The photo-induced damages of DNA in interaction with metal cations, which are found in various environments, still remain to be characterized. In this paper, we show how the complexation of a DNA base (cytosine (Cyt)) with a metal cation (Ag + ) changes its electronic properties. By means of UV photofragment spectroscopy of cold ions, it was found that the photoexcitation of the CytAg

It has been already established that DNA−silver fluorophores present two intense absorption bands, one in the visible and the other in the UV spectral region, and that the excitation of both bands produces a common fluorescence signal. Nevertheless, the detailed mechanism responsible for this coupling is still elusive. In this context, our work is a significant leap forward in this goal by providing a dynamic picture of the excitation and the processes that take place upon light absorption in these systems using a time-dependent density functional tight-binding approach. In the present work, we explore the electronic coupling mechanism in a cluster composed of a double string of deoxypolycytosine with six bases and a rod of six Ag atoms.

The proton-bound dimer of hydrogen sulfate and formate is an archetypal structure for ionic hydrogen-bonding complexes that contribute to biogenic aerosol nucleation. Of central importance for the structure and properties of this complex is the location of the bridging proton connecting the two conjugate base moieties. The potential energy surface for bridging proton translocation features two local minima, with the proton localized at either the formate or hydrogen sulfate moiety. However, electronic structure methods reveal a shallow potential energy surface governing proton translocation, with a barrier on the order of the zero-point energy. This shallow potential complicates structural assignment and necessitates a consideration of nuclear quantum effects. In this work, we probe the structure of this complex and its isotopologues, utilizing infrared (IR) action spectroscopy of ions captured in helium nanodroplets. The IR spectra indicate a structure in which a proton is shared between the hydrogen sulfate and formate moieties, HSO 4 – ···H + ··· – OOCH. However, because of the nuclear quantum effects and vibrational anharmonicities associated with the shallow potential for proton translocation, the extent of proton displacement from the formate moiety remains unclear, requiring further experiments or more advanced theoretical treatments for additional insight.

The UV-photofragmentation spectra of cold Cytosine-M + complexes (M + : Na + , K + , Ag+) were recorded and analyzed through comparison with geometry optimizations and frequency calculations of the ground and excited states at the SCS-CC2/Def2-SVPD level of theory. While in all complexes, the ground state minimum geometry is planar (Cs symmetry), the * state minimum geometry has the NH2 group slightly twisted and an out-of-plane metal cation. This was confirmed by comparing the simulated * Franck-Condon spectra with the vibrationally resolved photofragmentation spectra of CytNa + and CytK + . Vertical excitation transitions were also calculated to evaluate the energies of the CT states involving the transfer of an electron from the Cyt moiety to M + . For both CytK + and CytNa + complexes, the first CT state corresponds to an electron transfer from the cytosine aromatic  ring to the antibonding * orbital centered on the alkali cation. This * state is predicted to lie much higher in energy (> 6 V) than the band origin of the * electronic transition (around 4.3 eV) unlike what is observed for CytAg + complex for which the first excited state has a nO* electronic configuration. This is the reason for the absence of the Cyt + + M charge transfer fragmentation channel for CytK + and CytNa + complexes.