Change in electron and spin density upon electron transfer to haem

Area of Science:

• Biochemistry
• Quantum Chemistry
• Biophysics

Background:

• Haems are essential cofactors in cytochromes, catalyzing biological electron transfer.
• Haems feature a central iron atom within a porphyrin ring structure.
• Spectroscopy indicates ferric low-spin haem has one unpaired electron, which pairs upon reduction.

Purpose of the Study:

• To investigate the electronic changes in a haem model upon reduction using quantum chemical calculations.
• To understand the delocalization of electrons and charge distribution during haem reduction.
• To explore the implications of these changes for protein environments and electron transfer rates.

Main Methods:

• Quantum chemical calculations were performed on a model of haem.
• Analysis focused on electron delocalization and charge distribution upon reduction.
• Comparison of formal oxidation states with calculated charge changes.

Main Results:

• Reduction induces electron delocalization from the iron to the porphyrin periphery.
• The iron atom's charge change is minimal (~0.1 electrons) despite a unit change in formal oxidation state.
• Significant charge delocalization is observed, extending to porphyrin substituents.

Conclusions:

• Haem reduction involves extensive electron delocalization, not just localized charge change at the iron.
• This delocalization is crucial for haem's function within the low-dielectric environment of proteins.
• Quantum mechanical effects, like electron delocalization, play a vital role in biological electron transfer processes.