Solvent evaporation versus proton transfer in nucleobase–Pt(CN)4,62− dianion clusters: a collisional excitation and electronic laser photodissociation spectroscopy study

Abstract

Isolated molecular clusters of adenine, cytosine, thymine and uracil with Pt(CN)6(2-) and Pt(CN)4(2-) were studied for the first time to characterize the binding and reactivity of isolated transition metal complex ions with nucleobases. These clusters represent model systems for understanding metal complex-DNA adducts, as a function of individual nucleobases. Collisional excitation revealed that the clusters decay on the ground electronic surface by either solvent evaporation (i.e. loss of a nucleobase unit from the cluster) or via proton transfer from the nucleobase to the dianion. The Pt(CN)6(2-)-nucleobase clusters decay only by solvent evaporation, while the Pt(CN)4(2-) clusters fragment by both pathways. The enhanced proton-transfer reactivity of Pt(CN)4(2-) is attributed to the higher charge-density of the ligands in this transition metal anion. % fragmentation curves of the clusters reveal that the adenine clusters display distinctively higher fragmentation onsets, which are traced to the propensity of adenine to form the shortest intercluster H-bond. We also present laser electronic photodissociation measurements for the Pt(CN)6(2-)·Ur, Pt(CN)4(2-)·Ur and Pt(CN)4(2-)·Ur2 clusters to illustrate the potential of exploring metal complex DNA photophysics as a function of nucleobase within well-defined gaseous clusters. The spectra reported herein represent the first such measurements. We find that the electronic excited states decay with production of the same fragments (associated with solvent evaporation and proton transfer) observed upon collisional excitation of the electronic ground state, indicating ultrafast deactivation of the excited-state uracil-localized chromophore followed by vibrational predissociation.

Near-threshold electron transfer in anion-nucleobase clusters: does the identity of the anion matter?

“…UV photodissociation experiments were conducted in a laser-interfaced amaZon SL (Bruker Daltonics) ion-trap mass spectrometer as described in detail elsewhere [24,25]. The clusters were generated by electrospraying a solution of adenine (1 × 10 −4 mol dm -3 ) mixed with droplets of t-butyl ammonium iodide (1 × 10 −2 mol dm -3 ) or a NaH2PO3 solution in deionized water (1 × 10 −4 mol dm -3 ).…”

“…From the first-type of process, we expect the production of the adenine, A -(1a), or the deprotonated adenine anion [A-H] -(1c). In contrast, the adenine-centred transitions within the cluster are expected to produce the cluster fragments that are associated with thermal fragmentation of the ground-electronic state [25], following ultrafast decay of the initially populated nucleobase-localized electronic excited state.…”

Observation of Near-Threshold Resonances in the Flavin Chromophore Anions Alloxazine and Lumichrome

“…This leads to production of the m/z 170 and m/z 198 fragments as in collision induced dissociation (Section S6). 21 Recent work from Neumark and co-workers using time-resolved anion photoelectron spectroscopy has provided a number of examples of dipole-bound states which can either autodetach or evolve into a valence anion, which subsequently dissociates. 37,38 Thus it is now well acknowledged that the formation of a dipole-bound state can act as a doorway to valenceanion formation, and hence to molecular dissociation.…”

“…[AL-H]and [LC-H] -, were recorded in vacuo using action spectroscopy in a laser-interfaced mass spectrometer which is described in detail elsewhere and in the Supporting Information (SI). 20,21 The photodepletion spectrum can be considered to be equivalent to the gaseous absorption spectrum, in the limit where excited state fluorescence is negligible. 22…”

Unravelling the Keto-Enol Tautomer Dependent Photochemistry and Degradation Pathways of the Protonated UVA Filter Avobenzone

“…When fluorescence is negligible, 58 the UV excited gaseous ion will fragment upon excited state relaxation, yielding an action absorption spectrum by photodepletion (PD). 57,59,60 PD was measured as a function of the scanned wavelength, with the photofragment production (PF) also recorded simultaneously at each wavelength, both of which according to…”