Frequency and effect of the binding of Mg²⁺, Mn²⁺, and Co²⁺ ions on the guanine base in watson-crick and reverse watson-crick base pairs

We performed MP2 calculations to elucidate the structure and energetics of the Mg^2-, Mn²⁺, and Co^2- hexahydrated aquaions, and the effect of the metal binding to the N7 atom of (i) a single guanine, (ii) a guanine involved in a Watson-Crick pair, and (iii) a guanine involved in a reverse Watson-Crick base pair. Our comparative analysis of the three aquaions indicates a clear inverse correlation between the radius of the cation and the binding energy, that indeed increases in the order Mn²⁺ < Co ²⁺ < Mg^2-. The trend in the binding energies of the pentahydrated cations to the N7 atom of the guanine is instead Mg²⁺ < Mn^2- < Co²⁺, suggesting a rather different bonding scheme that, for the two transition metals, involves back-donation from the aromatic ring of the guanine to their empty d orbitais. In the gas phase, the three hydrated metals significantly stabilize both G-C base pair geometries, Watson-Crick and reverse Watson-Crick, we investigated. Inclusion of a continuous solvent model, however, remarkably reduces this additional stabilization, which becomes almost negligible in the case of the Mg ²⁺ cation coordinated to the guanine in the standard Watson-Crick geometry. Conversely, all three metal ions sensibly stabilize the reverse Watson-Crick geometry, also in water. Our results are supported by a screening of the structures available in the Protein Data Bank, which clearly indicates that the two transition metals we investigated have a tendency greater than Mg^2- to coordinate to the N7 atom of guanines, and that there is no clear correlation between the number of guanines in experimental structures with a metal bound to N7 atom and their involvement in Watson-Crick base pairs.