Excited States in Single-Stranded and i-Motif DNA with Silver Ions

We have studied the excited states and structural properties for the complexes of cytosine (dC)₁₀ chains with silver ions (Ag⁺) in a wide range of the Ag⁺ to DNA ratio (r) and pH conditions using circular dichroism, steady-state absorption, and fluorescence spectroscopy along with the ultrafast fluorescence upconversion technique. We also calculated vertical electronic transition energies and determined the nature of the corresponding excited states in some models of the cytosine-Ag⁺ complexes. We show that (dC)₁₀ chains in the presence of silver ions form a duplex stabilized by C-Ag⁺-C bonds. It is also shown that the i-motif structure formed by (dC)₁₀ chains is destabilized in the presence of Ag⁺ ions. The excited-state properties in the studied complexes depend on the amount of binding ions and the binding sites, which is supported by the calculations. In particular, new low-lying excited states appear when the second Ag⁺ ion interacts with the O atom of cytosine in the C-Ag⁺-C pairs. A similar picture is observed in the case when one Ag⁺ ion interacts with one cytosine via the N7 atom.