfer may be possible when PC is involved in hydrogen bonding, such as occurs in helical conformations