Influence of the R----T transition on the optical absorption and magnetic circular dichroism spectra of methemoglobin fluoride, aquomethemoglobin, and hydroxymethemoglobin.

From the observed absorption spectra of methemoglobin fluoride (metHbF) and hemoglobin monoxide (HbCO) and the magnetic circular dichoism (MCD) spectrum of HbCO, molecular quantities of the high-spin ferric heme group of metHbF and the MCD spectra of metHbF can be calculated [Rots, M. J. F., & Zandstra, P. J. (1982) Mol. Phys. 46, 1283]. In an essentially similar procedure and with the molecular quantities obtained for metHbF (in the quaternary R state), the contributions of the high-spin fractions to the absorption and MCD spectra of metHbF in the quaternary T structure and of aquo- and hydroxymethemoglobin (metHbH2O and metHbOH, respectively) in both the R and the T states are estimated. In these calculations, the energy of the iron egd orbital, which participates in the charge-transfer (CT) transitions, is used as an adjustable parameter. The changes in the absorption and MCD spectra of metHbF and metHbH2O upon the R----T transition are supposed to arise solely from the changes in the contributions of the high-spin fractions to the spectra, due to the change in energy of the iron egd orbital. The calculation shows that a decrease in the energy of the iron egd oribital of 0.017 and 0.008 eV for metHbF and metHbH2O, respectively, is likely, upon the R----T transition. The lowering of the egd orbital of the high-spin form of metHbOH is arbitrarily chosen as the average of these values. In the calculations for metHbOH, the increase of the high-spin fraction upon the R----T transition is taken into account.(ABSTRACT TRUNCATED AT 250 WORDS)