Synchrotron X-radiolysis of l-cysteine at the sulfur K-edge: Sulfurous products, experimental surprises, and dioxygen as an oxidoreductant

Abstract

In situ inventory of sulfurous products from the sulfur K-edge synchrotron X-radiolysis of L-cysteine in solid-phase and anaerobic (pH 5) and air-saturated (pH 5, 7, and 9) solutions without and with 40% glycerol is reported. Sequential K-edge X-ray Absorption Spectroscopic (XAS) spectra were acquired. L-cysteine degraded systematically in the X-ray beam. Radiolytic products were inventoried by fits using the XAS spectra of sulfur model compounds. Solid L-cysteine declined to 92% fraction after a single K-edge XAS scan. After six scans, 60% remained, accompanied by 14% cystine, 16% thioether, 5.4% elemental sulfur, and smaller fractions of more highly oxidized products. In air-saturated pH 5 solution, 73% of L-cysteine remained after ten scans, with 2% cystine and 19% elemental sulfur. Oxidation increased with 40% glycerol, yielding 67%, 5%, and 23% fractions, respectively, after ten scans. Higher pH solutions exhibited less radiolytic chemistry. All the reactivity followed first-order kinetics. The anaerobic experiment displayed two reaction phases, with sharp changes in kinetics and radiolytic chemistry. Unexpectedly, the radiolytic oxidation of L-cysteine was increased in anaerobic solution. After ten scans, only 60% of the L-cysteine remained, along with 17% cystine, 22% elemental sulfur, and traces of more highly oxidized products. A new aerobic reaction cycle is hypothesized, wherein dissolved dioxygen captures radiolytic H • or e − aq , enters HO 2 • /O 2 •− , reductively quenches cysteine thiyl radicals, and cycles back to O 2 . This cycle is suggested to suppress the radiolytic production of cystine in aerobic solution.

Microfluidic liquid sheets as large-area targets for high repetition XFELs

“…Water decomposes under ionizing radiation into a handful of major species, namely hydrated electrons (e − aq), hydroxyl radicals (OH*), the water radical cation (H 2 O + ), hydrogen radicals (H*), hydroxonium ions (H 3 O + ) and hydrogen peroxide (H 2 O 2 ) (Gunter, 1967;Plonka et al, 2000;Garrett et al, 2005;El Omar et al, 2011). Among these products, solvated electrons are likely most relevant to photoreduction (Frank et al, 2019;Stellato et al, 2019). Because the nozzle exit aperture is much larger than the sheet thickness, samples can run at high concentration with minimal risk of clogging.…”

Symmetry Breaking in Solution-Phase [Cu(tsc)2(H2O)2]2+: Emergent Asymmetry in Cu–S Distances and in Covalence

“…Measured sulfur or copper K-edge XAS spectra were background subtracted and normalized as has been described in detail elsewhere. ,,− EXAFS spectra were fit over the k -range 2–12.6 Å –1 , using Feff 8.5 within the program OPT, which is part of the EXAFSPAK software package . The number of statistically independent EXAFS data points was evaluated using the Stern equation N ind = (2Δ r Δ k /π) + 2 .…”