Mapping local protein electrostatics by EPR of pH-sensitive thiol-specific nitroxide

Area of Science:

• Biophysical Chemistry
• Chemical Biology
• Spectroscopy

Background:

• Development of novel spin-labels is crucial for studying biological systems.
• Existing labels have limitations in specificity and pH sensitivity.
• Thiol-specific labeling requires precise chemical tools.

Purpose of the Study:

• To synthesize and characterize a novel thiol-specific, pH-sensitive nitroxide spin-label, IMTSL.
• To evaluate its utility in determining pKa values and its reactivity with biological molecules.
• To assess its impact on protein structure.

Main Methods:

• Synthesis and characterization of IMTSL.
• Electron Paramagnetic Resonance (EPR) spectroscopy at X-band and W-band.
• pH and solvent polarity studies.
• Labeling of cysteine, glutathione, and iso-1-cytochrome c.
• Circular Dichroism (CD) spectroscopy.

Main Results:

• IMTSL was successfully synthesized and characterized as a thiol-specific, pH-sensitive spin-label.
• Accurate pKa determination was achieved using W-band EPR, independent of measurement frequency or spin exchange regime.
• IMTSL reacted specifically with yeast iso-1-cytochrome c, yielding EPR spectra similar to MTSL.
• CD data confirmed no significant perturbation of protein structure upon IMTSL labeling.
• Correlation between g iso and A iso differed for neutral and charged IMTSL forms, attributed to solvent effects.

Conclusions:

• IMTSL is a valuable new tool for thiol-specific labeling and accurate pKa determination in biological studies.
• Its specific reactivity and minimal structural impact make it suitable for protein studies.
• The observed spectral correlations provide insights into solvent effects on spin-label properties.