Spin-state equilibrium in the model complexes of azide hemoprotein

Area of Science:

• Biochemistry
• Spectroscopy
• Thermodynamics

Background:

• Ferric protohemin complexes with imidazole ligands are crucial models for hemoproteins.
• Azide binding to hemoproteins influences their spin state and function.

Purpose of the Study:

• To synthesize and characterize mixed ligand complexes of ferric protohemin with azide and methylimidazole ligands.
• To investigate the effect of methylimidazole substitution on the electronic and spin properties of azide-bound hemin models.
• To determine the thermodynamic parameters of thermal spin equilibria in these model systems.

Main Methods:

• Visible absorption spectroscopy to monitor complex formation and spectral changes.
• Infrared spectroscopy to identify high- and low-spin states of the iron-bound azide.
• Analysis of temperature-dependent spectral data to calculate thermodynamic values (ΔH and ΔS) for spin transitions.

Main Results:

• Formation of mixed ligand complexes, hemin-N3-1-MeIm and hemin-N3-2-MeIm, with distinct visible absorption spectra.
• Azide-bound hemin models exhibit characteristic infrared stretching bands indicative of spin states.
• Thermodynamic values for spin equilibria were determined: ΔH = -3920 cal/mol, ΔS = -11.1 e.u. for 1-MeIm and ΔH = -2150 cal/mol, ΔS = 7.9 e.u. for 2-MeIm.
• The 1-MeIm complex shows thermodynamic similarity to azide metmyoglobin, suggesting minimal contribution of nonbonded porphyrin-globin contacts.

Conclusions:

• The study successfully characterized novel azide-bound ferric protohemin model complexes.
• The thermodynamic data suggest that spin equilibrium mechanisms in hemin models share common features with those in hemoproteins.
• These findings contribute to understanding the fundamental principles governing spin transitions in heme-containing biological systems.