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Edward I. Solomon - One of the best experts on this subject based on the ideXlab platform.
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sulfur k edge xas studies of the effect of dna binding on the fe4s4 site in endoiii and muty
Journal of the American Chemical Society, 2017Co-Authors: Anna R Arnold, Britt Hedman, Keith O Hodgson, Nicole N Nunez, Phillip L Bartels, Andy Zhou, Sheila S David, Jacqueline K Barton, Edward I. SolomonAbstract:S K-edge X-ray absorption spectroscopy (XAS) was used to study the [Fe4S4] clusters in the DNA repair glycosylases EndoIII and MutY to evaluate the effects of DNA binding and solvation on Fe–S bond covalencies (i.e., the amount of S 3p character mixed into the Fe 3d valence orbitals). Increased covalencies in both iron–thiolate and iron–sulfide bonds would stabilize the oxidized state of the [Fe4S4] clusters. The results are compared to those on previously studied [Fe4S4] model complexes, ferredoxin (Fd), and to new data on high-potential iron–sulfur protein (HiPIP). A limited decrease in Covalency is observed upon removal of solvent water from EndoIII and MutY, opposite to the significant increase observed for Fd, where the [Fe4S4] cluster is solvent exposed. Importantly, in EndoIII and MutY, a large increase in Covalency is observed upon DNA binding, which is due to the effect of its negative charge on the iron–sulfur bonds. In EndoIII, this change in Covalency can be quantified and makes a significant ...
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Sulfur K‑Edge XAS Studies of the Effect of DNA Binding on the [Fe4S4] Site in EndoIII and MutY
2017Co-Authors: Anna R Arnold, Britt Hedman, Keith O Hodgson, Phillip L Bartels, Andy Zhou, Sheila S David, Jacqueline K Barton, Nicole N. Nuñez, Edward I. SolomonAbstract:S K-edge X-ray absorption spectroscopy (XAS) was used to study the [Fe4S4] clusters in the DNA repair glycosylases EndoIII and MutY to evaluate the effects of DNA binding and solvation on Fe–S bond covalencies (i.e., the amount of S 3p character mixed into the Fe 3d valence orbitals). Increased covalencies in both iron–thiolate and iron–sulfide bonds would stabilize the oxidized state of the [Fe4S4] clusters. The results are compared to those on previously studied [Fe4S4] model complexes, ferredoxin (Fd), and to new data on high-potential iron–sulfur protein (HiPIP). A limited decrease in Covalency is observed upon removal of solvent water from EndoIII and MutY, opposite to the significant increase observed for Fd, where the [Fe4S4] cluster is solvent exposed. Importantly, in EndoIII and MutY, a large increase in Covalency is observed upon DNA binding, which is due to the effect of its negative charge on the iron–sulfur bonds. In EndoIII, this change in Covalency can be quantified and makes a significant contribution to the observed decrease in reduction potential found experimentally in DNA repair proteins, enabling their HiPIP-like redox behavior
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anisotropic Covalency contributions to superexchange pathways in type one copper active sites
Journal of the American Chemical Society, 2014Co-Authors: Ryan G Hadt, Serge I Gorelsky, Edward I. SolomonAbstract:Type one (T1) Cu sites deliver electrons to catalytic Cu active sites: the mononuclear type two (T2) Cu site in nitrite reductases (NiRs) and the trinuclear Cu cluster in the multicopper oxidases (MCOs). The T1 Cu and the remote catalytic sites are connected via a Cys-His intramolecular electron-transfer (ET) bridge, which contains two potential ET pathways: P1 through the protein backbone and P2 through the H-bond between the Cys and the His. The high Covalency of the T1 Cu–S(Cys) bond is shown here to activate the T1 Cu site for hole superexchange via occupied valence orbitals of the bridge. This Covalency-activated electronic coupling (HDA) facilitates long-range ET through both pathways. These pathways can be selectively activated depending on the geometric and electronic structure of the T1 Cu site and thus the anisotropic Covalency of the T1 Cu–S(Cys) bond. In NiRs, blue (π-type) T1 sites utilize P1 and green (σ-type) T1 sites utilize P2, with P2 being more efficient. Comparing the MCOs to NiRs, the...
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solvation effects on s k edge xas spectra of fe s proteins normal and inverse effects on wt and mutant rubredoxin
Journal of the American Chemical Society, 2010Co-Authors: Zhiguang Xiao, Anthony G. Wedd, Britt Hedman, Keith O Hodgson, Edward I. SolomonAbstract:S K-edge X-ray Absorption Spectroscopy (XAS) was performed on wild type Cp rubredoxin and its Cys->Ser mutants in both solution and lyophilized forms. For wild type rubredoxin and for the mutants where an interior cysteine residue (C6 or C39) is substituted by serine, a normal solvent effect is observed, that is, the S Covalency increases upon lyophilization. For the mutants where a solvent accessible surface cysteine residue is substituted by serine, the S Covalency decreases upon lyophilization which is an inverse solvent effect. Density functional theory (DFT) calculations reproduce these experimental results and show that the normal solvent effect reflects the Covalency decrease due to solvent H-bonding to the surface thiolates and that the inverse solvent effect results from the Covalency compensation from the interior thiolates. With respect to the Cys->Ser substitution, the S Covalency decreases. Calculations indicate that the stronger bonding interaction of the alkoxide with the Fe relative to that of thiolate increases the energy of the Fe d orbitals and reduces their bonding interaction with the remaining cysteines. The solvent effects support a surface solvent tuning contribution to electron transfer and the Cys->Ser result provides an explanation for the change in properties of related iron-sulfur sites with this mutation.
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solvent tuning of electrochemical potentials in the active sites of hipip versus ferredoxin
Science, 2007Co-Authors: Francis E Jenney, Britt Hedman, Keith O Hodgson, Michael W W Adams, Elena Babini, Yasuhiro Takahashi, Keiichi Fukuyama, Edward I. SolomonAbstract:A persistent puzzle in the field of biological electron transfer is the conserved iron-sulfur cluster motif in both high potential iron-sulfur protein (HiPIP) and ferredoxin (Fd) active sites. Despite this structural similarity, HiPIPs react oxidatively at physiological potentials, whereas Fds are reduced. Sulfur K-edge x-ray absorption spectroscopy uncovers the substantial influence of hydration on this variation in reactivity. Fe-S Covalency is much lower in natively hydrated Fd active sites than in HiPIPs but increases upon water removal; similarly, HiPIP Covalency decreases when unfolding exposes an otherwise hydrophobically shielded active site to water. Studies on model compounds and accompanying density functional theory calculations support a correlation of Fe-S Covalency with ease of oxidation and therefore suggest that hydration accounts for most of the difference between Fd and HiPIP reduction potentials.
Nikolas Kaltsoyannis - One of the best experts on this subject based on the ideXlab platform.
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exceptional uranium vi nitride triple bond Covalency from 15n nuclear magnetic resonance spectroscopy and quantum chemical analysis
Nature Communications, 2021Co-Authors: John A Seed, Nikolas Kaltsoyannis, Victoria E J Berryman, Ralph W Adams, Daniel Lee, Stephen T LiddleAbstract:Determining the nature and extent of Covalency of early actinide chemical bonding is a fundamentally important challenge. Recently, X-ray absorption, electron paramagnetic, and nuclear magnetic resonance spectroscopic studies have probed actinide-ligand Covalency, largely confirming the paradigm of early actinide bonding varying from ionic to polarised-covalent, with this range sitting on the continuum between ionic lanthanide and more covalent d transition metal analogues. Here, we report measurement of the Covalency of a terminal uranium(VI)-nitride by 15N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15N nuclear magnetic resonance spectroscopy parameter space, and experimentally confirms a prior computational prediction that the uranium(VI)-nitride triple bond is not only highly covalent, but, more so than d transition metal analogues. These results enable construction of general, predictive metal-ligand 15N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies.
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29si nmr spectroscopy as a probe of s and f block metal ii silanide bond Covalency
Journal of the American Chemical Society, 2021Co-Authors: Benjamin L L Reant, Nikolas Kaltsoyannis, Victoria E J Berryman, Floriana Tuna, Annabel R Basford, Lydia E Nodaraki, Ashley J Wooles, David P Mills, Stephen T LiddleAbstract:We report the use of 29Si NMR spectroscopy and DFT calculations combined to benchmark the Covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(SitBu3)2(THF)2(THF)x] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(SitBu2Me)2(THF)2(THF)x] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and 29Si NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe3)3}--, {Si(SiMe2H)3}--, and {SiPh3}--substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound 29Si NMR isotropic chemical shifts, δSi, span a wide (∼225 ppm) range when the metal is kept constant, and direct, linear correlations are found between δSi and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond Covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The δSi is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a Covalency ordering of No(II) > Yb(II) > Ca(II) ≈ Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.
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computational analysis of m o Covalency in m oc6h5 4 m ti zr hf ce th u
Dalton Transactions, 2019Co-Authors: Victoria E J Berryman, Jacob J Shephard, Tatsumi Ochiai, Amy N Price, Polly L Arnold, Simon Parsons, Zoe J Whalley, Nikolas KaltsoyannisAbstract:A series of compounds M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, U) is studied with hybrid density functional theory, to assess M–O bond Covalency. The series allows for the comparison of d and f element compounds that are structurally similar. Two well-established analysis methods are employed: Natural Bond Orbital and the Quantum Theory of Atoms in Molecules. A consistent pattern emerges; the U–O bond is the most covalent, followed by Ce–O and Th–O, with those involving the heavier transition metals the least so. The Covalency of the Ti–O bond differs relative to Ce–O and Th–O, with the orbital-based method showing greater relative Covalency for Ti than the electron density-based methods. The deformation energy of r(M–O) correlates with the d orbital contribution from the metal to the M–O bond, while no such correlation is found for the f orbital component. f orbital involvement in M–O bonding is an important component of Covalency, facilitating orbital overlap and allowing for greater expansion of the electrons, thus lowering their kinetic energy.
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Does Covalency increase or decrease across the actinide series? Implications for minor actinide partitioning.
Inorganic chemistry, 2012Co-Authors: Nikolas KaltsoyannisAbstract:A covalent chemical bond carries the connotation of overlap of atomic orbitals between bonded atoms, leading to a buildup of the electron density in the internuclear region. Stabilization of the valence 5f orbitals as the actinide series is crossed leads, in compounds of the minor actinides americium and curium, to their becoming approximately degenerate with the highest occupied ligand levels and hence to the unusual situation in which the resultant valence molecular orbitals have significant contributions from both actinide and the ligand yet in which there is little atomic orbital overlap. In such cases, the traditional quantum-chemical tools for assessing the Covalency, e.g., population analysis and spin densities, predict significant metal–ligand Covalency, although whether this orbital mixing is really Covalency in the generally accepted chemical view is an interesting question. This review discusses our recent analyses of the bonding in AnCp3 and AnCp4 (An = Th–Cm; Cp = η5-C5H5) using both the trad...
Stephen T Liddle - One of the best experts on this subject based on the ideXlab platform.
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exceptional uranium vi nitride triple bond Covalency from 15n nuclear magnetic resonance spectroscopy and quantum chemical analysis
Nature Communications, 2021Co-Authors: John A Seed, Nikolas Kaltsoyannis, Victoria E J Berryman, Ralph W Adams, Daniel Lee, Stephen T LiddleAbstract:Determining the nature and extent of Covalency of early actinide chemical bonding is a fundamentally important challenge. Recently, X-ray absorption, electron paramagnetic, and nuclear magnetic resonance spectroscopic studies have probed actinide-ligand Covalency, largely confirming the paradigm of early actinide bonding varying from ionic to polarised-covalent, with this range sitting on the continuum between ionic lanthanide and more covalent d transition metal analogues. Here, we report measurement of the Covalency of a terminal uranium(VI)-nitride by 15N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15N nuclear magnetic resonance spectroscopy parameter space, and experimentally confirms a prior computational prediction that the uranium(VI)-nitride triple bond is not only highly covalent, but, more so than d transition metal analogues. These results enable construction of general, predictive metal-ligand 15N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies.
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29si nmr spectroscopy as a probe of s and f block metal ii silanide bond Covalency
Journal of the American Chemical Society, 2021Co-Authors: Benjamin L L Reant, Nikolas Kaltsoyannis, Victoria E J Berryman, Floriana Tuna, Annabel R Basford, Lydia E Nodaraki, Ashley J Wooles, David P Mills, Stephen T LiddleAbstract:We report the use of 29Si NMR spectroscopy and DFT calculations combined to benchmark the Covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(SitBu3)2(THF)2(THF)x] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(SitBu2Me)2(THF)2(THF)x] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and 29Si NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe3)3}--, {Si(SiMe2H)3}--, and {SiPh3}--substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound 29Si NMR isotropic chemical shifts, δSi, span a wide (∼225 ppm) range when the metal is kept constant, and direct, linear correlations are found between δSi and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond Covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The δSi is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a Covalency ordering of No(II) > Yb(II) > Ca(II) ≈ Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.
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Emergence of comparable Covalency in isostructural cerium(IV)- and uranium(IV)-carbon multiple bonds
Chemical Science, 2016Co-Authors: Matthew Gregson, Andrew Kerridge, Floriana Tuna, Eric J. L. Mcinnes, Christoph Hennig, Andreas C. Scheinost, Jonathan Mcmaster, William Lewis, Alexander J. Blake, Stephen T LiddleAbstract:We report comparable levels of Covalency in cerium- and uranium-carbon multiple bonds in the iso-structural carbene complexes [M(BIPMTMS)(ODipp)(2)] [M = Ce (1), U (2), Th (3); BIPMTMS = C(PPh2NSiMe3)(2); Dipp = C6H3-2,6-Pr-i(2)] whereas for M = Th the M=C bond interaction is much more ionic. On the basis of single crystal X-ray diffraction, NMR, IR, EPR, and XANES spectroscopies, and SQUID magnetometry complexes 1-3 are confirmed formally as bona fide metal(IV) complexes. In order to avoid the deficiencies of orbital-based theoretical analysis approaches we probed the bonding of 1-3 via analysis of RASSCF- and CASSCF-derived densities that explicitly treats the orbital energy near-degeneracy and overlap contributions to Covalency. For these complexes similar levels of Covalency are found for cerium(IV) and uranium(IV), whereas thorium(IV) is found to be more ionic, and this trend is independently found in all computational methods employed. The computationally determined trends in Covalency of these systems of Ce similar to U > Th are also reproduced in experimental exchange reactions of 1-3 with MCl4 salts where 1 and 2 do not exchange with ThCl4, but 3 does exchange with MCl4 (M = Ce, U) and 1 and 2 react with UCl4 and CeCl4, respectively, to establish equilibria. This study therefore provides complementary theoretical and experimental evidence that contrasts to the accepted description that generally lanthanide-ligand bonding in non-zero oxidation state complexes is overwhelmingly ionic but that of uranium is more covalent
Victoria E J Berryman - One of the best experts on this subject based on the ideXlab platform.
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exceptional uranium vi nitride triple bond Covalency from 15n nuclear magnetic resonance spectroscopy and quantum chemical analysis
Nature Communications, 2021Co-Authors: John A Seed, Nikolas Kaltsoyannis, Victoria E J Berryman, Ralph W Adams, Daniel Lee, Stephen T LiddleAbstract:Determining the nature and extent of Covalency of early actinide chemical bonding is a fundamentally important challenge. Recently, X-ray absorption, electron paramagnetic, and nuclear magnetic resonance spectroscopic studies have probed actinide-ligand Covalency, largely confirming the paradigm of early actinide bonding varying from ionic to polarised-covalent, with this range sitting on the continuum between ionic lanthanide and more covalent d transition metal analogues. Here, we report measurement of the Covalency of a terminal uranium(VI)-nitride by 15N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15N nuclear magnetic resonance spectroscopy parameter space, and experimentally confirms a prior computational prediction that the uranium(VI)-nitride triple bond is not only highly covalent, but, more so than d transition metal analogues. These results enable construction of general, predictive metal-ligand 15N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies.
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29si nmr spectroscopy as a probe of s and f block metal ii silanide bond Covalency
Journal of the American Chemical Society, 2021Co-Authors: Benjamin L L Reant, Nikolas Kaltsoyannis, Victoria E J Berryman, Floriana Tuna, Annabel R Basford, Lydia E Nodaraki, Ashley J Wooles, David P Mills, Stephen T LiddleAbstract:We report the use of 29Si NMR spectroscopy and DFT calculations combined to benchmark the Covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(SitBu3)2(THF)2(THF)x] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(SitBu2Me)2(THF)2(THF)x] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and 29Si NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe3)3}--, {Si(SiMe2H)3}--, and {SiPh3}--substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound 29Si NMR isotropic chemical shifts, δSi, span a wide (∼225 ppm) range when the metal is kept constant, and direct, linear correlations are found between δSi and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond Covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The δSi is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a Covalency ordering of No(II) > Yb(II) > Ca(II) ≈ Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.
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quantum chemical topology and natural bond orbital analysis of m o Covalency in m oc6h5 4 m ti zr hf ce th pa u np
Physical Chemistry Chemical Physics, 2020Co-Authors: Victoria E J Berryman, Jacob J Shephard, Tatsumi Ochiai, Amy N Price, Polly L Arnold, Simon ParsonsAbstract:Covalency is complex yet central to our understanding of chemical bonding, particularly in the actinide series. Here we assess Covalency in a series of isostructural d and f transition element compounds M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, Pa, U, Np) using scalar relativistic hybrid density functional theory in conjunction with the Natural Bond Orbital (NBO), quantum theory of atoms in molecules (QTAIM) and interacting quantum atoms (IQA) approaches. The IQA exchange–correlation Covalency metric is evaluated for the first time for actinides other than uranium, in order to assess its applicability in the 5f series. It is found to have excellent correlation with NBO and QTAIM Covalency metrics, making it a promising addition to the computational toolkit for analysing metal–ligand bonding. Our range of metrics agree that the actinide-oxygen bonds are the most covalent of the elements studied, with those of the heavier group 4 elements the least. Within the early actinide series, Th stands apart from the other three elements considered, being consistently the least covalent.
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computational analysis of m o Covalency in m oc6h5 4 m ti zr hf ce th u
Dalton Transactions, 2019Co-Authors: Victoria E J Berryman, Jacob J Shephard, Tatsumi Ochiai, Amy N Price, Polly L Arnold, Simon Parsons, Zoe J Whalley, Nikolas KaltsoyannisAbstract:A series of compounds M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, U) is studied with hybrid density functional theory, to assess M–O bond Covalency. The series allows for the comparison of d and f element compounds that are structurally similar. Two well-established analysis methods are employed: Natural Bond Orbital and the Quantum Theory of Atoms in Molecules. A consistent pattern emerges; the U–O bond is the most covalent, followed by Ce–O and Th–O, with those involving the heavier transition metals the least so. The Covalency of the Ti–O bond differs relative to Ce–O and Th–O, with the orbital-based method showing greater relative Covalency for Ti than the electron density-based methods. The deformation energy of r(M–O) correlates with the d orbital contribution from the metal to the M–O bond, while no such correlation is found for the f orbital component. f orbital involvement in M–O bonding is an important component of Covalency, facilitating orbital overlap and allowing for greater expansion of the electrons, thus lowering their kinetic energy.
Keith O Hodgson - One of the best experts on this subject based on the ideXlab platform.
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sulfur k edge xas studies of the effect of dna binding on the fe4s4 site in endoiii and muty
Journal of the American Chemical Society, 2017Co-Authors: Anna R Arnold, Britt Hedman, Keith O Hodgson, Nicole N Nunez, Phillip L Bartels, Andy Zhou, Sheila S David, Jacqueline K Barton, Edward I. SolomonAbstract:S K-edge X-ray absorption spectroscopy (XAS) was used to study the [Fe4S4] clusters in the DNA repair glycosylases EndoIII and MutY to evaluate the effects of DNA binding and solvation on Fe–S bond covalencies (i.e., the amount of S 3p character mixed into the Fe 3d valence orbitals). Increased covalencies in both iron–thiolate and iron–sulfide bonds would stabilize the oxidized state of the [Fe4S4] clusters. The results are compared to those on previously studied [Fe4S4] model complexes, ferredoxin (Fd), and to new data on high-potential iron–sulfur protein (HiPIP). A limited decrease in Covalency is observed upon removal of solvent water from EndoIII and MutY, opposite to the significant increase observed for Fd, where the [Fe4S4] cluster is solvent exposed. Importantly, in EndoIII and MutY, a large increase in Covalency is observed upon DNA binding, which is due to the effect of its negative charge on the iron–sulfur bonds. In EndoIII, this change in Covalency can be quantified and makes a significant ...
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Sulfur K‑Edge XAS Studies of the Effect of DNA Binding on the [Fe4S4] Site in EndoIII and MutY
2017Co-Authors: Anna R Arnold, Britt Hedman, Keith O Hodgson, Phillip L Bartels, Andy Zhou, Sheila S David, Jacqueline K Barton, Nicole N. Nuñez, Edward I. SolomonAbstract:S K-edge X-ray absorption spectroscopy (XAS) was used to study the [Fe4S4] clusters in the DNA repair glycosylases EndoIII and MutY to evaluate the effects of DNA binding and solvation on Fe–S bond covalencies (i.e., the amount of S 3p character mixed into the Fe 3d valence orbitals). Increased covalencies in both iron–thiolate and iron–sulfide bonds would stabilize the oxidized state of the [Fe4S4] clusters. The results are compared to those on previously studied [Fe4S4] model complexes, ferredoxin (Fd), and to new data on high-potential iron–sulfur protein (HiPIP). A limited decrease in Covalency is observed upon removal of solvent water from EndoIII and MutY, opposite to the significant increase observed for Fd, where the [Fe4S4] cluster is solvent exposed. Importantly, in EndoIII and MutY, a large increase in Covalency is observed upon DNA binding, which is due to the effect of its negative charge on the iron–sulfur bonds. In EndoIII, this change in Covalency can be quantified and makes a significant contribution to the observed decrease in reduction potential found experimentally in DNA repair proteins, enabling their HiPIP-like redox behavior
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solvation effects on s k edge xas spectra of fe s proteins normal and inverse effects on wt and mutant rubredoxin
Journal of the American Chemical Society, 2010Co-Authors: Zhiguang Xiao, Anthony G. Wedd, Britt Hedman, Keith O Hodgson, Edward I. SolomonAbstract:S K-edge X-ray Absorption Spectroscopy (XAS) was performed on wild type Cp rubredoxin and its Cys->Ser mutants in both solution and lyophilized forms. For wild type rubredoxin and for the mutants where an interior cysteine residue (C6 or C39) is substituted by serine, a normal solvent effect is observed, that is, the S Covalency increases upon lyophilization. For the mutants where a solvent accessible surface cysteine residue is substituted by serine, the S Covalency decreases upon lyophilization which is an inverse solvent effect. Density functional theory (DFT) calculations reproduce these experimental results and show that the normal solvent effect reflects the Covalency decrease due to solvent H-bonding to the surface thiolates and that the inverse solvent effect results from the Covalency compensation from the interior thiolates. With respect to the Cys->Ser substitution, the S Covalency decreases. Calculations indicate that the stronger bonding interaction of the alkoxide with the Fe relative to that of thiolate increases the energy of the Fe d orbitals and reduces their bonding interaction with the remaining cysteines. The solvent effects support a surface solvent tuning contribution to electron transfer and the Cys->Ser result provides an explanation for the change in properties of related iron-sulfur sites with this mutation.
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solvent tuning of electrochemical potentials in the active sites of hipip versus ferredoxin
Science, 2007Co-Authors: Francis E Jenney, Britt Hedman, Keith O Hodgson, Michael W W Adams, Elena Babini, Yasuhiro Takahashi, Keiichi Fukuyama, Edward I. SolomonAbstract:A persistent puzzle in the field of biological electron transfer is the conserved iron-sulfur cluster motif in both high potential iron-sulfur protein (HiPIP) and ferredoxin (Fd) active sites. Despite this structural similarity, HiPIPs react oxidatively at physiological potentials, whereas Fds are reduced. Sulfur K-edge x-ray absorption spectroscopy uncovers the substantial influence of hydration on this variation in reactivity. Fe-S Covalency is much lower in natively hydrated Fd active sites than in HiPIPs but increases upon water removal; similarly, HiPIP Covalency decreases when unfolding exposes an otherwise hydrophobically shielded active site to water. Studies on model compounds and accompanying density functional theory calculations support a correlation of Fe-S Covalency with ease of oxidation and therefore suggest that hydration accounts for most of the difference between Fd and HiPIP reduction potentials.
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ligand k edge x ray absorption spectroscopy Covalency of ligand metal bonds
Coordination Chemistry Reviews, 2005Co-Authors: Edward I. Solomon, Britt Hedman, Keith O Hodgson, Abhishek Dey, Robert K SzilagyiAbstract:The ligand K-edge probes the ligand 1s → valence np transitions. These transitions acquire intensity when the ligand is bound to an open shell metal ion. This intensity quantifies the amount of ligand character in the metal d orbitals, hence the Covalency of the ligand–metal bond. In this review the methodology is developed and applied to copper proteins, iron–sulfur sites and nickel dithiolene complexes, as examples. These illustrate the power and impact of this method in evaluating Covalency contributions to electron transfer pathways, reduction potentials, H-bond interactions, electron delocalization in mixed-valent systems and small molecule reactivity.
