Mössbauer studies of Fe2+ in anhydrous hemoglobin and its isolated subunits

Abstract

The Mössbauer spectra of dehydrated αSH and βSH deoxyhemoglobin subunits were obtained over a temperature range from 77 to 200°K and compared with anhydrous hemoglobin spectra. The Mössbauer data for both subunits show a superposition of a ferrous high spin (S = 2) and a ferrous low spin (S = 0) quadrupole doublet of approximately equal intensity. No differences either in the isomer shifts or the quadrupole splittings were detected between the three hemoproteins. Consequently, the model for AHb–A with the α and β chains in different spin states proposed by previous investigators is probably incorrect. The observed quadrupole splittings, ΔEQ(T), are analyzed in a ligand field approximation, which gives the electronic structure of the low lying eigenstates of the ferrous ion at the two spin sites. The principal components of the EFG for the low and high spin iron sites calculated from the ligand field model are normal to the heme plane and have the values at T = 80°K of −0.09e 〈r−3〉 and 0.21e 〈r−3〉, respectively. At T = 80°K the asymmetry parameter, η, is zero for the low spin iron site and 0.39 for the high spin site. A possible interpretation of the origin of the spin state mixture based on the disruption of the hydrophobic interaction upon dehydration is discussed.