Hyperfine shifts in low-spin iron(III) hemes: A ligand field analysis

The hyperfine shifts of heme nuclei in low-spin iron(III) porphyrins of known geometry are predicted from: (i) a ligand field analysis, and (ii)the Kurland and McGarvey theory (R.J. Kurland, B. R. McGarvey, J. Magn. Reson. 1970, 2, 286-301). The ligand field parameters are optimized to reproduce the experimentally available g values and magnetic susceptibility anisotropy, the latter being obtained from the analysis of pseudo-contact shifts. Simple known geometric relationships (I. Bcrtini, C. Luchinat, G. Parigi, EA. Walker, J. Biol. Inorg. Chem. 1999, 4, 515-519; N. V. Shokhirev, EA. Walker, J. Biol. Inorg. Chem. 1998, 3, 581-594), are used to relate the contact shifts to the various heme positions. The systems investigated are: metMyoglobin cyanide, cytochrome bs and cytochrome c. It is shown that the contact hyperfine coupling constants recalculated from this approach are different to those obtained through the simple and generally used McConnell equation, and that the deviation can be more or less severe, depending on the position on the heme and on the orientation of the axial ligands. The contact shift tensor is highly anisotropic and therefore the shift is dependent on the orientation of the molecule in a magnetic field. This theoretical analysis is necessary to interpret ENDOR spectra and NMR spectra in partially oriented systems.