Semi-empirical and linear-scaling DFT methods to characterize duplex DNA and G-quadruplexes in the presence of interacting small molecules.

The computational study of DNA and its interaction with ligands is a highly relevant area of research, with significant consequences for developing new therapeutic strategies. However, the computational description of such large and complex systems requires considering interactions of different types simultaneously in a balanced way, such as non-covalent weak interactions (namely hydrogen bonds and stacking), metal-ligand interactions, polarisation and charge transfer effects. All these considerations imply a real challenge for computational chemistry. The possibility of studying large biological systems using quantum methods for the entire system requires significant computational resources, with improvements in parallelisation and optimisation of theoretical strategies. Computational methods, such as Linear-Scaling Density Functional Theory (LS-DFT) and DLPNO-CCSD(T), may allow performing ab initio quantum mechanics calculations, including the electronic structure for large biological systems, in a reasonable computing time. In this work, we study the interaction of small molecules and cations with DNA (both duplex DNA and G-quadruplexes), comparing different computational methods: a LS-DFT method at the LMKLL/DZDP level of theory, semi-empirical methods (PM6-DH2 and PM7), mixed QM/MM, and DLPNO-CCSD(T). Our goal is to demonstrate the adequacy of LS-DFT to treat the different types of interactions present in DNA-dependent systems. We show that LMKLL/DZDP using SIESTA can yield very accurate geometries and energetics in all the different systems considered in this work: duplex DNA (dDNA), phenanthroline intercalating dDNA, G-quadruplexes, and metal-G-tetrads considering alkaline metals of different sizes. As far as we know, this is the first time that full G-quadruplex geometry optimisations have been carried out using a DFT method thanks to its linear-scaling capabilities. Moreover, we show that LS-DFT provides high-quality structures, and some semi-empirical Hamiltonians can also yield suitable geometries. However, DLPNO-CCSD(T) and LS-DFT are the only methods that accurately describe interaction energies for all the systems considered in our study.