G₄ nucleic acids. A) Chemical structure of a Hoogsteen hydrogen-bonded G-quartet. M+ represents a monovalent cation, typically Na⁺ or K⁺. B) Schematic representations of a parallel-strand intermolecular tetraplex (G4, 1), a monomeric intramolecular quadruplex (G4', 2), and two forms of dimeric hairpin quadruplexes (G'2, 3,4). Both RNA and DNA and mixtures of these nucleic acids can form G₄ structures. A G' structure contains only Hoogsteen hydrogen-bonded G•G base pairs and would correspond to a single hairpin (indicated in bold) in representation 4.