Revisiting the nitrosyl complex of myoglobin by high-field pulse EPR spectroscopy and quantum mechanical calculations

Abstract

The binding of NO to reduced myoglobin in solution results in the formation of two paramagnetic nitrosyl myoglobin (MbNO) complexes: one with a rhombic g-factor and the other with an axial one, referred to as the R- and A-forms. In spite of past extensive studies of MbNO by crystallography, spectroscopy and quantum chemical calculations it is still not clear what factors determine the appearance of the two forms. In this work we applied a combination of state of the art quantum chemical calculations and high field pulsed EPR spectroscopy (W-band, 3.4 T/95 GHz) to further characterize the two forms. Specifically, we have used (1)H and (2)H electron-nuclear double resonance (ENDOR) spectroscopy to identify and characterize the H-bond to the NO, and hyperfine sub-level correlation (HYSCORE) spectroscopy to determine the hyperfine and quadrupole interactions of the Fe(ii) coordinated (14)N of the proximal histidine (14)N(His93). The calculations employed quantum mechanics (QM), particularly density functional theory (DFT) methods in combination with molecular mechanics (MM) force-field to model the protein environment. Through QM/MM calculations of the EPR parameters we have explored their dependence on several geometrical factors of the Fe-NO bond and found those that reproduce the best experimental results. The spread of the W-band EPR spectrum of MbNO due to the g-anisotropy is large and there is a significant part of the spectrum where the R-form is the sole contributor. This allowed us to resolve some new characteristics of the R-form: (i) a NO-H hydrogen bond has been detected and characterized and through QM/MM calculations has been unambiguously assigned to (epsilon2)H(His64). (ii) The complete hyperfine and quadrupole interactions of (14)N(His93) have been determined and correlated with structural parameters again using QM/MM calculations. The agreement between the experimental results and calculations varied between excellent and good, depending on the EPR parameter in question. As for the more elusive A-form, the results only suggest that it does have a (14)N(His93) ligand with a hyperfine comparable to that of the R-form and it has less hydrogen bonding interaction with His(64). The calculations also established the orientation of the principal g-values, finding that they are closely related to the orientation of the NO bond. This information is essential for deriving structural information from the experimental orientation selective HYSCORE and ENDOR data.

Spectroscopic and Computational Study of a Nonheme Iron Nitrosyl Center in a Biosynthetic Model of Nitric Oxide Reductase

“…The two different components can be ascribed to either different orientations of NO bound to the Fe B site, or slight changes in the orientation of Fe II ligands upon NO binding. Multiple Fe‐NO symmetries with different rhombicities are commonly observed in heme nitrosyls 5g. 14 To test whether different components of the Fe B ‐NO complex observed here are a result of hyperfine interaction with the 14 N nucleus, EPR simulations were performed taking into consideration this interaction.…”

Conformationally Distinct Five-Coordinate Heme–NO Complexes of Soluble Guanylate Cyclase Elucidated by Multifrequency Electron Paramagnetic Resonance (EPR)

“…Recently the W-band EPR spectrum of the myoglobin six-coordinate Fe–NO complex was simulated as a superposition of a rhombic powder pattern and an axial powder pattern(30), and a similar analysis was applied to nitrite reductase (NIR)(31), also a six-coordinate nitrosylheme protein. The g tensors used for these simulations are included in Table 4.…”

“… e Six-coord Myoglobin from Morse and Chan(32). f Six coord Myoglobin from Radoul et al(30). g Six-coord Nitrite reductase from Radoul et al(31).…”

Oxidation of methane by an N-bridged high-valent diiron–oxo species: electronic structure implications on the reactivity

“…All spectroscopic parameter calculations incorporate a relativistic effect via a zeroth-order regular approximation method (ZORA) as implemented in ORCA suite. 66,67 The MBisomer shifts (IS) were calculated based on the calibration constants reported by Römelt et al and 0.16 barn was used for the calculation of quadruple moment of 57 Fe nuclei. 68 Calculation of g-anisotropy incorporates spin-orbit coupling using meanfield approximation and this methodology has been widely employed to compute the g-anisotropy.…”