Spin–orbit coupling in the excited states of ferricytochrome c and deoxyhemoglobin studied by magnetic circular dichroism

Abstract

Magnetic circular dichroism of ferricytochrome c and deoxyhemoglobin has been measured through their Soret bands at 300 and 80 °K. The MCD of both molecules is found to be temperature dependent (as 1/T) and to have derivative shape in respect to absorption band. The observed MCD is concluded to be the sum of closely spaced paramagnetic C-type effects of opposite signs. The most striking observation is the sign difference for MCD spectra of low spin (S=1/2) ferric heme in cytochrome c and high spin (S=2) ferrous heme in deoxyhemoglobin. The obtained results are interpreted in terms of spin orbit coupling. Two possible mechanisms of spin orbit coupling are discussed. One of them is the usual interaction of the angular momentum of an electron with its own spin, the other is the interaction of circular orbital motion of an electron with a foreign spin located at the metal atom inside the circular current. The two mechanisms give spin orbit coupling constants of opposite sign. The observed MCD seems to indicate the former mechanism to be dominant in the deoxyhemoglobin and the latter one to be dominant in ferricytochrome c.