Increased‐valence formulae and the bonding of oxygen to haemoglobin

Harcourt, Richard D.

doi:10.1002/qua.560050502

Cited by 29 publications

(23 citation statements)

References 33 publications

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“…Both are important in polarizing the Fe-O 2 π-bond. In previous studies on {FeNO} 6 , it was found that the degree of polarization is governed by the magnitude of the energy gap of the Fe-NO π-bonding and antibonding orbitals relative to the strength of the exchange interaction between electrons in these orbitals (44). When the energy gap is large relative to the exchange interaction, the bond remains unpolarized.…”

Section: Discussionmentioning

confidence: 99%

“…The iron oxygen-binding proteins contain an S = 0 {FeO 2 } 8 active site, denoting eight valence electrons delocalized among the Fe 3d and O 2 π*-orbitals in the Enemark-Feltham (1) notation used for metal-NO complexes. Three electronic structure models have been proposed by Pauling and Coryell (2, 3) (low-spin Fe II with singlet O 2 ), Weiss (4) (low-spin Fe III antiferromagnetically coupled to doublet O 2 − ), and McClure (5), Harcourt (6,7), and Goddard and Olafson (8) (S = 1 Fe II antiferromagnetically coupled to triplet O 2 , also known as the "ozone" model). Much computational work has been done, with all three models supported by different calculations (9)(10)(11)(12)(13).…”

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confidence: 99%

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Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex

Yan

Kröll

Baker

et al. 2019

Proc. Natl. Acad. Sci. U.S.A.

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Hemoglobin and myoglobin are oxygen-binding proteins with S = 0 heme {FeO2}8 active sites. The electronic structure of these sites has been the subject of much debate. This study utilizes Fe K-edge X-ray absorption spectroscopy (XAS) and 1s2p resonant inelastic X-ray scattering (RIXS) to study oxyhemoglobin and a related heme {FeO2}8 model compound, [(pfp)Fe(1-MeIm)(O2)] (pfp = meso-tetra(α,α,α,α-o-pivalamido-phenyl)porphyrin, or TpivPP, 1-MeIm = 1-methylimidazole) (pfpO2), which was previously analyzed using L-edge XAS. The K-edge XAS and RIXS data of pfpO2 and oxyhemoglobin are compared with the data for low-spin FeII and FeIII [Fe(tpp)(Im)2]0/+ (tpp = tetra-phenyl porphyrin) compounds, which serve as heme references. The X-ray data show that pfpO2 is similar to FeII, while oxyhemoglobin is qualitatively similar to FeIII, but with significant quantitative differences. Density-functional theory (DFT) calculations show that the difference between pfpO2 and oxyhemoglobin is due to a distal histidine H bond to O2 and the less hydrophobic environment in the protein, which lead to more backbonding into the O2. A valence bond configuration interaction multiplet model is used to analyze the RIXS data and show that pfpO2 is dominantly FeII with 6–8% FeIII character, while oxyhemoglobin has a very mixed wave function that has 50–77% FeIII character and a partially polarized Fe–O2 π-bond.

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Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex

Yan

Kröll

Baker

et al. 2019

Proc. Natl. Acad. Sci. U.S.A.

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“…The deoxygenated form of both proteins are high-spin Fe II , S = 2, and become diamagnetic upon dioxygen binding. Three possible descriptions of the spin singlet ground state of oxyhemoglobin are generally considered: a low spin (S=0) ferrous center with diamagnetic O 2 , as proposed by Pauling [72], a low-spin (S=1/2) ferric center antiferromagnetically coupled to a O 2 − doublet, as proposed by Weiss [73], and an intermediate (S=1) ferrous center antiferromagnetically coupled to an O 2 triplet as proposed by McClure [74], Harcout [75], and Goddard [76]. The sensitivity of L-edge absorption spectroscopy for probing electron delocalization was applied to shed new light on this long standing problem.…”

Section: L-edge Xasmentioning

confidence: 99%

K- and L-edge X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) determination of differential orbital covalency (DOC) of transition metal sites

Baker

Mara

Yan

et al. 2017

Coordination Chemistry Reviews

216

183

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Continual advancements in the development of synchrotron radiation sources have resulted in X-ray based spectroscopic techniques capable of probing the electronic and structural properties of numerous systems. This review gives an overview of the application of metal K-edge and L-edge X-ray absorption spectroscopy (XAS), as well as K resonant inelastic X-ray scattering (RIXS), to the study of electronic structure in transition metal sites with emphasis on experimentally quantifying 3d orbital covalency. The specific sensitivities of K-edge XAS, L-edge XAS, and RIXS are discussed emphasizing the complementary nature of the methods. L-edge XAS and RIXS are sensitive to mixing between 3d orbitals and ligand valence orbitals, and to the differential orbital covalency (DOC), that is, the difference in the covalencies for different symmetry sets of the d orbitals. Both L-edge XAS and RIXS are highly sensitive to and enable separation of and donor bonding and back bonding contributions to bonding. Applying ligand field multiplet simulations, including charge transfer via valence bond configuration interactions, DOC can be obtained for direct comparison with density functional theory calculations and to understand chemical trends. The application of RIXS as a probe of frontier molecular orbitals in a heme enzyme demonstrates the potential of this method for the study of metal sites in highly covalent coordination sites in bioinorganic chemistry.

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“…8 The precise nature of the Fe–O 2 bonding has been contentious. In addition to the Pauling model with its singlet dioxygen, 6, 9 Weiss proposed the antiferromagnetically coupled model of Fe III ( S =1/2)–O 2 − ( S =1/2); 10 and the ozone model Fe II ( S =1)–O 2 ( S =1) of McClure, 11 Harcourt, 12, 13 and Goddard 14 has also been put forward. Of these models, the Pauling model is closest to that supported by current data.…”

Section: Introductionmentioning

confidence: 99%

Correlated Ligand Dynamics in Oxyiron Picket Fence Porphyrins: Structural and Mössbauer Investigations

Noll

Oliver

et al. 2013

J. Am. Chem. Soc.

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Disorder in the position of the dioxygen ligand is a well-known problem in dioxygen complexes and in particular in those of picket fence porphyrin species. The dynamics of Fe–O2 rotation and tert-butyl motion in three different picket fence porphyrin derivatives has been studied by a combination of multi-temperature X-ray structural studies and Mössbauer spectroscopy. The structural studies show that the motions of the dioxygen ligand also require motions of the protecting pickets of the ligand binding pocket. The two motions appear to be correlated and the temperature-dependent change in the O2 occupancies can not be governed by a simple Boltzmann distribution. The three [Fe(TpivPP)(RIm)(O2)] derivatives studied have RIm = 1-methyl-, 1-ethyl-, or 2-methylimidazole. In all three species there is a preferred orientation of the Fe–O2 moiety with respect to the trans imidazole ligand and the population of this orientation increases with decreasing temperature. In the 1-MeIm and 1-EtIm species, the Fe–O2 unit is approximately perpendicular to the imidazole plane whereas in the 2-MeHIm species the Fe–O2 unit is approximately parallel. This reflects the low energy required for rotation of the Fe–O2 unit and the small energy differences in populating the possible pocket quadrants. All dioxygen complexes have a crystallographically required twofold axis of symmetry that limits the accuracy of the determined Fe–O2 geometry. However, the 80 K structure of the 2-MeHIm derivative allowed for the resolution of the two bonded oxygen atom positions and provided the best geometric description for the Fe–O2 unit. The values determined are: Fe–O = 1.811(5) Å, Fe–O–O = 118.2(9)°, O–O = 1.281(12) Å, and an off-axis tilt of 6.2°. The demonstration of the off-axis tilt is a first. We present detailed temperature-dependent simulations of the Mössbauer spectra that model the changing value of the quadrupole splitting and line widths. Residuals to fits are poorer at higher temperature. We believe that this is consistent with the idea that population of the two conformers are related to the concomitant motions of both Fe–O2 rotations and motions of the protecting tert-butyl pickets.

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Increased‐valence formulae and the bonding of oxygen to haemoglobin

Cited by 29 publications

References 33 publications

Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex

Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex

K- and L-edge X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) determination of differential orbital covalency (DOC) of transition metal sites

Correlated Ligand Dynamics in Oxyiron Picket Fence Porphyrins: Structural and Mössbauer Investigations

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