Guanine tetrads interacting with metal ions. An AIM topological analysis of the electronic densityElectronic Supplementary Information (ESI) available: Geometric data, topologic data at the BCP and integrated properties of the hydrogen atom of every H1⋯O6 and H2⋯N7 hydrogen bond. See http://www.rsc.org/suppdata/qu/b2/b210911e/