Electron detachment dynamics of the iodide-guanine cluster: does ionization occur from the iodide or from guanine?

Abstract

Laser photodissociation spectroscopy of the I -•guanine complex has been conducted for the first time across the regions above the electron detachment threshold to explore the excited states and whether vertical ionization occurs from the iodide or the nucleobase.The photofragment spectra reveal a prominent dipole-bound excited state (I) close to the calculated vertical electron detachment energy (~4.0 eV) and a second excited (II) centred around 4.8 eV, which we assign to -* nucleobase-localized transitions. The ionic photofragments are identified as Iand I -•[G-H], with the later fragment being produced significantly more strongly than the former. Both photofragments are observed across the two excited states, with production of the iodide being attributed to internal conversion to the ground state followed by evaporation. We trace the formation of the I -•[G-H] photofragment to initial vertical ionization of guanine, followed by ejection of a proton. This two-step process is important as it follows known steps in radiation-induced damage to DNA, namely initial formation of a guanine radical cation which then forms a free radical [G-H] moiety through deprotonation. Production of the I -•[G-H] photofragment is pronounced through II indicating that its formation is enhanced by coupling of the -* transitions to the electron detachment continuum.

Cryogenic “Iodide-Tagging” Photoelectron Spectroscopy: A Sensitive Probe for Specific Binding Sites of Amino Acids

“…Confirmation of the presence of multiple species can be achieved by varying their relative populations by carrying out temperature-dependent measurements. − For the Gly·I – cluster, five distinct structures with different binding motifs and varying in energy over a 3.93 kcal mol –1 range were successfully trapped and characterized. The importance of iodide in cluster anions has recently been demonstrated and explored as a way to initiate intracluster electron transfer and study excited-state dynamics as well as to probe various tautomers of a specific biomolecule to which the iodide binds. − …”

Observation of Enhanced Dissociative Photochemistry in the Non-Native Nucleobase 2-Thiouracil

“…The pattern of photofragment action spectra observed here for the m/z 58 and m/z 93 photofragments is reminiscent of behaviour we observed recently in studies of iodide-nucleobase complexes [55,56,64]. These complexes similarly display a dipole-bound excited state in the vicinity of the VDE, which decays with production of the respective valence anion produced upon low-energy electron attachment to the nucleobase.…”

“…However, these absorptions are not evident in our gaseous experimental spectrum presented in Figure 2, probably due to the dominance of electron detachment (Section S2). In previous anionic systems we studied, these excited states have been more clearly visible in the photofragment production spectra [44,55,56]. We therefore turn to inspecting the photofragmentation channels of [2-TU-H] − to further characterise the excited states and photochemistry.…”

Electron attachment dynamics following UV excitation of iodide-2-thiouracil complexes

“…Complementary experiments on the photodepletion of several of these species have been carried out by Dessent and co-workers. [15][16][17] The nature and dynamics of the TNI depend on the energy of the excitation pulse. In the I − ⋅uracil (I − ⋅U) cluster, which is one of the more intensively investigated systems, excitation near the vertical detachment energy (VDE) gives rise to both DB and VB states immediately after photoexcitation.…”