Magnetic circular dichroism and spin equilibrium of methemoglobin and its subunits.

The intensity of the Soret magnetic circular dichroism (MCD) spectra of various complexes of methemoglobin subunits (alpha + and beta +) as well as methemoglobin (metHb A) was correlated well with the spin states of ferric heme. Upon the subunit association, spin state transition toward higher spin was observed only in high spin derivatives and the changes in spin state were due to mainly those of beta + chains. The effect of an allostric effector, inositol hexaphosphate (IHP), on the MCD spectra of metHb A derivatives was observed much significantly for high spin forms than low spin ones.