title: Competitive binding exchange between alkali metal ions (K+, Rb+, and Cs+) and Na+ ions bound to the dimeric quadruplex [d(G4T4G4)]2: a 23Na and 1H NMR study.
abstract: A comparative study of the competitive cation exchange between the alkali metal ions K+, Rb+, and Cs+ and the Na+ ions bound to the dimeric quadruplex [d(G4T4G4)]2 was performed in aqueous solution by a combined use of the 23Na and 1H NMR spectroscopy. The titration data confirm the different binding affinities of these ions for the G-quadruplex and, in particular, major differences in the behavior of Cs+ as compared to the other ions were found. Accordingly, Cs+ competes with Na+ only for the binding sites at the quadruplex surface (primarily phosphate groups), while K+ and Rb+ are also able to replace sodium ions located inside the quadruplex. Furthermore, the 1H NMR results relative to the CsCl titration evidence a close approach of Cs+ ions to the phosphate groups in the narrow groove of [d(G4T4G4)]2. Based on a three-site exchange model, the 23Na NMR relaxation data lead to an estimate of the relative binding affinity of Cs+ versus Na+ for the quadruplex surface of 0.5 at 298 K. Comparing this value to those reported in the literature for the surface of the G-quadruplex formed by 5'-guanosinemonophosphate and for the surface of double-helical DNA suggests that topology factors may have an important influence on the cation affinity for the phosphate groups on DNA.
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Cagliari, Cittadella Universitaria di Monserrato, S.S. 554 (bivio per Sestu), I-09042, Monserrato, Cagliari, Italy. flaminia@unica.it
authors.forename: Flaminia
authors.initials: F
authors.lastname: Cesare Marincola
authors.forename: Ada
authors.initials: A
authors.lastname: Virno
authors.forename: Antonio
authors.initials: A
authors.lastname: Randazzo
authors.forename: Francesca
authors.initials: F
authors.lastname: Mocci
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Saba
authors.forename: Adolfo
authors.initials: A
authors.lastname: Lai
mesh_terms: D001667:Binding, Competitive; D004247:DNA / Q000737:chemistry*; D019281:Dimerization; D054856:G-Quadruplexes; D007477:Ions / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008672:Metals, Alkali / Q000737:chemistry*; D011522:Protons; D012015:Reference Standards
chemical_list: D007477:Ions; D008672:Metals, Alkali; D011522:Protons; D004247:DNA