Correlation of empirical magnetic susceptibility tensors and structure in low-spin haem proteins

Experimental magnetic susceptibility tensors are reported for eight haems c with bis-His coordination. These data, obtained by fitting the dipolar shi fts of backbone protons in the tetrahaem cytochromes c(3) from Desulfovibri o vulgaris and D. gigas, are analysed together with published values for ot her haem proteins. The x and y axes are found to rotate in the opposite sen se to the axial ligands and are also counter-rotated with respect to the fr ontier molecular orbitals of the haem. The magnetic z-axis is close to the normal to the haem plane in each case. The magnitudes of the magnetic aniso tropies are used to derive crystal field parameters and the rhombic splitti ng, V, is correlated with the dihedral angle between the axial ligands. Hen ce, it is apparent that the axial ligands are the dominant factor in determ ining the variation in magnetic properties between haems, and it is confirm ed that "high g(max)'' EPR signals are a reliable indicator of near-perpend icular ligands. These results are in full agreement with the analysis of no n-Curie effects and electronic structure in the His-Met coordinated cytochr omes c and C-551. Collectively, they show that the orientations of axial li gands to the haem may be estimated from single-crystal EPR data, from C-13 NMR shifts of the haem substituents, or from NMR dipolar shifts of the poly peptide.