Thieno analogues of RNA nucleosides: a detailed theoretical study.

We use first-principles density functional theory calculations to investigate the structural, energetic, bonding aspects, and optical properties of recently synthesized thieno-analogues of RNA nucleosides. The results are compared against the findings obtained for both the natural nucleosides as well as available experimental data. We find that the modified nucleosides form the hydrogen bonded Watson-Crick (WC) base pairing with similar H-bonding energy as obtained for the natural nucleosides. We have calculated and compared the charge transfer integrals for the H-bonded natural and thieno-modified nucleosides. We find that the thieno modification of these nucleosides strongly affects the charge transfer integrals due to the difference in extent of orbital delocalization in these two types of nucleosides. We also find that the degree of reduction of charge transfer integrals is larger for the H-bonded A-U pair than in the G-C pair. We also focus on the optical absorption properties of these thieno-modified nucleosides and their WC H-bonded base pairs in gas phase as well as with implicit water. Our calculated results show that the low energy peaks in the absorption spectra mainly arise because of the π-π* electronic transition for both the nucleosides, and the observed red shift for thieno-nucleosides compared to natural nucleosides are consistent with the calculated decrease in electronic gaps. Our results demonstrate that the thieno modification of natural nucleosides significantly modifies their electronic and optical properties, although the basic structural and bonding aspects remained the same. It also gives a microscopic understanding of the experimentally observed optical behaviors.