Magnetic-circular-dichroism studies of haem a and its derivatives

Area of Science:

• Biochemistry
• Spectroscopy
• Chemical Physics

Background:

• Heme proteins are crucial in biological systems.
• Understanding the electronic structure of heme derivatives is vital for elucidating their function.
• Magnetic circular dichroism (MCD) spectroscopy is a powerful tool for probing electronic transitions in molecules.

Purpose of the Study:

• To investigate the low-temperature magnetic circular dichroism (MCD) spectra of various haem a derivatives.
• To determine the spin state and electronic configuration of iron in reduced and oxidized haem a under different conditions.
• To assign electronic transitions within the haem a spectrum based on spectral changes.

Main Methods:

• Low-temperature MCD spectroscopy.
• UV-Vis absorption spectroscopy.
• Chemical reduction using sodium dithionite.
• Complexation with pyridine and imidazole.

Main Results:

• Reduced haem a in pyridine was identified as a low-spin ferrous species.
• Oxidized haem a in the presence of excess imidazole exhibited a low-spin ferric form.
• Addition of sodium dithionite to haem a in excess pyridine induced spectral changes similar to haems b and c, suggesting modification of the 8-formyl group.

Conclusions:

• The study provides insights into the electronic structure of haem a derivatives.
• The observed spectral changes indicate a reaction involving the 8-formyl group of haem a upon reduction in pyridine.
• These findings contribute to the partial assignment of the electronic spectrum of haem a.