Interaction of (G4)2 and (X4)2 DNA quadruplexes with Cu+, Ag+ and Au+ metal cations: a quantum chemical calculation on structural, energetic and electronic properties

The present study is focused to cast light on the structural, electronic and energetic properties of isolated (G4)2 and (X4)2 DNA quadruplexes with transition metal cations which are investigated using ab initio and density functional theory (DFT). The geometrical analysis revealed that (G4)2 and (X4)2 with metal cation can adopt the normal tetraplex Hoogsteen hydrogen bond, and no bifurcated hydrogen bonds are occurred. The addition of metal cation in the central cavity of isolated (G4)2 and (X4)2 bases can affect the structural chirality and their charge distribution. Binding energy and metal ion affinity of (G4)2 and (X4)2 metal complexes follow the interaction energy order of (G4)2-Cu+ > (G4)2-Ag+ > (G4)2-Au+ > and (X4)2-Cu+ > (X4)2-Ag+ > (X4)2-Au+. The quantum theory of atoms in molecule (QTAIM) analysis revealed that the electrostatic interaction has occurred between carbonyl group oxygen atom and metal cation (O6⋯M+). The natural bonding orbital (NBO) analysis reveals that the N-H⋯O and N-H⋯N hydrogen bond strength and charge transfer have been calculated for both (G4)2 and (X4)2 metal complexes. The higher wavelength and lower excitation energies are observed for both (G4)2 and (X4)2 metal complexes which exhibit that the hyperchromic shift occurred during metal ion interaction. The obtained results showed that the transition metal cations are strongly interacted with (G4)2 and (X4)2 bases, and it controls the telomerase activity. While comparing (G4)2 and (X4)2 bases, the (X4)2 bases are having similar properties of (G4)2 bases. Thus, the xanthine and its derivatives are very auspicious candidates for artificial quadruplex DNA.