Sign and Orientation Determination of the Principal Axis of the Electric Field Gradient in Fe57 Enriched Deoxygenated Myoglobin Single Crystals

Abstract

Single crystals of myoglobin enriched on Fe57 with a diameter of about 0.5 mm were prepared. After deoxygenation crystallographic alignment and Mössbauer measurements were carried out at 77°K. In the experiment eight different orientations were selected by rotating the crystal around the b axis, thus, b axis and γ-ray propagation direction remained perpendicular to each other. From the intensity ratio of the quadrupole split spectra we derive the sign and the orientation of the principal axis of the electric field gradient. We find Vzz to be positive and forming an angle of 40° to the b axis. From two alternatives consistent with our experiments we conclude that VZZ is most likely oriented along one of the four Fe-N directions in the heme.