Nonconventional Hydrogen Bonds: A Theoretical Study of [uracil-L]− (L = F, Cl, Br, I, Al, Ga, In) Complexes

Time-resolved radiation chemistry: femtosecond photoelectron spectroscopy of electron attachment and photodissociation dynamics in iodide-nucleobase clusters

Iodide-nucleobase (I-·N) clusters studied by time-resolved photoelectron spectroscopy (TRPES) are an opportune model system for examining radiative damage of DNA induced by low-energy electrons. By initiating charge transfer from iodide to the nucleobase and following the dynamics of the resulting transient negative ions (TNIs) with femtosecond time resolution, TRPES provides a novel window into the chemistry triggered by the attachment of low-energy electrons to nucleobases. In this Perspective, we examine and compare the dynamics of electron attachment, autodetachment, and photodissociation in a variety of I-·N clusters, including iodide-uracil (I-·U), iodide-thymine (I-·T), iodide-uracil-water (I-·U·H2O), and iodide-adenine (I-·A), to develop a more unified representation of our understanding of nucleobase TNIs. The experiments probe whether dipole-bound or valence-bound TNIs are formed initially and the subsequent time evolution of these species. We also provide an outlook for forthcoming applications of TRPES to larger iodide-containing complexes to enable the further investigation of microhydration dynamics in nucleobases, as well as electron attachment and photodissociation in more complex nucleic acid constituents.

Tautomerisation of thymine acts against the Hückel 4N + 2 rule. The effect of metal ions and H-bond complexations on the electronic structure of thymine

Not necessarily the π-electron delocalization is responsible for the stability of thymine tautomers.

Time-resolved photoelectron imaging of the iodide-thymine and iodide-uracil binary cluster systems

The energetics and dynamics of thymine and uracil transient negative ions were examined using femtosecond time-resolved photoelectron imaging. The vertical detachment energies (VDEs) of these systems were found to be 4.05 eV and 4.11 eV for iodide-thymine (I(-) x T) and iodide-uracil (I(-) x U) clusters, respectively. An ultraviolet pump pulse was used to promote intracluster charge transfer from iodide to the nucleobase. Subsequent electron detachment using an infrared probe pulse monitored the dynamics of the resulting transient negative ion. Photoelectron spectra reveal two primary features: a near-zero electron kinetic energy signal attributed to autodetachment and a transient feature representing photodetachment from the excited anion state. The transient state exhibits biexponential decay in both thymine and uracil complexes with short and long decay time constants ranging from 150-600 fs and 1-50 ps, respectively, depending on the excitation energy. However, both time constants are systematically shorter for I(-) x T. Vibrational autodetachment and iodine loss are identified as the primary decay mechanisms of the transient negative ions of thymine and uracil.

Time-resolved radiation chemistry: photoelectron imaging of transient negative ions of nucleobases

Time-resolved photoelectron imaging has been utilized to probe the energetics and dynamics of the transient negative ion of the nucleobase uracil. This species was created through charge transfer from an iodide anion within a binary iodide-uracil complex using a UV pump pulse; the ensuing dynamics were followed by photodetachment with a near-IR probe pulse. The photoelectron spectra show two time-dependent features, one from probe-induced photodetachment of the transient anion state and another from very low energy electron signal attributed to autodetachment. The transient anion was observed to decay biexponentially with time constants of hundreds of femtoseconds and tens of picoseconds, depending on the excitation energy. These dynamics are interpreted in terms of autodetachment from the initially excited state and a second, longer-lived species relaxed by iodine loss. Hydrogen loss from the N1 position may also occur in parallel.

Photoelectron and computational studies of the copper-nucleoside anionic complexes, Cu(-)(cytidine) and Cu(-)(uridine)

The copper-nucleoside anions, Cu(-)(cytidine) and Cu(-)(uridine), have been generated in the gas phase and studied by both experimental (anion photoelectron spectroscopy) and theoretical (density functional calculations) methods. The photoelectron spectra of both systems are dominated by single, intense, and relatively narrow peaks. These peaks are centered at 2.63 and 2.71 eV for Cu(-)(cytidine) and Cu(-)(uridine), respectively. According to our calculations, Cu(-)(cytidine) and Cu(-)(uridine) species with these peak center [vertical detachment energy (VDE)] values correspond to structures in which copper atomic anions are bound to the sugar portions of their corresponding nucleosides largely through electrostatic interactions; the observed species are anion-molecule complexes. The combination of experiment and theory also reveal the presence of a slightly higher energy, anion-molecule complex isomer in the case of the Cu(-)(cytidine). Furthermore, our calculations found that chemically bond isomers of these species are much more stable than their anion-molecule complex counterparts, but since their calculated VDE values are larger than the photon energy used in these experiments, they were not observed.