Oxidative stress produces a variety of radicals in DNA, including pyrimidine nucleobase radicals. The nitrogencentered DNA radical 2′-deoxycytidin-N4-yl radical (dC•) plays a role in DNA damage mediated by one electron oxidants, such as HOCl and ionizing radiation. However, the reactivity of dC• is not well understood. To reduce this knowledge gap, we photochemically generated dC• from a nitrophenyl oxime nucleoside and within chemically synthesized oligonucleotides from the same precursor. dC• formation is confirmed by transient UV-absorption spectroscopy in laser flash photolysis (LFP) experiments. LFP and duplex DNA cleavage experiments indicate that dC• oxidizes dG. Transient formation of the dG radical cation (dG +• ) is observed in LFP experiments. Oxidation of the opposing dG in DNA resultsin hole transfer when the opposing dG is part of a dGGG sequence. The sequence dependence is attributed to a competition between rapid proton transfer from dG +• to the opposing dC anion formed and hole transfer. Enhanced hole transfer when less acidic O6-methyl-2′-deoxyguanosine is opposite dC• supports this proposal. dC• produces tandem lesions in sequences containing thymidine at the 5′-position by abstracting a hydrogen atom from the thymine methyl group. The corresponding thymidine peroxyl radical completes tandem lesion formation by reacting with the 5′-adjacent nucleotide. As dC• is reduced to dC, its role in the process is traceless and is only detectable because of the ability to independently generate it from a stable precursor. These experiments reveal that dC• oxidizes neighboring nucleotides, resulting in deleterious tandem lesions and hole transfer in appropriate sequences.
Gold nanoparticle modifications for TiO2 (Au/TiO2) can extend the absorption wavelength from UV to visible (Vis) and enhance the photocatalytic performance, thus fueling increasing attention as an emerging photocatalysis strategy. To explore the plasmon-enhanced photocatalytic mechanism and directly unveil the intrinsic properties of Au/TiO2, the decay kinetics of photoelectrons upon UV (355 nm) or Vis (532 nm) excitation are monitored by means of nanosecond time-resolved infrared spectroscopy, which is a unique tool offering observations without interference of the holes. Under UV irradiation, the longer lifetime of photoelectrons observed in Au/TiO2 compared to that in bare TiO2 provides unambiguous evidence for the enhanced charge separation by AuNPs. Under Vis irradiation, the long-lived (hundreds of microseconds) electrons produced by injection from AuNPs into TiO2 upon plasmon excitation are here detected for the first time. Moreover, the effects of TiO2 phase composition and the amount of AuNPs loading on the decay kinetics of long-lived photoelectrons are examined.
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