A Highly Ordered Nitroxide Side Chain for Distance Mapping and Monitoring Slow Structural Fluctuations in Proteins.

Area of Science:

• Biophysics
• Structural Biology
• Biochemistry

Background:

• Site-directed spin labeling electron paramagnetic resonance (SDSL-EPR) is crucial for studying protein structure and dynamics.
• Existing nitroxide spin labels have limitations due to internal flexibility and disulfide linkage instability, hindering analysis of slow motions and use of reducing agents.

Purpose of the Study:

• To develop a novel spin label with hindered internal motion for enhanced SDSL-EPR applications.
• To introduce a non-reducible thioether-linked spin label, R9, for improved protein structural and dynamic studies.

Main Methods:

• Introduction of the R9 spin label via selective thioether linkage to solvent-exposed cysteine residues.
• X-ray crystallography to determine R9 structure in T4 Lysozyme.
• Electron paramagnetic resonance (EPR) techniques including spectral lineshape analysis, Saturation Transfer EPR, and Saturation Recovery EPR to characterize mobility.
• Interspin distance measurements using dipolar spectroscopy and relaxation enhancement methods.

Main Results:

• The R9 spin label demonstrates hindered internal motion and a stable, non-reducible thioether linkage.
• Structural and mobility characterization of R9 in T4 Lysozyme helical sites.
• Successful interspin distance measurements between R9 pairs, indicating utility for distance mapping.

Conclusions:

• The R9 spin label is a valuable tool for monitoring slow internal structural fluctuations in proteins.
• R9 is anticipated to advance distance mapping applications in structural biology using SDSL-EPR.