[Electron structure of the active center of paramagnetic hemoproteins from the data of low-temperature magnetic circular dichroism. High-spin ferric derivatives].

The temperature dependences of magnetic circular dichroism (MCD) spectra of the high-spin fluoride complexes of metmyoglobin, methemoglobin and horse-radish ferriperoxidase have been studied and analysed in spectral region 250-1100 nm at temperatures from 2 to 300 K. Upon lowering of the temperature the increase of the MCD intensity was observed over all spectral regions studied, indicating the presence of the so called C-type effects. In contrast to other paramagnetic hemoproteins studied previously the significant changes of the shape of MCD spectra of fluoride complexes in the near UV B (pi--pi*)-band (Soret band) was observed at the lowest temperatures. The changes are explained by the superposition of effects of two types: the usual C-type effects which have the shape of absorption dispersion and of the derivative shape CA-type effects. The CA-type effects in the Soret band were analysed in terms of the theoretical model suggested previously. The data are in qualitative agreement with the prediction of the model. New information on the energies of the 4E and 4A2 excited states of the heme iron, on the orbital contribution into ground state paramagnetism and on the value of the pi--d pi-splitting, typical for B(pi--pi*)-state and ground Kramers doublet, were obtained on the basis of quantitative analysis of the MCD temperature dependences. The parameter D of zero-field splitting was estimated from MCD spectra and its value (6 cm-1) agrees well with that obtained by other methods.