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1.
Nat Commun ; 12(1): 1086, 2021 02 17.
Artigo em Inglês | MEDLINE | ID: mdl-33597529

RESUMO

The dynamics of photodissociation and recombination in heme proteins represent an archetypical photochemical reaction widely used to understand the interplay between chemical dynamics and reaction environment. We report a study of the photodissociation mechanism for the Fe(II)-S bond between the heme iron and methionine sulfur of ferrous cytochrome c. This bond dissociation is an essential step in the conversion of cytochrome c from an electron transfer protein to a peroxidase enzyme. We use ultrafast X-ray solution scattering to follow the dynamics of Fe(II)-S bond dissociation and 1s3p (Kß) X-ray emission spectroscopy to follow the dynamics of the iron charge and spin multiplicity during bond dissociation. From these measurements, we conclude that the formation of a triplet metal-centered excited state with anti-bonding Fe(II)-S interactions triggers the bond dissociation and precedes the formation of the metastable Fe high-spin quintet state.


Assuntos
Citocromos c/metabolismo , Compostos Ferrosos/metabolismo , Ferro/metabolismo , Metais/metabolismo , Metionina/metabolismo , Citocromos c/química , Transporte de Elétrons/efeitos da radiação , Compostos Ferrosos/química , Heme/química , Heme/metabolismo , Ferro/química , Metais/química , Metionina/química , Simulação de Dinâmica Molecular , Fotólise , Espectrometria por Raios X
2.
Nat Chem ; 2021 Feb 15.
Artigo em Inglês | MEDLINE | ID: mdl-33589787

RESUMO

It is well known that the solvent plays a critical role in ultrafast electron-transfer reactions. However, solvent reorganization occurs on multiple length scales, and selectively measuring short-range solute-solvent interactions at the atomic level with femtosecond time resolution remains a challenge. Here we report femtosecond X-ray scattering and emission measurements following photoinduced charge-transfer excitation in a mixed-valence bimetallic (FeiiRuiii) complex in water, and their interpretation using non-equilibrium molecular dynamics simulations. Combined experimental and computational analysis reveals that the charge-transfer excited state has a lifetime of 62 fs and that coherent translational motions of the first solvation shell are coupled to the back electron transfer. Our molecular dynamics simulations identify that the observed coherent translational motions arise from hydrogen bonding changes between the solute and nearby water molecules upon photoexcitation, and have an amplitude of tenths of ångströms, 120-200 cm-1 frequency and ~100 fs relaxation time. This study provides an atomistic view of coherent solvent reorganization mediating ultrafast intramolecular electron transfer.

4.
Inorg Chem ; 60(2): 736-744, 2021 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-33373520

RESUMO

A series of ß-diketiminate Ni-NO complexes with a range of NO binding modes and oxidation states were studied by X-ray emission spectroscopy (XES). The results demonstrate that XES can directly probe and distinguish end-on vs side-on NO coordination modes as well as one-electron NO reduction. Density functional theory (DFT) calculations show that the transition from the NO 2s2s σ* orbital has higher intensity for end-on NO coordination than for side-on NO coordination, whereas the 2s2s σ orbital has lower intensity. XES calculations in which the Ni-N-O bond angle was fixed over the range from 80° to 176° suggest that differences in NO coordination angles of ∼10° could be experimentally distinguished. Calculations of Cu nitrite reductase (NiR) demonstrate the utility of XES for characterizing NO intermediates in metalloenzymes. This work shows the capability of XES to distinguish NO coordination modes and oxidation states at Ni and highlights applications in quantifying small molecule activation in enzymes.

5.
Artigo em Inglês | MEDLINE | ID: mdl-33283495

RESUMO

Layered lithium nickel, manganese, and cobalt oxides (NMC) are among the most promising commercial positive electrodes in the past decades. Understanding the detailed surface and bulk redox processes of Ni-rich NMC can provide useful insights into material design options to boost reversible capacity and cycle life. Both hard X-ray absorption (XAS) of metal K-edges and soft XAS of metal L-edges collected from charged LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Mn0.1Co0.1O2 (NMC811) showed that the charge capacity up to removing ∼0.7 Li/f.u. was accompanied with Ni oxidation in bulk and near the surface (up to 100 nm). Of significance to note is that nickel oxidation is primarily responsible for the charge capacity of NMC622 and 811 up to similar lithium removal (∼0.7 Li/f.u.) albeit charged to different potentials, beyond which was followed by Ni reduction near the surface (up to 100 nm) due to oxygen release and electrolyte parasitic reactions. This observation points toward several new strategies to enhance reversible redox capacities of Ni-rich and/or Co-free electrodes for high-energy Li-ion batteries.

6.
Phys Chem Chem Phys ; 2020 Dec 23.
Artigo em Inglês | MEDLINE | ID: mdl-33355326

RESUMO

Until recently, sulfur was known as a "spectroscopically silent" element because of a paucity of convenient spectroscopic probes suitable for in situ chemical speciation. In recent years the technique of sulfur K-edge X-ray absorption spectroscopy (XAS) has been used extensively in sulfur speciation in a variety of different fields. With an initial focus on reduced forms of organic sulfur, we have explored a complementary X-ray based spectroscopy - sulfur Kß X-ray emission spectroscopy (XES) - as a potential analytical tool for sulfur speciation in complex samples. We compare and contrast the sensitivity of sulfur Kß XES with that of sulfur K-edge XAS, and find differing sensitivities for the two techniques. In some cases an approach involving both sulfur K-edge XAS and sulfur Kß XES may be a powerful combination for deducing sulfur speciation in samples containing complex mixtures.

7.
Inorg Chem ; 59(22): 16567-16581, 2020 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-33136386

RESUMO

Cu(I) active sites in metalloproteins are involved in O2 activation, but their O2 reactivity is difficult to study due to the Cu(I) d10 closed shell which precludes the use of conventional spectroscopic methods. Kß X-ray emission spectroscopy (XES) is a promising technique for investigating Cu(I) sites as it detects photons emitted by electronic transitions from occupied orbitals. Here, we demonstrate the utility of Kß XES in probing Cu(I) sites in model complexes and a metalloprotein. Using Cu(I)Cl, emission features from double-ionization (DI) states are identified using varying incident X-ray photon energies, and a reasonable method to correct the data to remove DI contributions is presented. Kß XES spectra of Cu(I) model complexes, having biologically relevant N/S ligands and different coordination numbers, are compared and analyzed, with the aid of density functional theory (DFT) calculations, to evaluate the sensitivity of the spectral features to the ligand environment. While the low-energy Kß2,5 emission feature reflects the ionization energy of ligand np valence orbitals, the high-energy Kß2,5 emission feature corresponds to transitions from molecular orbitals (MOs) having mainly Cu 3d character with the intensities determined by ligand-mediated d-p mixing. A Kß XES spectrum of the Cu(I) site in preprocessed galactose oxidase (GOpre) supports the 1Tyr/2His structural model that was determined by our previous X-ray absorption spectroscopy and DFT study. The high-energy Kß2,5 emission feature in the Cu(I)-GOpre data has information about the MO containing mostly Cu 3dx2-y2 character that is the frontier molecular orbital (FMO) for O2 activation, which shows the potential of Kß XES in probing the Cu(I) FMO associated with small-molecule activation in metalloproteins.

8.
Sci Rep ; 10(1): 16837, 2020 Oct 08.
Artigo em Inglês | MEDLINE | ID: mdl-33033373

RESUMO

We report the time-resolved femtosecond evolution of the K-shell X-ray emission spectra of iron during high intensity illumination of X-rays in a micron-sized focused hard X-ray free electron laser (XFEL) beam. Detailed pulse length dependent measurements revealed that rapid spectral energy shift and broadening started within the first 10 fs of the X-ray illumination at intensity levels between 1017 and 1018 W cm-2. We attribute these spectral changes to the rapid evolution of high-density photoelectron mediated secondary collisional ionization processes upon the absorption of the incident XFEL radiation. These fast electronic processes, occurring at timescales well within the typical XFEL pulse durations (i.e., tens of fs), set the boundary conditions of the pulse intensity and sample parameters where the widely-accepted 'probe-before-destroy' measurement strategy can be adopted for electronic-structure related XFEL experiments.

9.
Inorg Chem ; 59(19): 13858-13874, 2020 Oct 05.
Artigo em Inglês | MEDLINE | ID: mdl-32936627

RESUMO

8-Hydroxyquinolines (8HQs) are a family of lipophilic metal ion chelators that have been used in a range of analytical and pharmaceutical applications over the last 100 years. More recently, CQ (clioquinol; 5-chloro-7-iodo-8-hydroxyquinoline) and PBT2 (5,7-dichloro-2-[(dimethylamino)methyl]-8-hydroxyquinoline) have undergone clinical trials for the treatment of Alzheimer's disease and Huntington's disease. Because CQ and PBT2 appear to redistribute metals into cells, these compounds have been redefined as copper and zinc ionophores. Despite the attention surrounding the clinical trials and the clear link between 8HQs and metals, the fundamental solution chemistry of how these compounds bind divalent metals such as copper and zinc, as well as their mechanism(s) of action in mammalian systems, remains poorly understood. In this study, we used a combination of X-ray absorption spectroscopy (XAS), high-energy resolution fluorescence detected (HERFD) XAS, electron paramagnetic resonance (EPR), and UV-visible absorption spectroscopies to investigate the aqueous solution chemistry of a range of 8HQ derivatives. To circumvent the known solubility issues with 8HQ compounds and their complexes with Cu(II), and to avoid the use of abiological organic solvents, we have devised a surfactant buffer system to investigate these Cu(II) complexes in aqueous solution. Our study comprises the first comprehensive investigation of the Cu(II) complexes formed with many 8HQs of interest in aqueous solution, and it provides the first structural information on some of these complexes. We find that halogen substitutions in 8HQ derivatives appear to have little effect on the Cu(II) coordination environment; 5,7-dihalogenated 8HQ conformers all have a pseudo square planar Cu(II) bound by two quinolin-8-olate anions, in agreement with previous studies. Conversely, substituents in the 2-position of the 8HQ moiety appear to cause significant distortions from the typical square-planar-like coordination of most Cu(II)-bis-8HQ complexes, such that the 8HQ moieties in the Cu(II)-bis-8HQ complex are rotated approximately 30-40° apart in a "propeller-like" arrangement.

10.
Phys Rev Lett ; 125(3): 037404, 2020 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-32745427

RESUMO

Kß x-ray emission spectroscopy is a powerful probe for electronic structure analysis of 3d transition metal systems and their ultrafast dynamics. Selectively enhancing specific spectral regions would increase this sensitivity and provide fundamentally new insights. Recently we reported the observation and analysis of Kα amplified spontaneous x-ray emission from Mn solutions using an x-ray free-electron laser to create the 1s core-hole population inversion [Kroll et al., Phys. Rev. Lett. 120, 133203 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.133203]. To apply this new approach to the chemically more sensitive but much weaker Kß x-ray emission lines requires a mechanism to outcompete the dominant amplification of the Kα emission. Here we report the observation of seeded amplified Kß x-ray emission from a NaMnO_{4} solution using two colors of x-ray free-electron laser pulses, one to create the 1s core-hole population inversion and the other to seed the amplified Kß emission. Comparing the observed seeded amplified Kß emission signal with that from conventional Kß emission into the same solid angle, we obtain a signal enhancement of more than 10^{5}. Our findings are the first important step of enhancing and controlling the emission of selected final states of the Kß spectrum with applications in chemical and materials science.

13.
Proc Natl Acad Sci U S A ; 117(22): 11981-11986, 2020 Jun 02.
Artigo em Inglês | MEDLINE | ID: mdl-32414927

RESUMO

Properties of liquid silicates under high-pressure and high-temperature conditions are critical for modeling the dynamics and solidification mechanisms of the magma ocean in the early Earth, as well as for constraining entrainment of melts in the mantle and in the present-day core-mantle boundary. Here we present in situ structural measurements by X-ray diffraction of selected amorphous silicates compressed statically in diamond anvil cells (up to 157 GPa at room temperature) or dynamically by laser-generated shock compression (up to 130 GPa and 6,000 K along the MgSiO3 glass Hugoniot). The X-ray diffraction patterns of silicate glasses and liquids reveal similar characteristics over a wide pressure and temperature range. Beyond the increase in Si coordination observed at 20 GPa, we find no evidence for major structural changes occurring in the silicate melts studied up to pressures and temperatures exceeding Earth's core mantle boundary conditions. This result is supported by molecular dynamics calculations. Our findings reinforce the widely used assumption that the silicate glasses studies are appropriate structural analogs for understanding the atomic arrangement of silicate liquids at these high pressures.

15.
Environ Sci Technol ; 54(10): 6021-6030, 2020 05 19.
Artigo em Inglês | MEDLINE | ID: mdl-32315524

RESUMO

Reaction conditions and mechanisms promoting or inhibiting U reduction exert a central control on U solubility and, therefore, U transport and its associated risks. Here, we vary and track common aqueous uranium species to show that a kinetic restriction inhibits homogeneous reduction of the calcium-uranyl-carbonato species (CaUO2(CO3)32- and Ca2UO2(CO3)3) by Fe(II)(aq), while ferrihydrite surface-catalyzed reduction of all aqueous uranyl by Fe(II) proceeds. Using U L3 high energy resolution fluorescence detection (HERFD) X-ray absorption near edge structure (XANES) spectroscopy, U L3 extended X-ray absorption fine structure (EXAFS) spectroscopy, and transmission electron microscopy (TEM), we also show that U(V) is generated and incorporated into ferrihydrite formed from homogeneous U(VI) reduction by Fe(II)(aq). Through elucidation of the mechanisms that inhibit reduction of the calcium-uranyl-carbonato species and promote stabilization of U(V), we advance our understanding of the controls on U solubility and thus improve prediction of U transport in surface and subsurface systems.


Assuntos
Cálcio , Urânio , Compostos Férricos , Compostos Ferrosos , Oxirredução
16.
Inorg Chem ; 59(5): 2711-2718, 2020 Mar 02.
Artigo em Inglês | MEDLINE | ID: mdl-32049511

RESUMO

Selenoenzymes, containing a selenocysteine (Sec) residue, fulfill important roles in biology. The mammalian thioredoxin reductase selenoenzymes are key regulators of antioxidant defense and redox signaling and are inhibited by methylmercury species and by the gold-containing drug auranofin. It has been proposed that such inhibition is mediated by metal binding to Sec in the enzyme. However, direct structural observations of these classes of inhibitors binding to selenoenzymes have been few to date. Here we therefore have used extended X-ray absorption fine structure as a direct structural probe to investigate binding to the selenium site in recombinant rat thioredoxin reductase 1 (TrxR1). The results demonstrate for the first time the direct and complete binding of the metal atom of the inhibitors to the selenium atom in TrxR1 for both methylmercury and auranofin, indicating that TrxR1 inhibition indeed can be attributed to such direct metal-selenium binding.


Assuntos
Auranofina/química , Auranofina/farmacologia , Compostos de Metilmercúrio/química , Compostos de Metilmercúrio/farmacologia , Selenocisteína/química , Tiorredoxinas/antagonistas & inibidores , Tiorredoxinas/química , Animais , Sítios de Ligação/efeitos dos fármacos , Ratos , Selenocisteína/metabolismo , Tiorredoxinas/metabolismo
17.
Nat Commun ; 11(1): 634, 2020 Jan 31.
Artigo em Inglês | MEDLINE | ID: mdl-32005815

RESUMO

The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kα and Kß X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278 fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110 fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kα XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces.

18.
ACS Appl Mater Interfaces ; 12(10): 11643-11656, 2020 Mar 11.
Artigo em Inglês | MEDLINE | ID: mdl-32057227

RESUMO

Understanding how structural and chemical transformations take place in particles under thermal conditions can inform designing thermally robust electrode materials. Such a study necessitates the use of diagnostic techniques that are capable of probing the transformations at multiple length scales and at different states of charge (SOC). In this study, the thermal behavior of LiNi0.6Mn0.2Co0.2O2 (NMC-622) was examined as a function of SOC, using an array of bulk and surface-sensitive techniques. In general, thermal stability decreases as lithium content is lowered and conversion in the bulk to progressively reduced metal oxides (spinels, rock salt) occurs as the temperature is raised. Hard X-ray absorption spectroscopy (XAS) and X-ray Raman spectroscopy (XRS) experiments, which probe the bulk, reveal that Ni and Co are eventually reduced when partially delithiated samples (regardless of the SOC) are heated, although Mn is not. Surface-sensitive synchrotron techniques, such as soft XAS and transmission X-ray microscopy (TXM), however, reveal that for 50% delithiated samples, apparent oxidation of nickel occurs at particle surfaces under some circumstances. This is partially compensated by reduction of cobalt but may also be a consequence of redistribution of lithium ions upon heating. TXM results indicate the movement of reduced nickel ions into particle interiors or oxidized nickel ions to the surface or both. These experiments illustrate the complexity of the thermal behavior of NMC cathode materials. The study also informs the importance of investigating the surface and bulk difference as a function of SOC when studying the thermal behaviors of battery materials.

19.
J Chem Phys ; 152(7): 074203, 2020 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-32087640

RESUMO

Valence-to-core x-ray emission spectroscopy (VtC XES) combines the sample flexibility and element specificity of hard x-rays with the chemical environment sensitivity of valence spectroscopy. We extend this technique to study geometric and electronic structural changes induced by photoexcitation in the femtosecond time domain via laser-pump, x-ray probe experiments using an x-ray free electron laser. The results of time-resolved VtC XES on a series of ferrous complexes [Fe(CN)2n(2, 2'-bipyridine)3-n]-2n+2, n = 1, 2, 3, are presented. Comparisons of spectra obtained from ground state density functional theory calculations reveal signatures of excited state bond length and oxidation state changes. An oxidation state change associated with a metal-to-ligand charge transfer state with a lifetime of less than 100 fs is observed, as well as bond length changes associated with metal-centered excited states with lifetimes of 13 ps and 250 ps.

20.
Environ Sci Technol ; 54(5): 2726-2733, 2020 03 03.
Artigo em Inglês | MEDLINE | ID: mdl-31951385

RESUMO

Industrial release of mercury into the local Minamata environment with consequent poisoning of local communities through contaminated fish and shellfish consumption is considered the classic case of environmental mercury poisoning. However, the mercury species in the factory effluent has proved controversial, originally suggested as inorganic, and more recently as methylmercury species. We used newly available methods to re-examine the cerebellum of historic Cat 717, which was fed factory effluent mixed with food to confirm the source. Synchrotron high-energy-resolution fluorescence detection-X-ray absorption spectroscopy revealed sulfur-bound organometallic mercury with a minor ß-HgS phase. Density functional theory indicated energetic preference for α-mercuri-acetaldehyde as a waste product of aldehyde production. The consequences of this alternative species in the "classic" mercury poisoning should be re-evaluated.


Assuntos
Intoxicação do Sistema Nervoso por Mercúrio , Intoxicação por Mercúrio , Mercúrio , Compostos de Metilmercúrio , Animais , Gatos , Japão , Frutos do Mar
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