The significance of the metal cation in guanine-quartet - metalloporphyrin complexes.

The planarity and the appropriate size of the porphyrin ring make porphyrin derivatives ideal ligands for stacking to guanine quartets and they could thus be used as anti-cancer drugs. In this contribution we analyzed complexes of a guanine quartet with a porphyrin molecule, magnesium porphyrin and calcium porphyrin. As magnesium and calcium ions are located in the center and above the porphyrin ring, respectively, the two metalloporphyrins are expected to have different impacts on the target. The optimized structures of the three systems revealed geometrical changes in the guanine quartet upon complexation: while stacking of porphyrin and magnesium porphyrin does not induce significant changes, calcium porphyrin considerably distorts the quartet's structure, which has significant implications for the binding properties among guanine molecules. Ab initio molecular dynamics simulations revealed that the systems perform small fluctuations around the equilibrium structures. The largest atom displacements are performed by the calcium ion. The interacting quantum atoms methodology enabled analysis of the binding properties in the studied complexes. Interestingly, although the proximity of the calcium ion is responsible for the quartet's pronounced deformation and weakening of guanine-guanine binding, it also enables stronger binding of the metal ion to the quartet, resulting in a more stable complex. These results imply that metalloporphyrin-like ligands with out-of-plane central ions might represent promising drug candidates in anti-tumor treatment.