Recently, the successful incorporation of artificial base pairs in genetics has made a significant progress in synthetic biology. The present work reports the proton transfer and photoisomerization of unnatural base pair ZP, which is synthesized from the pyrimidine analog 6-amino-5-nitro-3-(1-β-D-2′-deoxyribo-furanosyl)-2 (1H)-pyridone (Z) and paired with its Watson-Crick complement, the purine analog 2-amino-8-(1′-β-D-2′- deoxyribofuranosyl)-imidazo[1,2-a]-1,3,5-triazin-4(8H)-one (P). To explain the mechanism of proton transfer process, we constructed the relaxed potential energy surfaces (PESs) linking the different tautomers in both gas phase and solution. Our results show that the double proton transfer in the gas phase occurs in a concerted way both in S0 and S1 states, while the stepwise mechanism becomes more favorable in solution. The solvent effect can promote the single proton transfer, which undergoes a lower energy barrier in S1 state due to the strengthened hydrogen bond. In contrast to the excited state ultrafast deactivation process of the natural bases, there is no conical intersection between S0 and S1 states along the proton transfer coordinate to activate the decay mechanism in ZP. Of particular relevance to the photophysical properties, charge-transfer character is obviously related to the nitro rotation in S1 state. We characterized the molecular vibration effect on the electronic properties, which reveals the electronic excitation can be tuned by the rotation-induced structural distortion accompanied with the electron localization on nitro group.
Energy Barrier Reduction for the Double Proton-Transfer Reaction in Guanine–Cytosine DNA Base Pair on a Gold Surface