| 1. |
- Kositzki, Ramona, et al.
(författare)
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Electronic and molecular structure relations in diiron compounds mimicking the [FeFe]-hydrogenase active site studied by X-ray spectroscopy and quantum chemistry
- 2017
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Ingår i: Dalton Transactions. - : ROYAL SOC CHEMISTRY. - 1477-9226 .- 1477-9234. ; 46:37, s. 12544-12557
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Tidskriftsartikel (refereegranskat)abstract
- Synthetic diiron compounds of the general formula Fe-2(mu-S2R)(CO)(n)(L)(6-n) (R = alkyl or aromatic groups; L = CN- or phosphines) are versatile models for the active-site cofactor of hydrogen turnover in [FeFe]-hydrogenases. A series of 18 diiron compounds, containing mostly a dithiolate bridge and terminal ligands of increasing complexity, was characterized by X-ray absorption and emission spectroscopy in combination with density functional theory. Fe K-edge absorption and K beta main-line emission spectra revealed the varying geometry and the low-spin state of the Fe(I) centers. Good agreement between experimental and calculated core-to-valence-excitation absorption and radiative valence-to-core-decay emission spectra revealed correlations between spectroscopic and structural features and provided access to the electronic configuration. Four main effects on the diiron core were identified, which were preferentially related to variation either of the dithiolate or of the terminal ligands. Alteration of the dithiolate bridge affected mainly the Fe-Fe bond strength, while more potent donor substitution and ligand field asymmetrization changed the metal charge and valence level localization. In contrast, cyanide ligation altered all relevant properties and, in particular, the frontier molecular orbital energies of the diiron core. Mutual benchmarking of experimental and theoretical parameters provides guidelines to verify the electronic properties of related diiron compounds.
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| 2. |
- Boniolo, Manuel, et al.
(författare)
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Water Oxidation by Pentapyridyl Base Metal Complexes? : A Case Study
- 2022
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:24, s. 9104-9118
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Tidskriftsartikel (refereegranskat)abstract
- The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O2 release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]+ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)3]2+/S2O82– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the MIII/MII oxidation, but none was able to yield a detectable amount of a MIV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl– or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond MIII. On that basis, design features for next-generation molecular WOCs are suggested.
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| 3. |
- Chernev, Petko, et al.
(författare)
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Light-driven formation of manganese oxide by today's photosystem II supports evolutionarily ancient manganese-oxidizing photosynthesis
- 2020
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Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- Water oxidation and concomitant dioxygen formation by the manganese-calcium cluster of oxygenic photosynthesis has shaped the biosphere, atmosphere, and geosphere. It has been hypothesized that at an early stage of evolution, before photosynthetic water oxidation became prominent, light-driven formation of manganese oxides from dissolved Mn(2+) ions may have played a key role in bioenergetics and possibly facilitated early geological manganese deposits. Here we report the biochemical evidence for the ability of photosystems to form extended manganese oxide particles. The photochemical redox processes in spinach photosystem-II particles devoid of the manganese-calcium cluster are tracked by visible-light and X-ray spectroscopy. Oxidation of dissolved manganese ions results in high-valent Mn(III, IV)-oxide nanoparticles of the birnessite type bound to photosystem II, with 50-100 manganese ions per photosystem. Having shown that even today's photosystem II can form birnessite-type oxide particles efficiently, we propose an evolutionary scenario, which involves manganese-oxide production by ancestral photosystems, later followed by downsizing of protein-bound manganese-oxide nanoparticles to finally yield today's catalyst of photosynthetic water oxidation.
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| 4. |
- Griese, Julia J., et al.
(författare)
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Assembly of a heterodinuclear Mn/Fe cofactor is coupled to tyrosine-valine ether cross-link formation in the R2-like ligand-binding oxidase
- 2019
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Ingår i: Journal of Biological Inorganic Chemistry. - : Springer Science and Business Media LLC. - 0949-8257 .- 1432-1327. ; 24:2, s. 211-221
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Tidskriftsartikel (refereegranskat)abstract
- R2-like ligand-binding oxidases (R2lox) assemble a heterodinuclear Mn/Fe cofactor which performs reductive dioxygen (O-2) activation, catalyzes formation of a tyrosine-valine ether cross-link in the protein scaffold, and binds a fatty acid in a putative substrate channel. We have previously shown that the N-terminal metal binding site 1 is unspecific for manganese or iron in the absence of O-2, but prefers manganese in the presence of O-2, whereas the C-terminal site 2 is specific for iron. Here, we analyze the effects of amino acid exchanges in the cofactor environment on cofactor assembly and metalation specificity using X-ray crystallography, X-ray absorption spectroscopy, and metal quantification. We find that exchange of either the cross-linking tyrosine or the valine, regardless of whether the mutation still allows cross-link formation or not, results in unspecific manganese or iron binding at site 1 both in the absence or presence of O-2, while site 2 still prefers iron as in the wild-type. In contrast, a mutation that blocks binding of the fatty acid does not affect the metal specificity of either site under anoxic or aerobic conditions, and cross-link formation is still observed. All variants assemble a dinuclear trivalent metal cofactor in the aerobic resting state, independently of cross-link formation. These findings imply that the cross-link residues are required to achieve the preference for manganese in site 1 in the presence of O-2. The metalation specificity, therefore, appears to be established during the redox reactions leading to cross-link formation.
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| 5. |
- Griese, Julia J., et al.
(författare)
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Structural Basis for Oxygen Activation at a Heterodinuclear Manganese/Iron Cofactor
- 2015
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Ingår i: Journal of Biological Chemistry. - : American Society for Biochemistry and Molecular Biology. - 0021-9258 .- 1083-351X. ; 290:42, s. 25254-25272
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Tidskriftsartikel (refereegranskat)abstract
- Two recently discovered groups of prokaryotic di-metal carboxylate proteins harbor a heterodinuclear Mn/Fe cofactor. These are the class Ic ribonucleotide reductase R2 proteins and a group of oxidases that are found predominantly in pathogens and extremophiles, called R2-like ligand-binding oxidases (R2lox). We have recently shown that the Mn/Fe cofactor of R2lox self-assembles from Mn-II and Fe-II in vitro and catalyzes formation of a tyrosine-valine ether cross-link in the protein scaffold (Griese, J. J., Roos, K., Cox, N., Shafaat, H. S., Branca, R.M., Lehtio , J., Graslund, A., Lubitz, W., Siegbahn, P. E., and Hogbom, M. (2013) Proc. Natl. Acad. Sci. U.S.A. 110, 1718917194). Here, we present a detailed structural analysis of R2lox in the nonactivated, reduced, and oxidized resting Mn/Fe- and Fe/Fe-bound states, as well as the nonactivated Mn/Mn-bound state. X-ray crystallography and x-ray absorption spectroscopy demonstrate that the active site ligand configuration of R2lox is essentially the same regardless of cofactor composition. Both the Mn/Fe and the diiron cofactor activate oxygen and catalyze formation of the ether cross-link, whereas the dimanganese cluster does not. The structures delineate likely routes for gated oxygen and substrate access to the active site that are controlled by the redox state of the cofactor. These results suggest that oxygen activation proceeds via similar mechanisms at the Mn/Fe and Fe/Fe center and that R2lox proteins might utilize either cofactor in vivo based on metal availability.
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| 6. |
- Kositzki, Ramona, et al.
(författare)
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y Protonation State of MnFe and FeFe Cofactors in a Ligand-Binding Oxidase Revealed by X-ray Absorption, Emission, and Vibrational Spectroscopy and QM/MM Calculations
- 2016
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 55:19, s. 9869-9885
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Tidskriftsartikel (refereegranskat)abstract
- Enzymes with a dimetalcarboxylate cofactor catalyze reactions among the top challenges in chemistry such as methane and dioxygen (O-2) activation. Recently described proteins bind a manganeseiron cofactor (MnFe) instead of the classical diiron cofactor (FeFe). Determination of atomic-level differences of homo- versus hetero-bimetallic cofactors is crucial to understand their diverse redox reactions. We studied a ligand-binding oxidase from the bacterium Geobacillus kaustophilus (R2lox) loaded with a FeFe or MnFe cofactor, which catalyzes O-2 reduction and an unusual tyrosinevaline ether cross-link formation, as revealed by X-ray crystallography. Advanced X-ray absorption, emission, and vibrational spectroscopy methods and quantum chemical and molecular mechanics calculations provided relative Mn/Fe contents, X-ray photoreduction kinetics, metalligand bond lengths, metalmetal distances, metal oxidation states, spin configurations, valence-level degeneracy, molecular orbital composition, nuclear quadrupole splitting energies, and vibrational normal modes for both cofactors. A protonation state with an axial water (H2O) ligand at Mn or Fe in binding site 1 and a metal-bridging hydroxo group (OH) in a hydrogen-bonded network is assigned. Our comprehensive picture of the molecular, electronic, and dynamic properties of the cofactors highlights reorientation of the unique axis along the MnOH2 bond for the Mn1(III) JahnTeller ion but along the Fe-mu OH bond for the octahedral Fe1(III). This likely corresponds to a more positive redox potential of the Mn(III)Fe(III) cofactor and higher proton affinity of its mu OH group. Refined model structures for the Mn(III)Fe(III) and Fe(III)Fe(III) cofactors are presented. Implications of our findings for the site-specific metalation of R2lox and performance of the O-2 reduction and cross-link formation reactions are discussed.
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| 7. |
- Kutin, Yury, et al.
(författare)
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Chemical flexibility of heterobimetallic Mn/Fe cofactors : R2lox and R2c proteins
- 2019
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Ingår i: Journal of Biological Chemistry. - 0021-9258 .- 1083-351X. ; 294:48, s. 18372-18386
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Tidskriftsartikel (refereegranskat)abstract
- A heterobimetallic Mn/Fe cofactor is present in the R2 subunit of class Ic ribonucleotide reductases (R2c) and in R2-like ligand-binding oxidases (R2lox). Although the protein-derived metal ligands are the same in both groups of proteins, the connectivity of the two metal ions and the chemistry each cofactor performs are different: in R2c, a one-electron oxidant, the Mn/Fe dimer is linked by two oxygen bridges (?-oxo/?-hydroxo), whereas in R2lox, a two-electron oxidant, it is linked by a single oxygen bridge (?-hydroxo) and a fatty acid ligand. Here, we identified a second coordination sphere residue that directs the divergent reactivity of the protein scaffold. We found that the residue that directly precedes the N-terminal carboxylate metal ligand is conserved as a glycine within the R2lox group but not in R2c. Substitution of the glycine with leucine converted the resting-state R2lox cofactor to an R2c-like cofactor, a ?-oxo/?-hydroxo?bridged Mn-III/Fe-III dimer. This species has recently been observed as an intermediate of the oxygen activation reaction in WT R2lox, indicating that it is physiologically relevant. Cofactor maturation in R2c and R2lox therefore follows the same pathway, with structural and functional divergence of the two cofactor forms following oxygen activation. We also show that the leucine-substituted variant no longer functions as a two-electron oxidant. Our results reveal that the residue preceding the N-terminal metal ligand directs the cofactor's reactivity toward one- or two-electron redox chemistry, presumably by setting the protonation state of the bridging oxygens and thereby perturbing the redox potential of the Mn ion.
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| 8. |
- Kutin, Yuri, et al.
(författare)
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Divergent assembly mechanisms of the manganese/iron cofactors in R2lox and R2c proteins
- 2016
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Ingår i: Journal of Inorganic Biochemistry. - : Elsevier BV. - 0162-0134 .- 1873-3344. ; 162, s. 164-177
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Tidskriftsartikel (refereegranskat)abstract
- A manganese/iron cofactor which performs multi-electron oxidative chemistry is found in two classes of ferritin-like proteins, the small subunit (R2) of dass Ic ribonucleotide reductase (R2c) and the R2-like ligand-binding oxidase (R2lox). It is undear how a heterodimeric Mn/Fe metallocofactor is assembled in these two related proteins as opposed to a homodimeric Fe/Fe cofactor, especially considering the structural similarity and proximity of the two metal-binding sites in both protein scaffolds and the similar first coordination sphere ligand preferences of Mn-II and Fe-II. Using EPR and Mfissbauer spectroscopies as well as X-ray anomalous dispersion, we examined metal loading and cofactor activation of both proteins in vitro (in solution). We find divergent cofactor assembly mechanisms for the two systems. In both cases, excess Mn-II promotes heterobimetallic cofactor assembly. In the absence of Fe-II, R2c cooperatively binds Mn-II at both metal sites, whereas R2lox does not readily bind Mn-II at either site. Heterometallic cofactor assembly is favored at substoichiometric Feu concentrations in R2lox. Fe-II and Mn-II likely bind to the protein in a stepwise fashion, with Feu binding to site 2 initiating cofactor assembly. In R2c, however, heterometallic assembly is presumably achieved by the displacement of Mn-II by Fe-II at site 2. The divergent metal loading mechanisms are correlated with the putative in vivo functions of R2c and R2lox, and most likely with the intracellular Mn-II/Fe-II concentrations in the host organisms from which they were isolated.
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| 9. |
- Leidel, Nils, et al.
(författare)
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Electronic Structure of an [FeFe] Hydrogenase Model Complex in Solution Revealed by X-ray Absorption Spectroscopy Using Narrow-Band Emission Detection
- 2012
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 134:34, s. 14142-14157
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection, supported by density functional theory calculations (XAES-DFT), was used to study a model complex, ([Fe-2(mu-adt)(CO)(4)(PMe3)(2)] (1, adt = S-CH2-(NCH2Ph)-CH2-S), of the [FeFe] hydrogenase active site. For 1 in powder material (1(powder)), in MeCN solution (1'), and in its three protonated states (1H, 1Hy, 1HHy; H denotes protonation at the adt-N and Hy protonation of the Fe-Fe bond to form a bridging metal hydride), relations between the molecular structures and the electronic configurations were determined. EXAFS analysis and DFT geometry optimization suggested prevailing rotational isomers in MeCN, which were similar to the crystal structure or exhibited rotation of the (CO) ligands at Fe1 (1(CO), 1Hy(CO)) and in addition of the phenyl ring (1H(CO,ph), 1HHy(CO,ph)), leading to an elongated solvent-exposed Fe-Fe bond. Isomer formation, adt-N protonation, and hydride binding caused spectral changes of core-to-valence (pre-edge of the Fe K-shell absorption) and of valence-to-core (K beta(2,5) emission) electronic transitions, and of K alpha RIXS data, which were quantitatively reproduced by DFT. The study reveals (1) the composition of molecular orbitals, for example, with dominant Fe-d character, showing variations in symmetry and apparent oxidation state at the two Fe ions and a drop in MO energies by similar to 1 eV upon each protonation step, (2) the HOMO-LUMO energy gaps, of similar to 2.3 eV for 1(powder) and similar to 2.0 eV for 1', and (3) the splitting between iron d(z(2)) and d(x(2-)y(2)) levels of similar to 0.5 eV for the nonhydride and similar to 0.9 eV for the hydride states. Good correlations of reduction potentials to LUMO energies and oxidation potentials to HOMO energies were obtained. Two routes of facilitated bridging hydride binding thereby are suggested, involving ligand rotation at Fe1 for 1Hy(CO) or adt-N protonation for 1HHy(CO,ph). XAES-DFT thus enables verification of the effects of ligand substitutions in solution for guided improvement of [FeFe] catalysts.
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| 10. |
- Leidel, Nils, et al.
(författare)
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High-valent [MnFe] and [FeFe] cofactors in ribonucleotide reductases
- 2012
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1817:3, s. 430-444
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Tidskriftsartikel (refereegranskat)abstract
- Ribonucleotide reductases (RNRs) are essential for DNA synthesis in most organisms. In class-Ic RNR from Chlamydia trachomatis (Ct), a MnFe cofactor in subunit R2 forms the site required for enzyme activity, instead of an FeFe cofactor plus a redox-active tyrosine in class-la RNRs, for example in mouse (Mus musculus, Mm). For R2 proteins from Ct and Mm, either grown in the presence of, or reconstituted with Mn and Fe ions, structural and electronic properties of higher valence MnFe and FeFe sites were determined by X-ray absorption spectroscopy and complementary techniques, in combination with bond-valence-sum and density functional theory calculations. At least ten different cofactor species could be tentatively distinguished. In Cr R2, two different Mn(IV)Fe(III) site configurations were assigned either L4MnIV(mu O)(2)(FeL4)-L-III (metal-metal distance of similar to 2.75 angstrom, L = ligand) prevailing in metal-grown R2, or L4MnIV(mu O)(mu OH)(FeL4)-L-III (similar to 2.90 angstrom) dominating in metal-reconstituted R2. Specific spectroscopic features were attributed to an Fe(IV)Fe(III) site (similar to 2.55 angstrom) with a L4FeIV(mu O)(2)(FeL3)-L-III core structure. Several Mn,Fe(III)Fe(III) (similar to 2.9-3.1 angstrom) and Mn,Fe(III)Fe(II) species (similar to 3.3-3.4 angstrom) likely showed 5-coordinated Mn(III) or Fe(III). Rapid X-ray photoreduction of iron and shorter metal-metal distances in the high-valent states suggested radiation-induced modifications in most crystal structures of R2. The actual configuration of the MnFe and FeFe cofactors seems to depend on assembly sequences, bound metal type, valence state, and previous catalytic activity involving subunit RI. In Ct R2, the protonation of a bridging oxide in the Mn-IV(mu O)(mu OH)Fe-III core may be important for preventing premature site reduction and initiation of the radical chemistry in R1.
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| 11. |
- Leidel, Nils, et al.
(författare)
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Site-Selective X-ray Spectroscopy on an Asymmetric Model Complex of the [FeFe] Hydrogenase Active Site
- 2012
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 51:8, s. 4546-4559
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Tidskriftsartikel (refereegranskat)abstract
- The active site for hydrogen production in [FeFe] hydrogenase comprises a diiron unit. Bioinorganic chemistry has modeled important features of this center, aiming at mechanistic understanding and the development of novel catalysts. However, new assays are required for analyzing the effects of ligand variations at the metal ions. By high-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection (XAS/XES = XAES) and density functional theory (DFT), we studied an asymmetrically coordinated [FeFe] model complex, [(CO)(3)Fe(I)1-(bdtCl(2))-Fe-2(I)(CO)(Ph2P-CH2-NCH3-CH2-PPh2)] (1, bdt = benzene-1,2-dithiolate), in comparison to iron-carbonyl references. K beta emission spectra (K beta(1,3), K beta') revealed the absence of unpaired spins and the low-spin character for both Fe ions in 1. In a series of low-spin iron compounds, the K beta(1,3) energy did not reflect the formal iron oxidation state, but it decreases with increasing ligand field strength due to shorter iron-ligand bonds, following the spectrochemical series. The intensity of the valence-to-core transitions (K beta(2,5)) decreases for increasing Fe-ligand bond length, certain emission peaks allow counting of Fe-CO bonds, and even molecular orbitals (MOs) located on the metal-bridging bdt group of 1 contribute to the spectra. As deduced from 3d -> 1s emission and 1s -> 3d absorption spectra and supported by DFT, the HOMO-LUMO gap of 1 is about 2.8 eV. K beta-detected XANES spectra in agreement with DFT revealed considerable electronic asymmetry in 1; the energies and occupancies of Fe-d dominated MOs resemble a square-pyramidal F(0) for Fe1 and an octahedral Fe(II) for Fe2. EXAFS spectra for various K beta emission energies showed considerable site-selectivity; approximate structural parameters similar to the crystal structure could be determined for the two individual iron atoms of 1 in powder samples. These results suggest that metal site- and spin-selective XAES on [FeFe] hydrogenase protein and active site models may provide a powerful tool to study intermediates under reaction conditions.
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| 12. |
- Liu, Si, et al.
(författare)
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Electrocatalytic Water Oxidation at Neutral pH–Deciphering the Rate Constraints for an Amorphous Cobalt‐Phosphate Catalyst System
- 2022
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Ingår i: Advanced Energy Materials. - : Wiley-VCH Verlagsgesellschaft. - 1614-6832 .- 1614-6840. ; 12:46
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Tidskriftsartikel (refereegranskat)abstract
- The oxygen evolution reaction (OER) is pivotal in sustainable fuel production. Neutral-pH OER reduces operational risks and enables direct coupling to electrochemical CO2 reduction, but typically is hampered by low current densities. Here, the rate limitations in neutral-pH OER are clarified. Using cobalt-based catalyst films and phosphate ions as essential electrolyte bases, current–potential curves are recorded and simulated. Operando X-ray spectroscopy shows the potential-dependent structural changes independent of the electrolyte phosphate concentration. Operando Raman spectroscopy uncovers electrolyte acidification at a micrometer distance from the catalyst surface, limiting the Tafel slope regime to low current densities. The electrolyte proton transport is facilitated by diffusion of either phosphate ions (base pathway) or H3O+ ions (water pathway). The water pathway is not associated with an absolute current limit but is energetically inefficient due to the Tafel-slope increase by 60 mV dec−1, shown by an uncomplicated mathematical model. The base pathway is a specific requirement in neutral-pH OER and can support high current densities, but only with accelerated buffer-base diffusion. Catalyst internal phosphate diffusion or other internal transport mechanisms do not limit the current densities. A proof-of-principle experiment shows that current densities exceeding 1 A cm−2 can also be achieved in neutral-pH OER.
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| 13. |
- Löscher, Simone, et al.
(författare)
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Facilitated hydride binding in an Fe-Fe hydrogenase active-site biomimic revealed by X-ray absorption spectroscopy and DFT calculations
- 2007
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 46:26, s. 11094-11105
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Tidskriftsartikel (refereegranskat)abstract
- Iron-only hydrogenases are high-efficiency biocatalysts for the synthesis and cleavage of molecular hydrogen. Their active site is a diiron center, which carries CO and CN ligands. Remarkably, the two iron atoms likely are connected by a non-protein azadithiolate (adt = S-CH2-NH-CH2-S). To dwell on the role of the adt in H-2 catalysis, a specific biomimetic diiron compound, 1 = [Fe-2(mu-adt-CH2-Ph)(CO)(4)(PMe3)(2)], with unprecedented positive reduction potential, has been synthesized and crystallized previously. It comprises two protonation sites, the N-benzyl-adt nitrogen that can hold a proton (H) and the Fe-Fe bond that will formally carry a hydride (Hy). We investigated changes in the solution structure of 1 in its four different protonation states (1', [1H](+), [1HHy](2+), and [1Hy](+)) by X-ray absorption spectroscopy at the iron K-edge. EXAFS reveals that already protonation at the adt nitrogen atom causes a change of the ligand geometry involving a significant lengthening of the Fe-Fe distance and CO and PMe3 repositioning, respectively, thereby facilitating the subsequent binding of a bridging hydride. Hydride binding clearly is discernible in the XANES spectra of [1HHy](2+) and [1Hy](+). DIFT calculations are in excellent agreement with the experimentally derived structural parameters and provide complementary insights into the electronic structure of the four protonation states. In the iron-only hydrogenases, protonation of the putative adt ligand may cause the bridging CO to move to a terminal position, thereby preparing the active site for hydride binding en route to H2 formation.
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| 14. |
- Mebs, Stefan, et al.
(författare)
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Fate of oxygen species from O-2 activation at dimetal cofactors in an oxidase enzyme revealed by Fe-57 nuclear resonance X-ray scattering and quantum chemistry
- 2019
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1860:12
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Tidskriftsartikel (refereegranskat)abstract
- Oxygen (O-2) activation is a central challenge in chemistry and catalyzed at prototypic dimetal cofactors in biological enzymes with diverse functions. Analysis of intermediates is required to elucidate the reaction paths of reductive O-2 cleavage. An oxidase protein from the bacterium Geobacillus kaustophilus, R2lox, was used for aerobic in-vitro reconstitution with only Fe-57(II) or Mn(II) plus Fe-57(II) ions to yield [FeFe] or [MnFe] cofactors under various oxygen and solvent isotopic conditions including O-16/18 and H/D exchange. Fe-57-specific X-ray scattering techniques were employed to collect nuclear forward scattering (NFS) and nuclear resonance vibrational spectroscopy (NRVS) data of the R2lox proteins. NFS revealed Fe/Mn(III)Fe(III) cofactor states and Mossbauer quadrupole splitting energies. Quantum chemical calculations of NRVS spectra assigned molecular structures, vibrational modes, and protonation patterns of the cofactors, featuring a terminal water (H2O) bound at iron or manganese in site 1 and a metal-bridging hydroxide (mu OH-) ligand. A procedure for quantitation and correlation of experimental and computational NRVS difference signals due to isotope labeling was developed. This approach revealed that the protons of the ligands as well as the terminal water at the R2lox cofactors exchange with the bulk solvent whereas O-18 from O-18(2) cleavage is incorporated in the hydroxide bridge. In R2lox, the two water molecules from four-electron O-2 reduction are released in a two-step reaction to the solvent. These results establish combined NRVS and QM/MM for tracking of iron-based oxygen activation in biological and chemical catalysts and clarify the reductive O-2 cleavage route in an enzyme.
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| 15. |
- Németh, Brigitta, et al.
(författare)
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[FeFe]-hydrogenase maturation : H-cluster assembly intermediates tracked by electron paramagnetic resonance, infrared, and X-ray absorption spectroscopy
- 2020
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Ingår i: Journal of Biological Inorganic Chemistry. - : Springer Nature. - 0949-8257 .- 1432-1327. ; 25:5, s. 777-788
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Tidskriftsartikel (refereegranskat)abstract
- [FeFe]-hydrogenase enzymes employ a unique organometallic cofactor for efficient and reversible hydrogen conversion. This so-called H-cluster consists of a [4Fe-4S] cubane cysteine linked to a diiron complex coordinated by carbon monoxide and cyanide ligands and an azadithiolate ligand (adt =NH(CH2S)(2)).[FeFe]-hydrogenase apo-protein binding only the [4Fe-4S] sub-complex can be fully activated in vitro by the addition of a synthetic diiron site precursor complex ([2Fe](adt)). Elucidation of the mechanism of cofactor assembly will aid in the design of improved hydrogen processing synthetic catalysts. We combined electron paramagnetic resonance, Fourier-transform infrared, and X-ray absorption spectroscopy to characterize intermediates of H-cluster assembly as initiated by mixing of the apo-protein (HydA1) from the green alga Chlamydomonas reinhardtii with [2Fe](adt). The three methods consistently show rapid formation of a complete H-cluster in the oxidized, CO-inhibited state (Hox-CO) already within seconds after the mixing. Moreover, FTIR spectroscopy support a model in which Hox-CO formation is preceded by a short-lived Hred'-CO-like intermediate. Accumulation of Hox-CO was followed by CO release resulting in the slower conversion to the catalytically active state (Hox) as well as formation of reduced states of the H-cluster. [GRAPHICS] .
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| 16. |
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| 17. |
- Pasquini, Chiara, et al.
(författare)
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Operando tracking of oxidation-state changes by coupling electrochemistry with time-resolved X-ray absorption spectroscopy demonstrated for water oxidation by a cobalt-based catalyst film
- 2021
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Ingår i: Analytical and Bioanalytical Chemistry. - : Springer Nature. - 1618-2642 .- 1618-2650. ; 413:21, s. 5395-5408
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal oxides are promising electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), which is critical in electrochemical production of non-fossil fuels. The involvement of oxidation state changes of the metal in OER electrocatalysis is increasingly recognized in the literature. Tracing these oxidation states under operation conditions could provide relevant information for performance optimization and development of durable catalysts, but further methodical developments are needed. Here, we propose a strategy to use single-energy X-ray absorption spectroscopy for monitoring metal oxidation-state changes during OER operation with millisecond time resolution. The procedure to obtain time-resolved oxidation state values, using two calibration curves, is explained in detail. We demonstrate the significance of this approach as well as possible sources of data misinterpretation. We conclude that the combination of X-ray absorption spectroscopy with electrochemical techniques allows us to investigate the kinetics of redox transitions and to distinguish the catalytic current from the redox current. Tracking of the oxidation state changes of Co ions in electrodeposited oxide films during cyclic voltammetry in neutral pH electrolyte serves as a proof of principle.
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| 18. |
- Planas, Nora, et al.
(författare)
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Electronic Structure of Oxidized Complexes Derived from cis-Ru(II)(bpy)(2)(H(2)O)(2)](2+) and Its Photoisomerization Mechanism
- 2011
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 50:21, s. 11134-11142
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Tidskriftsartikel (refereegranskat)abstract
- The geometry and electronic structure of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) and its higher oxidation state species up formally to Ru(VI) have been studied by means of UV-vis, EPR, XAS, and DFT and CASSCF/CASPT2 calculations. DFT calculations of the molecular structures of these species show that, as the oxidation state increases, the Ru-O bond distance decreases, indicating increased degrees of Ru-O multiple bonding. In addition, the O-Ru-O valence bond angle increases as the oxidation state increases. EPR spectroscopy and quantum chemical calculations indicate that low-spin configurations are favored for all oxidation states. Thus, cis-[Ru(IV)(bpy)(2)(OH)(0)](2+) (d(4)) has a singlet ground state and is EPR-silent at low temperatures, while cis-[Ru(V)(bpy)(2)(O)(OH)](2+) (d(3)) has a doublet ground state. XAS spectroscopy of higher oxidation state species and DFT calculations further illuminate the electronic structures of these complexes, particularly with respect to the covalent character of the O-Ru-O fragment. In addition, the photochemical isomerization of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) to its trans-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) isomer has been fully characterized through quantum chemical calculations. The excited-state process is predicted to involve decoordination of one aqua ligand, which leads to a coordinatively unsaturated complex that undergoes structural rearrangement followed by recoordination of water to yield the trans isomer.
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| 19. |
- Redman, Holly J., et al.
(författare)
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Lewis acid protection turns cyanide containing [FeFe]-hydrogenase mimics into proton reduction catalysts
- 2022
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Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 51:12, s. 4634-4643
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Tidskriftsartikel (refereegranskat)abstract
- Sustainable sources of hydrogen are a vital component of the envisioned energy transition. Understanding and mimicking the [FeFe]-hydrogenase provides a route to achieving this goal. In this study we re-visit a molecular mimic of the hydrogenase, the propyl dithiolate bridged complex [Fe2(μ-pdt)(CO)4(CN)2]2−, in which the cyanide ligands are tuned via Lewis acid interactions. This system provides a rare example of a cyanide containing [FeFe]-hydrogenase mimic capable of catalytic proton reduction, as demonstrated by cyclic voltammetry. EPR, FTIR, UV-vis and X-ray absorption spectroscopy are employed to characterize the species produced by protonation, and reduction or oxidation of the complex. The results reveal that biologically relevant iron-oxidation states can be generated, potentially including short-lived mixed valent Fe(I)Fe(II) species. We propose that catalysis is initiated by protonation of the diiron complex and the resulting di-ferrous bridging hydride species can subsequently follow two different pathways to promote H2 gas formation depending on the applied reduction potential.
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| 20. |
- Roy, Souvik, et al.
(författare)
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Light-driven hydrogen evolution catalyzed by a cobaloxime catalyst incorporated in a MIL-101(Cr) metal-organic framework
- 2018
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Ingår i: Sustainable Energy & Fuels. - : Royal Society of Chemistry (RSC). - 2398-4902. ; 2:6, s. 1148-1152
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Tidskriftsartikel (refereegranskat)abstract
- A cobaloxime H-2 evolution catalyst with a hydroxo-functionalized pyridine ligand, Co(dmgH)(2)(4-HEP)Cl [dmgH = dimethylglyoxime, 4-HEP = 4-(2-hydroxyethyl)pyridine] was immobilized on a chromium terephthalate metal-organic framework (MOF), MIL-101(Cr), to construct a MOF-catalyst hybrid which displays good photocatalytic H-2 evolution activity. The longevity of the cobaloxime catalyst is increased by MOF incorporation, but limited by the stability of the cobalt-pyridine bond under turnover conditions.
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| 21. |
- Schuth, Nils, et al.
(författare)
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K alpha X-ray Emission Spectroscopy on the Photosynthetic Oxygen-Evolving Complex Supports Manganese Oxidation and Water Binding in the S-3 State
- 2018
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Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 57:16, s. 10424-10430
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Tidskriftsartikel (refereegranskat)abstract
- The unique manganese calcium-catalyst in photosystem II (PSII) is the natural paragon for efficient light driven water oxidation to yield O-2. The oxygen-evolving complex (OEC) in the dark-stable state (S-1) comprises a Mn4CaO4 core with five metal-bound water species. Binding and modification of the water molecules that are substrates of the water-oxidation reaction is mechanistically crucial but controversially debated. Two recent crystal structures of the OEC in its highest oxidation state (S-3) show either a vacant Mn coordination site or a bound peroxide species. For purified PSII at room temperature, we collected Mn K alpha X-ray emission spectra of the S-0, S-1, S-2, and S-3 intermediates in the OEC cycle, which were analyzed by comparison to synthetic Mn compounds, spectral simulations, and OEC models from density functional theory. Our results contrast both crystallographic structures. They indicate Mn oxidation in three S-transitions and suggest additional water binding at a previously open Mn coordination site. These findings exclude Mn reduction and render peroxide formation in S-3 unlikely.
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| 22. |
- Senger, Moritz, et al.
(författare)
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Trapping an Oxidized and Protonated Intermediate of the [FeFe]-Hydrogenase Cofactor under Mildly Reducing Conditions
- 2022
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:26, s. 10036-10042
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Tidskriftsartikel (refereegranskat)abstract
- The H-cluster is the catalytic cofactor of [FeFe]-hydrogenase, a metalloenzyme that catalyzes the formation of dihydrogen (H-2). The catalytic diiron site of the H-cluster carries two cyanide and three carbon monoxide ligands, making it an excellent target for IR spectroscopy. In previous work, we identified an oxidized and protonated H-cluster species, whose IR signature differs from that of the oxidized resting state (Hox) by a small but distinct shift to higher frequencies. This "blue shift" was explained by a protonation at the [4Fe-4S] subcomplex of the H-cluster. The novel species, denoted HoxH, was preferentially accumulated at low pH and in the presence of the exogenous reductant sodium dithionite (NaDT). When HoxH was reacted with H-2, the hydride state (Hhyd) was formed, a key intermediate of [FeFe]-hydrogenase turnover. A recent publication revisited our protocol for the accumulation of HoxH in wild-type [FeFe]-hydrogenase, concluding that inhibition by NaDT decay products rather than cofactor protonation causes the spectroscopic "blue shift". Here, we demonstrate that HoxH formation does not require the presence of NaDT (or its decay products), but accumulates also with the milder reductants tris(2-carboxyethyl)phosphine, dithiothreitol, or ascorbic acid, in particular at low pH. Our data consistently suggest that HoxH is accumulated when deprotonation of the H-cluster is impaired, thereby preventing the regain of the oxidized resting state Hox in the catalytic cycle.
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| 23. |
- Sigfridsson Clauss, Kajsa, et al.
(författare)
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Rapid X-ray photoreduction of dimetal-oxygen cofactors in ribonucleotide reductase.
- 2013
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Ingår i: Journal of Biological Chemistry. - 1083-351X .- 0021-9258.
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Tidskriftsartikel (refereegranskat)abstract
- Prototypic dinuclear metal cofactors with varying metallation constitute a class of O2-activating catalysts in numerous enzymes such as ribonucleotide reductase (RNR1). Reliable structures are required to unravel the reaction mechanisms. However, protein crystallography data may be compromised by X-ray photoreduction (XPR). We studied XPR of Fe(III)Fe(III) and Mn(III)Fe(III) sites in the R2 subunit of Chlamydia trachomatis RNR using X-ray absorption spectroscopy. Rapid and biphasic XPR kinetics at 20 K and 80 K for both cofactor types suggested sequential formation of (III,II) and (II,II) species and similar redox potentials of Fe and Mn sites. Comparing with typical X-ray doses in crystallography implies that (II,II) states are reached in <1 s in such studies. First-sphere metal coordinations and metal-metal distances differed after chemical reduction at room temperature and after XPR at cryogenic temperatures, as corroborated by model structures from density functional theory calculations. The inter-metal distances in the (II,II) states, however, are similar to R2 crystal structures. Therefore, crystal data of initially oxidized R2-type proteins mostly contain photoreduced (II,II) cofactors, which deviate from the native structures functional in O2-activation, explaining observed variable metal ligation motifs. This situation may be remedied by novel femtosecond free-electron-laser protein crystallography techniques.
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| 24. |
- Silvano, Alessandro, et al.
(författare)
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Observing Antarctic Bottom Water in the Southern Ocean
- 2023
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Ingår i: Frontiers in Marine Science. - 2296-7745. ; 10
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Forskningsöversikt (refereegranskat)abstract
- Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW's key role in regulating Earth's climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system.
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| 25. |
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