| 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 guaninemolecules. 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/D2Ocontrast variation technique, has been applied to study the cation structural features of quadruplexes in hexagonalphase at different hydrations and counterion concentrations. The guanosine 5‘-monophosphate, dipotassiumsalt, was considered and G-quadruplexes in hexagonal phase were prepared in the different experimentalconditions (contrast, hydration and KCl solution concentration) by using the osmotic stress technique. Thecalculated scattering length density distribution maps show that counterions fill the helix inner cavity andthat atmospheric cations are bound to a second site, close to the external phosphate groups. The site occupancyturned out to be very high: we found on the inner site 0.87 K ions per tetramer in G-quadruplexes preparedin pure water and 0.97 K ions per tetramer in G-quadruplexes prepared in KCl 0.5 M, while in all cases 6ions per unit cell were detected to occupy the outer sites, partially neutralizing the two formal negative chargesper phosphate group. The very large K ions concentration difference between the binding sites and the bulksolution demonstrates that counterions are structurally involved in the formation and in the stabilization ofthe helices.
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