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

In X-band electron paramagnetic resonance spectra from single crystals of horse ferric hemoglobin, observed line widths at the low- and high-field extrema are 30 and 24 g, and as much as 400 G in the intermediate region. This behavior is similar to that of ferric myoglobin. Due to large anisotropy in the g-tensors, the line width variation can be accounted for on the basis of heme orientation disorder. This disorder is characterized by an angle, determined here by two independent methods. In these computations Gaussian disorder on a sphere is assumed. The disorder angle is found to be constant on the sphere and about 4 degrees for both alpha- and beta- chains. Treatment of crystals with heavy water (buffer) increases the disorder. Since ligand nitrogen hyperfine couplings are available from hemoglobin electron nuclear double resonance, single crystal electron paramagnetic resonance spectra can be simulated by superimposing hyperfine bands, where the line width of the component bands is a variable and the disorder model above is employed. Comparison with observed resonances fixes the hyperfine component line widths. These component line widths from ferric hemoglobin in the crystalline state are found to be smaller than those in frozen solution.