Abstract
The variation of electron spin resonance lineshapes with orientation of single-crystal myoglobins are studied for high-spin (S=52) Fe3+ in met-myoglobin and low-spin (S=½) Fe3+ in myoglobin azide. It is found that for both these crystals random variations of about 2° in the orientation of the symmetry axes contribute significantly to the observed linewidths. For met-myoglobin, these misorientations are sufficient to explain the observed angular dependence of the linewidths. In the case of myoglobin azide, however, there is an additional broadening due to a 3.5% random variation in the rhombic molecular field. Theoretical expressions describing the effects of random distributions of rhombic field parameters on linewidth are derived and fitted to the observed linewidths. Further lineshape changes are interpreted in terms of an anisotropic-exchange interaction between equivalent paramagnetic sites. A brief account of attempts to study the electron nuclear-double resonance (ENDOR) spectrum of the myoglobin is given.
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