Crystalline state disorder and hyperfine component line widths in ferric hemoglobin chains.

Area of Science:

• Biophysics
• Structural Biology
• Spectroscopy

Background:

• Horse ferric hemoglobin exhibits complex X-band electron paramagnetic resonance (EPR) spectra.
• Line width variations in EPR spectra are influenced by molecular structure and dynamics.

Purpose of the Study:

• To investigate heme orientation disorder in horse ferric hemoglobin single crystals using EPR.
• To quantify the disorder angle and its impact on spectral line widths.
• To compare spectral properties in crystalline state versus frozen solution.

Main Methods:

• X-band electron paramagnetic resonance (EPR) spectroscopy on single crystals.
• Analysis of line width variations and g-tensor anisotropy.
• Modeling of Gaussian disorder on a sphere.
• Simulation of EPR spectra using hyperfine couplings and disorder model.

Main Results:

• Observed line widths varied significantly, up to 400 G, consistent with heme orientation disorder.
• A constant disorder angle of approximately 4 degrees was determined for both alpha- and beta-chains.
• Treatment with heavy water increased the observed disorder.
• Component line widths in crystalline ferric hemoglobin were smaller than in frozen solution.

Conclusions:

• Heme orientation disorder significantly impacts EPR spectra of horse ferric hemoglobin.
• The determined disorder angle provides a quantitative measure of structural heterogeneity.
• EPR spectroscopy is a valuable tool for studying molecular disorder in biological systems.