Stabilisation of non-canonical structures of nucleic acids by metal ions and small molecules

•Non-B duplex stability depends on charge density and inner- vs. outer-sphere coordination.•For triplexes diffuse- or site-binding mode by the ion plays a role.•Crescent-shaped, flexible, planar intercalators provide triplex stabilisation.•For quadruplexes ion size, dehydration energy, and coordination number are important.•For metal complexes/quadruplexes size and H-bond interactions are important.

A general overview is provided about the recent and state of the art advances in the field of metal ions, metal complexes and small aromatic molecules use as stabilising agents for non-canonical nucleotide forms (non-B duplex, triplexes, quadruplexes). Despite the broad topic, the collection and comparison of the large amount of data available enable some general remarks to be done. The ability of metal ions to stabilise a particular form is discussed on the basis of ion size, charge density, coordination and binding modes and/or dehydration energy. In the case of small molecules (intercalators and/or groove binders) charge, flexibility, hydrophobicity and/or geometry are considered.

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