Nucleic acid G-quartets: Insights into diverse patterns and optical properties

Structures of various conformers of G-quartets (G4) with different types of hydrogen bonding patterns have been investigated through various levels of Density Functional Theory (DFT). Their structure and stability has been compared with the diad (G2), triad (G3), pentad (G5) and hexad (G6) of guanine. The calculations show that G4 has the highest stabilization through hydrogen-bond interaction which explains the tendency of guanine rich strands in the telomeric region to favor the formation of the quadruplex structure. We have also performed calculations for Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺ and Ca²⁺ complexes of G4. Calculations show that for an isolated quartet, the metal ion with the smallest ionic radius in their respective groups (IA and IIA) form more stable complexes. Other properties such as the HOMO-LUMO gap and polarizability have also been analyzed. The variation in the polarizability has been studied with respect to the movement of cations along the central cavity of the quartet. Such movement leads to a large anisotropy of polarization and hence the refractive index (η) thereby creating optical birefringence which have potential applications in biomolecular imaging.