Structural insights into G-quadruplex binding by metal complexes: implications for drug design

G-quadruplex DNA secondary structures are formed in guanine-rich sequences and have been found to play an important role in regulating different biological processes. Indeed, guanine-rich sequences with the potential to form G-quadruplexes are present in different regions in the human genome, such as telomeres and the promoter region of different genes, including oncogene promoters. Thus, the rational design of small molecules capable of interacting, stabilising or damaging with high specificity these secondary structures represents an important strategy for the development of potent anticancer drugs. In this review, we highlight the interaction between G-quadruplex structures and their ligands, specifically emphasising the role of metal complexes. We provide detailed structural insight into the binding modes of metal complex-G-quadruplex interaction by analysing 18 sets of coordinates from X-ray and NMR currently available in the Protein Data Bank (PDB), with a primary focus on X-ray structural data.