Photophysical properties of fluorescent nucleobase P-analogues expected to monitor DNA replication.

In this work, we computationally design a series of fluorescent purine analogues based on the 2-amino-8-(1'-β-D-2'-deoxyribofuranosyl)-imidazo[1,2-a]-1,3,5-triazin-4(8H)-one (P) to monitor the DNA replication process with merely a minimal perturbation to the natural structure of nucleic acid. The P-modified fluorescent probes present red-shifted absorption spectra and enhanced photoluminescence due to the additional π-conjugation resulting from the fluorophore modification and the ring-expansion. Efficient fluorescence quenching of P-analogues occurs upon pairing with the complementary 6-amino-5-nitro-3-(1'-β-D-2'-deoxyribofuranosyl)-2(1H)-pyridone (Z) due to the nonradiative relaxation from the low-lying dark excited state to the ground state of Z moiety. Especially, the P3 and the P7, which have high fluorescence intensity in both gas and liquid phases, are proposed as the sensors for studying conformational switching in the presence and absence of a complementary sequence. Also examined are the influences of hydration and the linking to deoxyribose on absorption and emission processes. Besides, the potential phosphorescence emission of these modified base pairs is taken into account by constructing the relaxed potential energy curves of S, T and S states.