TEMPERATURE-DEPENDENCE OF THE SPIN S=1 2 FLUCTUATION RATE AND THE LOW-SPIN VERSUS HIGH-SPIN TRANSITION IN NATIVE CHLOROPEROXIDASE/

We analyzed two preparations of native, low-spin ferric chlroperoxides as a function of temperature with the following results. (i) The spin lattice transition rate W(T) is relative slow, following a power law W = 0.035 (T/K)(4.98) (rad/s) for one of the samples. (ii) The quadrup oIe splitting is strongly temperature dependent, dropping from \Delta E(Q)\ similar to 2.94 mm/s at 100 K to similar to 2 mm/s at 250 K. (ii i) Starting at 190 K, the low-spin heme iron in frozen adqueous soluti on converts reversibly to a high-spin form, reaching similar to 40% hi gh-spin at 250 K. The two forms appear to be in thermal equilibrium (i v) Optical data indicate that in a 70% glycerol glass, the conversion starts at lower temperature and reaches similar to 50% highspin at 190 K.