X-ray free electron laser (XFEL) experiments using self-amplified spontaneous emission (SASE) pulses typically achieve temporal resolutions of order several femtoseconds, as the pulse duration puts a practical limit to pump-probe or probe-probe schemes. Even with the emerging capabilities to generate pulses with attosecond durations with new single-spike SASE schemes, direct access to attosecond electron dynamics remains an experimental challenge. Here we show how X-ray coherent attosecond pulse-pair spectroscopy (X-CAPPS) provides a powerful new approach to access the ultrashort time-delay window. Coherent attosecond pulse pairs with time delays varying from 500 as to 5 fs are generated with Cu Kα_1 stimulated X-ray emission from a gain medium pumped by intense SASE XFEL pulses. These pulse pairs are analyzed with two subsequent Bragg crystal spectrometers, and the resulting interference spectrum is captured on two sequential 2D image detectors encoding their time separation, relative amplitudes, and phases with high precision. X-CAPPS requires no split-and-delay X-ray optics, nor XFEL pulse modifications, making it broadly implementable across existing facilities. This technique enables the investigation of attosecond processes with Angström resolution, providing a new tool for probing ultrafast dynamics across a wide range of atomic, molecular, and solid systems.

The electronic structure of iridium carbonyl complexes is known to be fundamental to their ability to activate alkane C-H bonds following UV photolysis. Here, we investigate three prototypical iridium complexes with different ancillary ligands using valence-to-core resonant inelastic X-ray scattering measurements at the Ir L3-edge in combination with optical absorption spectroscopy and calculations based on time-dependent density functional theory. We characterize experimentally how the nature and degree of metal-ligand hybridization impact valence-excited state energetics as well as how changes in ionic vs. covalent metal-ligand interactions for different ancillary ligands modulate charge densities at the central metal atom. The selectivity of our methods to the valence-excited state manifold allows us to observe and quantify shifts in the d-d and charge-transfer manifold of excited-states, which are both thought to influence the yield of photochemical C-H bond activation. Our combined experimental and theoretical study of this series of iridium complexes reveals the interplay of ligand structure, metal-ligand bonding or covalency and valence-excited state landscape, which allows to deduce a general understanding of how these properties impact photochemical pathways and reactivity in C-H activation and other photocatalytic applications.

Lead exposure remains a major global health concern, contributing to an estimated 1.5% of annual deaths worldwide and causing widespread, irreversible neurodevelopmental and cardiovascular damage, particularly in children. A central challenge in understanding and mitigating lead toxicity is the determination of its chemical speciation in complex biological and environmental systems, where conventional Pb LIII-edge X-ray absorption spectroscopy (XAS) is fundamentally limited by severe core-hole lifetime broadening. Here we show that Pb Lα1 high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) provides a practical and experimentally accessible route to overcoming this limitation. Pb Lα1 HERFD-XAS yields an approximately 2-fold improvement in spectral resolution relative to conventional XAS, enabling enhanced sensitivity for quantitative speciation in heterogeneous and dilute systems. The method shows negligible chemical shifts (<0.06 eV) in the emission energy, simplifying implementation, distinguishes holodirected and hemidirected Pb(II) coordination environments, resolves biologically relevant Pb binding modes in zinc-finger model peptides, and identifies hemidirected Pb-substitution in hydroxyapatite, a key long-term reservoir of lead in humans. These results establish Pb Lα1 HERFD-XAS as a sensitive and experimentally accessible tool for determining Pb speciation in environmental and biological systems.

Formate dehydrogenases catalyze the reversible two-electron interconversion of formate and carbon dioxide and generally are assumed to operate through metal-centered redox chemistry at their molybdenum or tungsten active sites. The enzymes also are thought to possess a terminal sulfido ligand which is expected to play a central role in the catalytic mechanism. Here, we investigate the molybdenum-containing formate dehydrogenase FdsDABG from Cupriavidus necator using a combination of Mo K-edge X-ray absorption spectroscopy (XAS), high-energy-resolution fluorescence-detected (HERFD) XAS, extended X-ray absorption fine structure analysis (EXAFS), and density functional theory (DFT) calculations. We observe only subtle spectroscopic changes upon reduction of the oxidized enzyme by formate, suggesting that redox chemistry does not involve the metal; moreover, EXAFS analysis shows no evidence for a terminal sulfido ligand in the oxidized enzyme. DFT calculations support these findings and suggest that the oxidized enzyme possesses a cysteine persulfido structure, which is reduced by cleavage of the S-S bond to form an Mo-SH species, leaving the formal oxidation state of molybdenum unchanged. Collectively, these results suggest that the catalytic reaction of formate dehydrogenases involves ligand-based redox chemistry at a metal center formally reduced to the Mo(IV) state in the oxidized enzyme.

Determining the electronic structure of transition metal complexes with non-innocent ligands is challenging, both experimentally and theoretically. In this study, we investigate the electronic structure of iron corrole nitrosyl (Fe[TPC](NO), TPC = meso-triphenylcorrole) using a combination of Fe L-edge and K pre-edge X-ray absorption spectroscopy (XAS), Kβ X-ray emission spectroscopy (XES), and multiconfigurational calculations. A key debate revolves around the distribution of radical character on the ligand and the spin/oxidation state of the iron center. The experimental spectra reveal that the Fe center adopts a low-spin configuration, characterized as either FeII or FeIII with strong π back-bonding and little spin polarization. Calculations identified Fe[TPC](NO) primarily as {FeNO}6 with a corrole3-, where the {FeNO}6 unit exhibits FeIII character. However, no localized hole was found in the iron t2g orbital due to strong covalent mixing with NO π* orbitals. The occupation of the Fe 3d z 2 orbital, and thus the radical character on the corrole ligand, is highly sensitive to the axial ligand environment. This was supported by wavefunction analysis over varying Fe-NO distances and comparison with iron corrole chloride (Fe[TPC]Cl), which displayed significant radical character on the corrole ligand due to the weak axial ligand. These findings provide critical insights into ligand non-innocence and covalency in metal corrole systems, offering a foundation for understanding other highly covalent transition metal systems.

We report the successful resolution of the multiplet structure of Kα 1 x-ray emission in manganese (Mn) complexes through seeded stimulated x-ray emission spectroscopy (seeded S-XES). Using a femtosecond pump pulse above the Mn K edge to generate simultaneous 1s core holes, and a second-color tunable seed pulse to initiate the stimulated emission process, we were able to enhance individual lines within the Kα 1 emission. This approach allows to resolve the fine multiplet features that are obscured by lifetime broadening in conventional (spontaneous) Mn Kα XES. The work builds on our previous observation that S-XES from Mn(II) and Mn(VII) complexes pumped at high intensities can exhibit stimulated emission without sacrificing the chemical sensitivity to oxidation states. This technique opens the door to controlled high-resolution electronic structure spectroscopy in transition-metal complexes beyond the core-hole lifetime, with potential applications in catalysis, inorganic chemistry, and materials science.

Divalent lanthanide ions can adopt complex 4fn5d1 electron configurations that formally violate the Aufbau principle. The blending of contracted 4f and diffuse 5d character strongly influences lanthanide frontier molecular orbitals, yet quantitative characterization of 5d electron participation in bonding remains challenging. To address this, we report L3-edge high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) and valence to core resonant inelastic X-ray scattering (V2C-RIXS) for a series of divalent lanthanide complexes [M(NHAriPr6)2] (M = Ce, Tb, Dy and Yb; NHAriPr6 2=N(H)C6H3-2,6-(C6H2-2,4,6-iPr3)2) and their trivalent precursors [M(NHAriPr6)2(I)], which all feature close metal–arene contacts. It is demonstrated that V2C-RIXS can be applied to quantify relative 5d covalency contributions to lanthanide–arene bonding. Analysis of divalent cerium, terbium, and dysprosium complexes reveals non-Aufbau (5d1) configurations, which show significant 5d–arene δ-bond covalency and stabilization of the associated bonding orbitals. V2C- RIXS spectra are accurately simulated and compared with UV-vis-NIR results using DFT/ROCIS calculations. A detailed assignment of spectral features and correlation to predicted electron configurations define a trend of increased 5d covalency for these complexes relative to their trivalent precursors. Conversely, the divalent ytterbium complex has an Aufbau configuration, 4f14, and shows markedly reduced 5d covalency relative to its trivalent precursor.

Cathodic corrosion is an electrochemical phenomenon that etches metals at moderately negative potentials. Although cathodic corrosion probably occurs by forming a metal-containing anion, such intermediate species have not yet been observed. Here, aiming to resolve this long-standing debate, our work provides such evidence through X-ray absorption spectroscopy. High-energy-resolution X-ray absorption near-edge structure experiments are used to characterize platinum nanoparticles during cathodic corrosion in 10 mol l−1 NaOH. These experiments detect minute chemical changes in the Pt during corrosion that match first-principles simulations of X-ray absorption spectra of surface platinum multilayer hydrides. Thus, this work supports the existence of hydride-like platinum during cathodic corrosion. Notably, these results provide a direct observation of these species under conditions where they are highly unstable and where prominent hydrogen bubble formation interferes with most spectroscopy methods. Therefore, this work identifies the elusive intermediate that underlies cathodic corrosion. Cathodic corrosion of platinum is investigated using operando high-resolution X-ray absorption spectroscopy matched with density functional calculations. Platinum hydrides are found to be reaction intermediates in conditions at which these species are expected to be unstable.

Since the invention of the laser, nonlinear effects such as filamentation1, Rabi cycling2,3 and collective emission4 have been explored in the optical regime, leading to a wide range of scientific and industrial applications5-8. X-ray free-electron lasers (XFELs) have extended many optical techniques to X-rays for their advantages of ångström-scale spatial resolution and elemental specificity9. An example is XFEL-driven inner-shell Kα1 (2p3/2 → 1s1/2) X-ray lasing in elements ranging from neon to copper, which has been used for nonlinear spectroscopy and development of new X-ray laser sources10-16. Here we show that strong lasing effects similar to those in the optical regime can occur at 1.5-2.1 Å wavelengths during high-intensity (>1019 W cm-2) XFEL-driven Kα1 lasing of copper and manganese. Depending on the temporal XFEL pump pulse substructure, the resulting X-ray pulses (about 106-108 photons) can exhibit strong spatial inhomogeneities and spectral splitting, inhomogeneities and broadening. Three-dimensional Maxwell-Bloch calculations17 show that the observed spatial inhomogeneities result from X-ray filamentation and that the broad spectral features are driven by sub-femtosecond Rabi cycling. Our simulations indicate that these X-ray pulses can have pulse lengths of less than 100 attoseconds and coherence properties that provide opportunities for quantum X-ray optics applications.

High energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) can dramatically reduce core-hole lifetime broadening that smears out spectroscopic detail in conventional X-ray absorption spectroscopy (XAS). This increased spectroscopic resolution can directly translate to increased speciation capabilities in analysis of complex mixtures. However, spectral distortions due to fluorescence self-absorption are inherent in the HERFD-XAS method and can negate improvements. We discuss a method for correcting self-absorption.

We report the successful resolution of the multiplet structure of the Kα1 x-ray emission in manganese (Mn) complexes through seeded stimulated X-ray emission spectroscopy (seeded S-XES). By employing a femtosecond pump pulse above the Mn K edge to generate simultaneous 1s core-holes, and a second-color tunable seed pulse to initiate the stimulated emission process, we were able to enhance individual lines within the Kα1 emission. This approach allows to resolve the fine multiplet features that are obscured by the life-time broadening in conventional Mn Kα XES. The work builds on our previous observation that S-XES from Mn(II) and Mn(VII) complexes pumped at high intensities can exhibit stimulated emission without sacrificing the chemical sensitivity to oxidation states. This technique opens the door to controlled high-resolution electronic structure spectroscopy in transition metal complexes beyond core hole life time broadening with potential applications in catalysis, inorganic chemistry, and materials science.

The causative agent of Covid-19 is the SARS-CoV-2 virus. Initiation of cell entry by SARS-CoV-2 is critically dependent upon binding of the SARS-CoV-2 spike protein to angiotensin-converting enzyme 2 (ACE2, EC 3.4.17.23). The mechanism of binding of the SARS-CoV-2 spike receptor binding domain to ACE2 is among the most intensively studied infection mechanisms of any pathogen, including a very large number of structural studies. ACE2 is a membrane-associated zinc carboxypeptidase, comprising three domains, the protease domain, a neck domain, and a membrane-spanning alpha-helical domain. In addition to its role as a carboxypeptidase, ACE2 is also a chaperone for a Na+-amino acid cotransporter called B0AT1, and in the presence of B0AT1, full-length ACE2 forms dimers. Most studies to date related to Covid-19 have employed just the ACE2 protease domain and have neglected any possible roles of the Zn2+-containing ACE2 active site. We show here that ACE2, including the neck domain in addition to the protease domain (and in the absence of B0AT1), is dimeric and shows distinctive allostery in its catalytic activity. In contrast, the intensively studied protease domain is monomeric and shows no allostery. Binding of the spike receptor binding domain (RBD) to dimeric ACE2 eliminates its allostery. X-ray absorption spectroscopy of Zn2+ ACE2 shows distinctive changes in the active site structure upon binding of spike RBD but only in the dimeric form. Taken together, our results indicate that the Zn2+-containing active site exhibits a notable level of flexibility and that the dimeric form of ACE2, including both protease and neck domains, likely presents a superior model for the study of ACE2-spike interactions than the monomeric ACE2.

X-ray absorption spectroscopy (XAS) of 3d transition metals provides important electronic structure information for many fields. However, X-ray-induced radiation damage under physiological temperature has prevented using this method to study dilute aqueous systems, such as metalloenzymes, as the catalytic reaction proceeds. Here we present a new approach to enable operando XAS of dilute biological samples and demonstrate its feasibility with K-edge XAS spectra from the Mn cluster in photosystem II and the Fe-S centers in photosystem I. This approach combines highly efficient sample delivery strategies and a robust signal normalization method with high-transmission Bragg diffraction-based spectrometers at X-ray free-electron lasers (XFELs) in a damage-free, shot-by-shot mode. These photon-out spectrometers have been optimized for discriminating the metal Mn/Fe Kα fluorescence signals from the overwhelming scattering background present on currently available detectors for XFELs that lack suitable energy discrimination. We quantify the enhanced performance metrics of the spectrometer and discuss its potential applications for acquiring time-resolved XAS spectra of biological samples during their reactions at XFELs.

Human manganese superoxide dismutase (MnSOD) plays a crucial role in controlling levels of reactive oxygen species (ROS) by converting superoxide ( $${{{{\rm{O}}}}}_{2}^{\bullet -}$$ ) to molecular oxygen (O2) and hydrogen peroxide (H2O2) with proton-coupled electron transfers (PCETs). A key catalytic residue, Tyr34, determines the activity of human MnSOD and also becomes post-translationally inactivated by nitration in various diseases associated with mitochondrial dysfunction. Tyr34 has an unusual pKa due to its proximity to the Mn metal and undergoes cyclic deprotonation and protonation events to promote the electron transfers of MnSOD. Neutron diffraction, X-ray spectroscopy, and quantum chemistry calculations in oxidized, reduced and product inhibited enzymatic states shed light on the role of Tyr34 in MnSOD catalysis. The data identify the contributions of Tyr34 in MnSOD activity that support mitochondrial function and give a thorough characterization of how a single tyrosine modulates PCET catalysis. Product inhibition occurs by an associative displacement mechanism. The activity of human manganese superoxide dismutase (MnSOD) is determined by the state of a key catalytic residue, Tyr34, which was reported to undergo cyclic deprotonation and protonation events to promote the electron transfers of MnSOD. Here, the authors performed neutron diffraction, X-ray spectroscopy, and quantum chemistry calculations of Tyr34Phe MnSOD in oxidized, reduced and product inhibited enzymatic states, to elucidate the role of Tyr34 in MnSOD catalysis.

Marine propellers are studied in design and off-design modes of operation like crashback, where the propeller rotates in reverse while the vehicle is in forward motion. Past experiments (Jessup et al., Proceedings of the 25th Symposium on Naval Hydrodynamics, St John's, Canada, 2004; Proceedings of the 26th Symposium on Naval Hydrodynamics, Rome, Italy, 2006) studied the marine propeller David Taylor Model Basin 4381 in the open-jet test section of the 36-inch variable pressure water tunnel (VPWT). In crashback, a significant discrepancy with unclear sources exists between the mean propeller loads from the VPWT and open-water towing tank (OW) experiments (Ebert et al., 2007 ONR Propulsor S & T Program Review, October, 2007). We perform large-eddy simulation at $Re=561\,000$ and advance ratios $J=-0.50$ and $-0.82$ with the VPWT geometry included, contrasting to the unconfined (OW) case at those same $J$ and $Re=480\,000$. We identify and delineate the water tunnel interference effects responsible, and demonstrate that these effects resemble those of a symmetric solid model or bluff body. Solid blockage due to jet expansion and nozzle blockage due to proximity to the tunnel nozzle are identified as the primary interference effects. Their impact varies with the advance ratio $J$ and strengthens for higher magnitudes of $J$. The effective length scale to assess the severity of interference effects is found to be larger than the vortex ring diameter.

The high abundance and molecular versatility of iron have led to its universal presence in biological systems, yet its absorption is exceptionally challenging. Animals and yeasts use divalent metal transporters to import iron, but yeasts also employ the multicopper oxidase Fet3p for high-affinity iron uptake when iron-starved. Using long-term iron depletion in Drosophila, we identified four components involved in iron absorption: Multicopper oxidase-4 (Mco4), a Fet3p ortholog, is essential for surviving iron starvation, whereas the cytochrome b561 enzymes Fire (Ferric Iron Reductase) and Fire-like, as well as cytochrome b5 protein Firewood, are required for iron absorption under normal conditions. This study reports the presence of a high-affinity iron uptake system in an animal, a cytochrome b5 electron donor for ferric iron reduction, and intestinal ferric reductases, and provides a valuable resource for further exploration of genes involved in iron homeostasis, transport, and absorption.

Molecular metal complexes offer opportunities for developing artificial photocatalytic systems. The search for efficient molecular photocatalytic systems, which involves a vast number of photosensitizer–catalyst combinations, is extremely time consuming via a conventional trial and error approach, while high-throughput virtual screening has not been feasible owing to a lack of reliable descriptors. Here we present a machine learning-accelerated high-throughput screening protocol for molecular photocatalytic CO2 reduction systems using multiple descriptors incorporating the photosensitization, electron transfer and catalysis steps. The protocol rapidly screened 3,444 molecular photocatalytic systems including 180,000 conformations of photosensitizers and catalysts during their interaction, enabling the prediction of six promising candidates. Then, we experimentally validated the screened photocatalytic systems, and the optimal one achieved a turnover number of 4,390. Time-resolved spectroscopy and first-principles calculation further validated not only the relevance of the descriptors within certain screening scopes but also the role of dipole coupling in triggering dynamic catalytic reaction processes. High-throughput computational screening of multicomponent molecular photocatalytic systems offers a strategy to minimize the screening of large numbers of photosensitizer–catalyst combinations. Here a machine learning-accelerated approach using multiple descriptors shows strong predictive power in experimentally validated systems for CO2 reduction.

High-valent FeIV=O species are common intermediates in biological and artificial catalysts. Heme and nonheme S=1 FeIV=O sites have been synthesized and studied for decades but little quantitative experimental comparison of their electronic structures has been available, due to the lack of direct methods focused on the iron. This study allows a rigorous determination of the electronic structure of a nonheme FeIV=O center and its comparison to an FeIV=O heme site using 1s2p resonant inelastic X-ray scattering (RIXS) and Fe L-edge X-ray absorption spectroscopy (XAS). Further, variable temperature magnetic circular dichroism (VT-MCD) of the ligand field transitions, combined with nuclear resonance vibrational spectroscopy of the two S=1 FeIV=O systems show that the equatorial ligand field decreases from a nonheme to a heme FeIV=O site. Alternatively, RIXS and Fe L-edge XAS combined with MCD show that the Fe dπ orbitals are unperturbed in the FeIV=O heme relative to the nonheme site because the strong axial Fe-O bond uncouples the Fe dπ orbitals from the porphyrin π-system. As a consequence, the thermodynamics and kinetics of the H-atom abstraction reactions are actually very similar for heme compound II and nonheme FeIV=O active sites.

Human mitochondrial manganese superoxide dismutase (MnSOD) converts superoxide into hydrogen peroxide and molecular oxygen, serving as a key defence against oxidative damage. Despite extensive studies, the full structural characterization of H2O2-binding sites in MnSOD remains largely unexplored. Previous H2O2-soaked MnSOD structures have identified two distinct H2O2-binding sites: one directly ligated to the catalytic manganese (LIG position) and another at the active-site gateway (PEO position) between the second-shell residues Tyr34 and His30. In this study, a kinetically impaired Gln143Asn MnSOD variant is used to trap and explore additional H2O2-binding sites beyond the second-shell solvent gate. In the wild-type enzyme, Gln143 mediates proton transfers with the manganese-bound solvent (WAT1) to drive redox cycling of the metal, which is necessary for effective superoxide dismutation. Substitution with Asn stalls catalysis because the increased distance from WAT1 disrupts critical proton-coupled electron-transfer (PCET) events, and the redox cycling of the active-site metal is impaired. This, in turn, stalls the electrostatic cycling of positive charge on the enzyme surface and enhances the likelihood of trapping transient H2O2-bound states in this variant. The results reveal several H2O2 molecules leading up to the active site, in addition to the canonical LIG and PEO positions.