Tautomeric effect of guanine on stability, spectroscopic and absorbance properties in cytosine–guanine base pairs: a DFT and TD-DFT perspective

Abstract

A comparative study of the tautomeric effect of guanine (G) on stability, spectral properties, and optical response in canonical and noncanonical cytosine–guanine (C–G) base pairs has been investigated by employing density functional theory (DFT) and time-dependent DFT. Three keto-G tautomers (G1, G2, G3) have taken to interact with the C monomer of W–C type which in turn gives three C–G base pairs (C–G1, C–G2, and C–G3) with three conventional H-bonds. The geometry optimization with various ground-state properties and spectroscopic details has been estimated in the gas phase using Cam-B3LYP functional with two different basis sets 6-311++G (d, p) and Aug-cc-pVDZ at the DFT level. The W–C base pair C–G1 possesses the lowest electronic energy although G1 is not the lowest energy structure of guanine. C–G3 is found to have maximum binding and H-bond energies. Significant changes have been observed in IR and Raman spectra of C–G clusters due to the tautomeric effect of G monomer. All the absorption spectra of C–G clusters belong to the UV region, and the electronic excitations are associated with π → π* transition. The molecular orbitals associated with the electronic transitions reveal the intermolecular charge transfer characteristic of G1, G3 to C monomer. Besides this, the electrostatic potential surface plots have been used to explore the possible binding sites of isolated bases and base pairs. As the tautomeric effect influences the chemical reactions, this study can be useful for drug designing and to find the best fit between the receptors and important ligands.