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"Mara, Michael"
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Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy
The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe–S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe–S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.
Journal Article
Selective nitrogen adsorption via backbonding in a metal–organic framework with exposed vanadium sites
Industrial processes prominently feature π-acidic gases, and an adsorbent capable of selectively interacting with these molecules could enable important chemical separations
1
–
4
. Biological systems use accessible, reducing metal centres to bind and activate weakly π-acidic species, such as N
2
, through backbonding interactions
5
–
7
, and incorporating analogous moieties into a porous material should give rise to a similar adsorption mechanism for these gaseous substrates
8
. Here, we report a metal–organic framework featuring exposed vanadium(
ii
) centres capable of back-donating electron density to weak π acids to successfully target π acidity for separation applications. This adsorption mechanism, together with a high concentration of available adsorption sites, results in record N
2
capacities and selectivities for the removal of N
2
from mixtures with CH
4
, while further enabling olefin/paraffin separations at elevated temperatures. Ultimately, incorporating such π-basic metal centres into porous materials offers a handle for capturing and activating key molecular species within next-generation adsorbents.
Nitrogenases use transition metals to selectively capture weak π acids such as N
2
by employing backbonding interactions. Here, a metal–organic framework with exposed vanadium sites is presented that uses this approach for selective capture of N
2
from CH
4
, with impressive selectivity and capacity.
Journal Article
Short-lived metal-centered excited state initiates iron-methionine photodissociation in ferrous cytochrome c
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 1
s
3
p
(Kβ) 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.
The dissociation mechanism of the heme axial ligand in heme proteins is not yet fully understood. The authors investigate the photodissociation dynamics of the bond between heme Fe and methionine S in ferrous cytochrome c using femtosecond time-resolved X-ray solution scattering and X-ray emission spectroscopy, simultaneously tracking electronic and nuclear structure changes.
Journal Article
Chemical and elemental mapping of spent nuclear fuel sections by soft X‐ray spectromicroscopy
Soft X‐ray spectromicroscopy at the O K‐edge, U N4,5‐edges and Ce M4,5‐edges has been performed on focused ion beam sections of spent nuclear fuel for the first time, yielding chemical information on the sub‐micrometer scale. To analyze these data, a modification to non‐negative matrix factorization (NMF) was developed, in which the data are no longer required to be non‐negative, but the non‐negativity of the spectral components and fit coefficients is largely preserved. The modified NMF method was utilized at the O K‐edge to distinguish between two components, one present in the bulk of the sample similar to UO2 and one present at the interface of the sample which is a hyperstoichiometric UO2+x species. The species maps are consistent with a model of a thin layer of UO2+x over the entire sample, which is likely explained by oxidation after focused ion beam (FIB) sectioning. In addition to the uranium oxide bulk of the sample, Ce measurements were also performed to investigate the oxidation state of that fission product, which is the subject of considerable interest. Analysis of the Ce spectra shows that Ce is in a predominantly trivalent state, with a possible contribution from tetravalent Ce. Atom probe analysis was performed to provide confirmation of the presence and localization of Ce in the spent fuel. Soft X‐ray spectromicroscopy at the O K‐edge, U N4,5‐edges and Ce M4,5‐edges has been performed on focused ion beam sections of spent nuclear fuel for the first time. Analysis of oxygen spectra using a modified non‐negative matrix factorization method is consistent with a thin layer of hyperstoichiometric uranium oxide having been formed at the interface of a sample consisting of primarily UO2, probably formed after sample preparation. The cerium oxidation state is shown to be predominantly trivalent, with a possible small contribution from tetravalent Ce.
Journal Article
Interplays of electron and nuclear motions along CO dissociation trajectory in myoglobin revealed by ultrafast X-rays and quantum dynamics calculations
Ultrafast structural dynamics with different spatial and temporal scales were investigated during photodissociation of carbon monoxide (CO) from iron(II)–heme in bovine myoglobin during the first 3 ps following laser excitation. We used simultaneous X-ray transient absorption (XTA) spectroscopy and X-ray transient solution scattering (XSS) at an X-ray free electron laser source with a time resolution of 80 fs. Kinetic traces at different characteristic X-ray energies were collected to give a global picture of the multistep pathway in the photodissociation of CO from heme. In order to extract the reaction coordinates along different directions of the CO departure, XTA data were collected with parallel and perpendicular relative polarizations of the laser pump and X-ray probe pulse to isolate the contributions of electronic spin state transition, bond breaking, and heme macrocycle nuclear relaxation. The time evolution of the iron K-edge X-ray absorption near edge structure (XANES) features along the two major photochemical reaction coordinates, i.e., the iron(II)–CO bond elongation and the heme macrocycle doming relaxation were modeled by time-dependent density functional theory calculations. Combined results from the experiments and computations reveal insight into interplays between the nuclear and electronic structural dynamics along the CO photodissociation trajectory. Time-resolved small-angle X-ray scattering data during the same process are also simultaneously collected, which show that the local CO dissociation causes a protein quake propagating on different spatial and temporal scales. These studies are important for understanding gas transport and protein deligation processes and shed light on the interplay of active site conformational changes and large-scale protein reorganization.
Journal Article
Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy
The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe–S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe–S(Met) bond enthalpy is 4 kcal/mol stronger than in the absence of protein constraints. As a result, the 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.
Journal Article
Joint Staff Budget Formulation Process
Budget formulation begins during the programming phase of the Department of Defense Planning, Programming, Budgeting, and Execution (PPBE) process and ends during the budgeting phase with the allocation of resources in accordance with mission priorities. Here, the Joint Staff's budget formulation process, which may be suitable for defense agencies is described. Particular focus is given on the first year of the six-year Future Years Defense Program. The end products of this process, the Joint Staff Program Objective Memorandum (POM) and Budget Estimate Submission (BES), serve as a statement of the Chairman's priorities to both internal and external stakeholders. As a benchmark, the Joint Staff budget includes four appropriations, spanning 23 program elements with an annual topline of approximately $1.3 billion. Of that topline, $630M goes directly to a Combatant Commanders' exercise program managed jointly by the Joint Staff and Under Secretary of Defense, Personnel & Readiness, and another $100M goes to the National Defense University, leaving a $520M for the Joint Staff portfolio. In the Joint Staff, the program and budget branch within the Comptroller's office performs the budget formulation function.
Journal Article
Reducing time from presentation to diagnosis of scaphoid fractures with cone beam CT: a before-and-after study
BackgroundScaphoid fractures comprise approximately 50–70% of carpal bone fractures but can be difficult to detect on initial plain film radiographs. A delayed diagnosis can lead to a high rate of non-union, avascular necrosis and Complex Regional Pain Syndrome. Current literature supports cone beam CT (CBCT) (within 10–14 days) as an effective method for diagnosing scaphoid fractures. We implemented an early outpatient CBCT pathway, prior to specialist review, with the aim to increase the proportion of patients with suspected scaphoid fracture undergoing CBCT within 7 days.MethodsWe designed an ambulatory pathway for suspected scaphoid fractures in the Emergency Department (ED) in which outpatient CBCT was requested by emergency medicine clinicians. A retrospective audit of current management of these patients was performed between 1 August 2022 and 31 October 2022 (prepathway period). A list of patients who underwent CBCT performed for the indication ‘suspected scaphoid or carpal bone fracture’ in the hospital was obtained and screened. Implementation of the pathway took place in February 2023 and was reviewed by continuous audit monitoring from 1 March 2023 to 31 May 2023 (postpathway period).ResultsPrepathway implementation, 54 patients underwent CBCT. Following implementation of our pathway, the number of CBCTs performed in the hospital for this clinical indication increased to 111 (postpathway). The proportion of patients undergoing CBCT within 7 days increased from 11.1% (6/54) to 91.8% (102/111) (p<0.000). There was a 71.9% reduction in fracture clinic attendances (50/54 (92.6%) prepathway and 23/111 (22.5%) post pathway (p<0.000).ConclusionWe successfully implemented an ambulatory pathway for suspected scaphoid fractures in the ED that significantly increased the proportion of patients with suspected scaphoid fractures undergoing early (<7 days) CBCT and definitive care.
Journal Article