Analysis of the principal g-tensors in single crystals of ferrimyoglobin complexes.

This paper describes a method of determining the directions of the principal axes of the g-tensors in single crystals from measurements of the g-value variation in three crystalline planes (ab, bc*, ac) and of the principal values of g-tensors. Measurements of paramagnetic resonance spectra of ferrimyoglobin (Mb(Fe3+)) complexes (Mb(Fe3+).CN-, .N3-, .imidazole(Im.) and .OCN-) in single crystals provided detailed information on the electronic state of the Fe3+. The direction of the z axis in Mb(Fe3+).CN-, .N3- and .Im. is not parallel to that in Mb(Fe3+).H2O, which has been used as the haem normal, where the z axis is one of the principal axes of the g-tensors corresponding to the maximal (in low-spin state of Fe3+ in haem) or minimal (in high-spin state of Fe3+ in haem) g-value. In Mb(Fe3+).OCN-, however, which is in thermal equilibrium between high-spin and low-spin states, the directions of the z axis in both states seem to be perpendicular to the haem plane. The direction of the x axis is not parallel to the plane of the linked histidine ring.