Electromerism and linkage isomerism in biologically-relevant Fe-SO complexes

Area of Science:

• Bioinorganic Chemistry
• Computational Chemistry
• Enzymology

Background:

• Sulfur monoxide (SO) is an unstable molecule with limited study in bioinorganic chemistry.
• The catalytic cycle of sulfite reductases involves heme-SO/OS adducts.
• Understanding SO's coordination properties is key to elucidating sulfite reductase mechanisms.

Purpose of the Study:

• To investigate the electronic structure and stability of heme-SO/OS adducts using computational methods.
• To determine the preferred isomers and oxidation states of SO in the context of sulfite reductase catalysis.
• To explore the influence of the active site environment on iron-ligand bonding and electron transfer.

Main Methods:

• Density Functional Theory (DFT) calculations were employed.
• Analysis of four possible oxidation states for heme-SO/OS adducts.
• Investigation of FeOS and FeSO isomer energies and spin states.

Main Results:

• FeOS and FeSO isomers exhibit similar energies, indicating potential observability.
• FeSO isomers are more likely during the catalytic cycle, initiated by sulfur binding.
• Higher spin states, favoring weaker iron-ligand bonds, are prevalent in oxidized models relevant to early catalytic steps.

Conclusions:

• The study provides insights into the coordination chemistry of sulfur monoxide with heme.
• Computational findings support the proposed catalytic cycle of sulfite reductases, highlighting the role of SO isomers and spin states.
• The proximity of the iron-sulfur cluster enhances electron delivery to the heme, facilitating SO protonation and enzyme function.