Effective intermediate-spin iron in O ₂ -transporting heme proteins

Abstract

Proteins carrying an iron-porphyrin (heme) cofactor are essential for biological O2 management. The nature of Fe-O2 bonding in hemoproteins is debated for decades. We used energy-sampling and rapid-scan X-ray Kβ emission and K-edge absorption spectroscopy as well as quantum chemistry to determine molecular and electronic structures of unligated (deoxy), CO-inhibited (carboxy), and O2-bound (oxy) hemes in myoglobin (MB) and hemoglobin (HB) solutions and in porphyrin compounds at 20–260 K. Similar metrical and spectral features revealed analogous heme sites in MB and HB and the absence of low-spin (LS) to high-spin (HS) conversion. Amplitudes of Kβ main-line emission spectra were directly related to the formal unpaired Fe(d) spin count, indicating HS Fe(II) in deoxy and LS Fe(II) in carboxy. For oxy, two unpaired Fe(d) spins and, thus by definition, an intermediate-spin iron center, were revealed by our static and kinetic X-ray data, as supported by (time-dependent) density functional theory and complete-active-space self-consistent-field calculations. The emerging Fe-O2 bonding situation includes in essence a ferrous iron center, minor superoxide character of the noninnocent ligand, significant double-bond properties of the interaction, and three-center electron delocalization as in ozone. It resolves the apparently contradictory classical models of Pauling, Weiss, and McClure/Goddard into a unifying view of O2 bonding, tuned toward reversible oxygen transport.

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

“…spectrum of the HbO 2 sample used here and found a significantly weaker satellite feature that is not consistent with an S = 1 Fe (SI Appendix, Fig. S15) (17). The Fe II S = 1 description of HbO 2 also requires polarized Fe-O π-backbonding and polarized O-Fe σ-donation.…”

“…An analysis of the iron K preedge suggested that HbO 2 has an electronic structure similar to the Weiss model in solution, but that crystalline HbO 2 has an electronic structure more similar to the Pauling model (16). Alternatively, a recent X-ray emission Kβ study suggested that the iron center has an S = 1 Fe II spin state, similar to the ozone model (17). However, neither technique provides a quantitative analysis of the electronic structure of iron centers.…”

What Fractionates Oxygen Isotopes During Respiration? Insights from Multiple Isotopologues and Theory

“…72 Cytochrome c oxidase, for example, binds to O 2 in an endon configuration with distal copper instead of the distal histidine residue found in hemoglobin, but the bonding scheme may be similar. 71,[73][74][75][76][77] Becke-Johnson damping (B3LYP-D3BJ), [78][79][80] (2) the one-parameter hybrid Tao-Perdew-Staroverov-Scuseria functional (TPSSh), 81,82 and (3) the second-order generalized gradient approximation functional of Pervati, Zhao, and Truhlar (SOGGA11). 83 Each level of theory is paired with the Weigand-Ahlrichs split-valence Gaussian basis sets def2-SVP and def2-TZVP,…”

Tetrapyrroles at near-ambient pressure: porphyrins and phthalocyanines beyond the pressure gap

“…In the specific, the reactivity is mainly determined by four properties: (i) the embedded metal species, (ii) the peripheral structure of the tetrapyrrolic cage, (iii) the nature of the axial ligand (exploiting the trans-effect for tuning purposes), and (iv) the nature of the distal superstructure of the hosting protein [4]. These elements are of paramount importance, since they can be exploited for tailoring purposes in biomimetic systems (figure 9) [10,45]. At surfaces, the tuning role of axial ligands is played by the surface trans-effect [10], where hybridization of the LUMO with the substrate states takes place [46,47] while the surrounding metalorganic coordination environment (lateral interactions among tectons) governs the selfassembly [9,26,30,48,49], the charge delocalization [37], and the chemical/thermal stability [50].…”