Spin and electron distributions in heme-cyanide models and hemeproteins

Proton NMR spectra of low-spin Fe(III) cyanoprotoheme as prosthetic group in a number of proteins are presented. The diagonally positioned 1-, 5- and 3-, 8-methyl groups obey shifts proportional to the Fe(III)/(II) reduction potential Em7, which indicates a pseudo-contact interaction. The correlation with Em7 is understandable if one postulates an enhanced rhombic distortion, dominating the Fe-methyl dipolar interactions. Hartree-Fock-Slater quantum chemical calculations show no significant changes of spin density as a function of the Fe-L5 distance, except at the iron atom and predominantly in the 3dxz and 3dyz orbitals. 4p orbitals, on the other hand, uphold most of the changes of electron density. We also observe a principal difference in the amino acid sequences in the heme-accommodating pocket of oxygen carriers and two-electron transmitters.