Paramagnetic anisotropy measurements on a single crystal of deoxyhemoglobin

Abstract

The temperature dependence of the paramagnetic anisotropy of a single deoxyhemoglobin crystal (Form II) was measured by the torque method with a sensitivity of 10 −4 dyne·cm in the temperature range of 2 to 77 °K. Torque curves were measured in the three perpendicular planes of the crystal by a rotating magnetic field. The torque curves obtained were of a sinusoidal shape per half rotation of the magnetic field. The phases of the torque curves as well as the amplitudes changed with temperature. This means that the three principal values of magnetic susceptibility of heme depended on temperature and these temperature dependencies were not the same. The amplitude of the observed torque was very small when the magnetic field was rotated almost parallel to the heme planes in the unit cell, while it was very large when the magnetic field was rotated perpendicular to the heme planes. The directions of the magnetic field which gave zero torque values were nearly perpendicular and parallel to the heme plane. From these results, it was concluded that the magnetic susceptibility of heme in deoxyhemoglobin was fairly axially symmetrical, taking the axis to be nearly perpendicular to the plane of the heme, though in detail a small asymmetry in the heme plane existed. The temperature dependence of amplitude measured over a wide temperature range was analyzed on the basis of the spin Hamiltonian; H=DS Z 2+E(S X 2−S y 2)+β S g ̃ H . Experimental data were best represented by the theoretical curves which were calculated from the values of D = 5.3 and E = 0.9 cm −1 . This study made it clear that the previously reported result of D = −18 cm −1 was incorrect (M. Kotani, Ann. N.Y. Acad. Sci., 158 (1969) 20).