A Photochemically Generated Selenyl Free Radical Observed by High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy

Abstract

Selenium-based selenyl free radicals are chemical entities that may be involved in a range of biochemical processes. We report the first X-ray spectroscopic observation of a selenyl radical species generated photochemically by X-ray irradiation of low-temperature solutions of l-selenocysteine. We have employed high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) and electron paramagnetic resonance (EPR) spectroscopy, coupled with density functional theory calculations, to characterize and understand the species. The HERFD-XAS spectrum of the selenyl radical is distinguished by a uniquely low-energy transition with a peak energy at 12 659.0 eV, which corresponds to a 1s → 4p transition to the singly occupied molecular orbital of the free radical. The EPR spectrum shows the broad features and highly anisotropic g-values that are expected for a selenium free radical species. The availability of spectroscopic probes for selenyl radicals may assist in understanding why life chooses selenium over sulfur in selected biochemical processes.

Copper(II) Binding to PBT2 Differs from That of Other 8-Hydroxyquinoline Chelators: Implications for the Treatment of Neurodegenerative Protein Misfolding Diseases

“…The concentration of copper in the human brain has been previously found to be 3.32 ± 1.50 μg/g, with copper concentrations in amyloid plaques reported to be as high as 25.0 ± 7.8 μg/g in AD patients . [The study by Lovell et al used formalin fixed brain tissue, which is known to result in metal contamination in some cases (e.g., copper) and elemental depletion in other cases (e.g., potassium). , Therefore, the reported copper concentrations may be somewhat overestimated .…”

“…63 [The study by Lovell et al 63 used formalin fixed brain tissue, which is known to result in metal contamination in some cases (e.g., copper) and elemental depletion in other cases (e.g., potassium). 64,65 Therefore, the reported copper concentrations may be somewhat overestimated. 65 ] PBT2 concentrations were reported to be 318.1 ± 75.1 ng/g in the brain of Alzheimer's model mice 2 h after an oral dose of 30 mg/kg.…”

“…64,65 Therefore, the reported copper concentrations may be somewhat overestimated. 65 ] PBT2 concentrations were reported to be 318.1 ± 75.1 ng/g in the brain of Alzheimer's model mice 2 h after an oral dose of 30 mg/kg. 2 This would seem to suggest that there is likely to be at least 10 times more copper in the AD brain than PBT2.…”

“…Near-edge HERFD-XAS measurements were carried out on BL 6-2 at the Stanford Synchrotron Radiation Lightsource (SSRL) with the SPEAR3 storage ring containing 500 mA at 3.0 GeV. Using a setup similar to that previously described for our Se HERFD-XAS, 65 a Si(311) doublecrystal monochromator with an energy resolution at the Cu K-edge of ∼0.3 eV was used with a six-element Johann-type array of Si(444) crystal analyzers to record the Cu Kα 1 emission. 66 Harmonic rejection was achieved by setting the cutoff energy of the upstream Rh-coated mirror to 18 keV.…”

Discussions of Fluorescence in Selenium Chemistry: Recently Reported Probes, Particles, and a Clearer Biological Knowledge

“…However, naturally there are also many reports which we do not describe, but to which we simply refer. See for example references [80], [81], [82], [83], [84], [85], [86], [87], [88]. Devi et al designed carbon quantum dots (CQDs) (Figure 42) from pyrolysis of citric acid with a nitrogen-rich supramolecular ligand (SL) to detect the extremely toxic selenite species (SeO3 2− ) reference.…”