UV laser photoactivation of hexachloroplatinate bound to individual nucleobases in vacuo as molecular level probes of a model photopharmaceutical

Abstract

Isolated molecular clusters of adenine, cytosine, thymine and uracil bound to hexachloroplatinate, PtCl6(2-), have been studied using laser electronic photodissociation spectroscopy to investigate photoactivation of a platinum complex in the vicinity of a nucleobase. These metal complex-nucleobase clusters represent model systems for identifying the fundamental photochemical processes occurring in photodynamic platinum drug therapies that target DNA. This is the first study to explore the specific role of a strongly photoactive platinum compound in the aggregate complex. Each of the clusters studied displays a broadly similar absorption spectra, with a strong λmax ∼ 4.6 eV absorption band and a subsequent increase in the absorption intensity towards higher spectral-energy. The absorption bands are traced to ligand-to-metal-charge-transfer excitations on the PtCl6(2-) moiety within the cluster, and result in Cl(-)·nucleobase and PtCl5(-) as primary photofragments. These results demonstrate how selective photoexcitation can drive distinctive photodecay channels for a model photo-pharmaceutical. In addition, cluster absorption due to excitation of nucleobase-centred chromophores is observed in the region around 5 eV. For the uracil cluster, photofragments consistent with ultrafast decay of the excited state and vibrational predissociation on the ground-state surface are observed. However, this decay channel becomes successively weaker on going from thymine to cytosine to adenine, due to differential coupling of the excited states to the electron detachment continuum. These effects demonstrate the distinctive photophysical characteristics of the different nucleobases, and are discussed in the context of the recently recorded photoelectron spectra of theses clusters.

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 ).…”

Photoexcitation of iodide ion-pyrimidine clusters above the electron detachment threshold: Intracluster electron transfer versus nucleobase-centred excitations

“…UV photodissociation experiments were conducted in an AmaZon (Bruker) ion-trap mass spectrometer that has been converted for laser experiments as described in detail elsewhere. 19,20 The I -•M clusters were generated by electrospraying solutions of nucleobase and iodide in deionized water (nucleobase solutions were 1 × 10 −4 mol dm -3 , mixed with droplets of t-butyl ammonium iodide (TBAI) at 1 × 10 −2 mol dm -3 ). All chemicals were purchased from Sigma The I -•M clusters were mass-selected and isolated in an ion-trap prior to laser irradiation.…”

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

“…[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…”