Calculated electron densities and experimental isomer shifts of Fe57 in the deoxy- and CO-compounds of myoglobin and hemoglobin

Abstract

Relativistic calculations of various electronic configurations of the iron atom were used in conjunction with Huckel-type self consistent field molecular orbital calculations for CO-myoglobin and CO-hemoglobin (mbCO, hbCO) and with limited configuration interaction calculations for deoxymyoglobin and deoxy-hemoglobin (mb, hb) to determine electron densities at the iron nucleus, ϱ(0). The calculations included all effects of overlap of iron core and next nearest neighbour (ligand) orbitals, and the effect of potential distortion of iron core orbitals due to molecular configurations 3d m4s n. From the calculated electron densities we found that the change in the experimental Mossbauer isomer shift, Δδ =δmbCO, mbCO −δmb,hb, was mainly due to changes in the so-called overlap distortion of iron cores orbitals. The considerably higher electron density ϱ(0) in mbCO, hbCO than in mb, hb corresponds to the stronger interaction between iron and ligands in mbCO, hbCO compared to mb, hb. From the calculated values for ϱ(0) and the experimental isomer shifts we derived an isomer shift calibration constant, α = Δδ/Δϱ(0), of the value −0.242 ± 0.039a 0 3 mm sec−1, which agrees reasonably well with the work of other investigators.