Nitrosyliron(III) hemoglobin: autoreduction and spectroscopy

Area of Science:

• Bioinorganic Chemistry
• Biophysics
• Spectroscopy

Background:

• Nitrosyl complexes of heme proteins (myoglobin, hemoglobin) and synthetic porphyrins are crucial for understanding nitric oxide (NO) interactions.
• Iron's electronic state and coordination environment significantly influence the spectral and redox properties of these complexes.

Purpose of the Study:

• To prepare and characterize nitrosyl complexes of various iron(III) heme proteins and synthetic porphyrins.
• To investigate the electronic structure, spectral properties, and autoreduction kinetics of these nitrosyl complexes.
• To elucidate the mechanism of nitric oxide reactions with ferric heme proteins.

Main Methods:

• Preparation of nitrosyl complexes of myoglobin, human hemoglobin, Glycera dibranchiata hemoglobins (Hbm, Hbh), and synthetic iron(II)/iron(III) porphyrins (e.g., OEP).
• Electron Spin Resonance (ESR) spectroscopy to probe paramagnetic states and coordination structures.
• Optical absorption and Circular Dichroism (CD) spectroscopy to analyze electronic transitions and chiral contributions.
• Kinetic studies to determine the rates and mechanism of autoreduction.

Main Results:

• Iron(III) heme proteins were ESR silent, while hexacoordinate structures were indicated for certain complexes.
• Distinct optical spectral features (band splitting) were observed in Mb(III)NO and Hbh(III)NO, attributed to distal histidine influence.
• Autoreduction of Hb(III)NO to Hb(II)NO was observed, following first-order kinetics, with varying rates across different hemoglobins (Mb < Hbm < Hbα < HbA).
• The reaction mechanism involves reversible formation of an iron(III) adduct followed by reaction with another NO molecule, potentially via a trans-heme-(NO)2 intermediate.
• The overall reaction was found to be photoenhanced.

Conclusions:

• The distal histidine plays a significant role in modulating the electronic properties of nitrosyl heme complexes.
• Autoreduction of ferric nitrosyl heme proteins is a key process in NO-heme interactions, with protein structure dictating the rate.
• A multi-step mechanism involving NO nitrosylation is proposed for the reaction of NO with ferric heme, influenced by light.