Photophysical Properties Of The Isomorphic Emissive Rna Nucleobase Analogues And Effect Of Water Solution, Ribose, And Base Pairing: A Theoretical Study

In the present work, a comprehensive theoretical investigation on the excited state properties of the isomorphic emissive RNA nucleobase analogues, namely (tz)A, (tz)G, C-tz, and U-tz, was performed. Vertical transition energies are determined with the time-dependent density functional theory method at both the B3LYP and CAM-B3LYP levels using the 6-311++G(d, p) basis set. The nature of the low-lying singlet excited states is discussed and the results are compared with the findings from experiment and those for thieno analogues and natural bases. In gas phase, it was found that the S-1 state is pi pi* in nature for all the tz-bases except for (tz)A, for which the S-1 state is predicted to be n pi* in nature with the pi pi* state being the S-2. While in water solution, the S-1 state for all tz-bases are predicted to be pi pi* dominated by the configuration HOMO-LUMO. Compared with natural bases, the lowest pi pi* states are about 0.85-1.22 eV red-shifted. When compared with thieno analogues, it is interesting to note that the S-1 state (pi pi*) transition energies of the two counterparts from the two alphabets are nearly equal due to the very little differences of their HOMO-LUMO gaps. In addition, it was found that the hydration + PCM model can perfectly reproduce the photophysical properties of the tz-bases since the calculated excitation maxima and fluorescence are in good agreement with the experimental data. The microenvironment effects of linking to ribose, base pairing, and further hydration of base pairs were also studied.