Crossing of low-lying electronic levels of high-spin ferrous ion in deoxyhemoglobin and deoxymyoglobin

Abstract

The electronic structure of high-spin S  2 ferrous ion in deoxy forms of hemoglobin and myoglobin is considered in terms of spin Hamiltonian formalism. Spin Hamiltonian parameters of the second order B 0 (2)( D), B 2 (2)( E) and, for the first time in the available literature, of fourth order B 0 (4), B 2 (4) and B 4 (4), are calculated for the rhombic symmetry case of Fe 2+. The Hamiltonian matrix is diagonalized for several sets of B ( k) q parameters compatible with other experimental data. The low-lying Fe 2+ levels exhibit crossings in a high magnetic field, applied along the z-axis perpendicular to the heme plane. The cross-over values of the magnetic field are determined to be H c1 = 46 kOe and H c2 = 168 kOe for D = 5.2, E = 0.6 cm −1 (close to the magnetic data of Nakano, N., Otsuka, J. and Tasaki, A. (1972) Biochim. Biophys. Acta 278, 355–371) and with B (4) 0 = 0.037, B (4) 2 = 0.005, B (4) 4 = 0.013 cm −1 and g z = 2.028. Experimental techniques for measurement of the crossing effects are discussed.