title: Locating counterions in guanosine quadruplexes: a contrast-variation neutron diffraction experiment in condensed hexagonal phase.
abstract: Guanosine, one of the primary components of nucleic acids, self-associates in water to form G-quadruplexes, four-stranded helicoidal aggregates, made by stacked planar tetramers, consisting of four planar guanine molecules. Essential for the stability of these supramolecular aggregates is the presence of monovalent cations. As G-quadruplexes show a lyotropic polymorphism, neutron diffraction, in combination with the H2O/D2O contrast variation technique, has been applied to study the cation structural features of quadruplexes in hexagonal phase at different hydrations and counterion concentrations. The guanosine 5'-monophosphate, dipotassium salt, was considered and G-quadruplexes in hexagonal phase were prepared in the different experimental conditions (contrast, hydration and KCl solution concentration) by using the osmotic stress technique. The calculated scattering length density distribution maps show that counterions fill the helix inner cavity and that atmospheric cations are bound to a second site, close to the external phosphate groups. The site occupancy turned out to be very high: we found on the inner site 0.87 K ions per tetramer in G-quadruplexes prepared in pure water and 0.97 K ions per tetramer in G-quadruplexes prepared in KCl 0.5 M, while in all cases 6 ions per unit cell were detected to occupy the outer sites, partially neutralizing the two formal negative charges per phosphate group. The very large K ions concentration difference between the binding sites and the bulk solution demonstrates that counterions are structurally involved in the formation and in the stabilization of the helices.
authors.affiliation: Dipartimento di Scienze Applicate ai Sistemi Complessi and INFM, Università Politecnica delle Marche, Via Ranieri 65, I-60131 Ancona, Italy. federic@alisfl.univpm.it
authors.forename: Francesco
authors.initials: F
authors.lastname: Federiconi
authors.forename: Pamela
authors.initials: P
authors.lastname: Ausili
authors.forename: Giovanna
authors.initials: G
authors.lastname: Fragneto
authors.forename: Claudio
authors.initials: C
authors.lastname: Ferrero
authors.forename: Paolo
authors.initials: P
authors.lastname: Mariani
mesh_terms: D002412:Cations; D017666:Deuterium Oxide; D006151:Guanosine / Q000737:chemistry*; D006157:Guanosine Monophosphate / Q000737:chemistry; D007477:Ions; D008956:Models, Chemical; D008968:Molecular Conformation; D033363:Neutron Diffraction; D009502:Neutrons; D009692:Nucleic Acid Heteroduplexes / Q000737:chemistry*; D011188:Potassium / Q000737:chemistry; D011189:Potassium Chloride / Q000494:pharmacology; D012542:Scattering, Radiation; D014965:X-Rays
chemical_list: D002412:Cations; D007477:Ions; D009692:Nucleic Acid Heteroduplexes; D006151:Guanosine; D011189:Potassium Chloride; D006157:Guanosine Monophosphate; D017666:Deuterium Oxide; D011188:Potassium