Quantitative evaluation of contributions to electron paramagnetic resonance line widths in ferric hemoglobin single crystals

The contributions to the dipolar broadening of ferric magnetic resonances, from crystals of hemoglobin for which the atomic coordinates are known, have been calculated. The total second moment of the g = 2 resonance so determined is about 50 (MHz)2 or 5.0 G (peak-to-trough), figures consistent with the range of values found from analysis of experimental data. Two-thirds of this second moment comes from the two protons of the H2O molecule coordinated to the iron. Treatment with D2O is predicted to reduce the total second moment at g = 2 to about 25 (MHz)2, whereas the experimental measurements on single crystals show no decrease. If the structure of the tetramer is assumed to be the same when in solution as in the crystal, the total second moment is readily redetermined for hemoglobin in solution; the value so obtained is found to be significantly smaller than that from analysis of the g = 2 resonance measured in frozen solution. These two unexpected observations can be explained in terms of distributions in spin Hamiltonian parameters, the spread depending upon the nature of the sample--crystal or solution, ordinary or heavy water-treated. This distribution in H2O and D2O solutions appears to be about the same, since the measured differences in component line width agree with the calculated difference in dipolar contributions.