Magnetic circular dichroism of myoglobin-thiolate complexes

Area of Science:

• Biochemistry
• Spectroscopy
• Bioinorganic Chemistry

Background:

• Myoglobin (Mb) is a heme protein involved in oxygen transport.
• Thiolate ligands can interact with heme iron in proteins.
• Magnetic Circular Dichroism (MCD) spectroscopy is a sensitive tool for studying electronic transitions in metalloproteins.

Purpose of the Study:

• To characterize the interaction of myoglobin with various thiolate and sulfur-containing compounds using MCD spectroscopy.
• To compare the MCD spectra of myoglobin complexes with those of known heme proteins like cytochrome P-450 and cytochrome c.
• To investigate the influence of pH and reducing agents on myoglobin-thiolate complex formation and stability.

Main Methods:

• Preparation of myoglobin complexes with ethyl, n-propyl, isopropylmercaptan, cysteine, cysteine methyl ester, 2-mercaptoethanol, and sodium sulfide.
• Measurement of Magnetic Circular Dichroism (MCD) spectra of these complexes across different pH values.
• Reduction of complexes using sodium dithionite to observe spectral changes indicative of ligand release or protein reduction.

Main Results:

• Metmyoglobin (MetMb) complexes with alkylmercaptans exhibited MCD spectra similar to cytochrome P-450, independent of pH.
• Cysteine and cysteine methyl ester formed complexes with MetMb only at pH 9.18, showing P-450-like MCD spectra.
• 2-Mercaptoethanol and sodium sulfide showed complex interactions, including reduction of Mb and potential saturation of porphyrin ring double bonds by sulfur.

Conclusions:

• Myoglobin's interaction with thiolate ligands is pH-dependent and can yield spectral characteristics resembling cytochrome P-450.
• Reduction of MetMb-thiolate complexes by sodium dithionite leads to thiolate anion release.
• Sulfur binding to myoglobin may involve saturation of porphyrin ring conjugation, altering spectral properties.