Dynamics of the spin transition in camphor-bound ferric cytochrome P-450 versus temperature, pressure and viscosity.

The kinetics and equilibrium of the low-spin (LS) to high-spin (HS) transition in camphor-bound ferric cytochrome P-450 have been measured versus temperature, pressure and viscosity at selected pH and KCl concentration. The results cannot be described by a single set of delta E degrees, delta V degrees and delta S degrees, since these parameters change with the solvent conditions. A three-state model can explain a large bulk of the data but it is clear that other substates are involved. The activation energy and volume are practically zero for the high to low spin transition, while the low to high spin transition parameters vary from 20 kJ/mol to 80 kJ/mol for delta E psi and from 5 cm3/mol to 80 cm3/mol for delta V psi. The observed relaxation time could be varied from about 10 ms to more than 1000 s by changing temperature, pressure and viscosity. Both kLH and kHL rate coefficients scale roughly as 1/eta, indicating that the slow spin change involves protein motions which are influenced by the solvent viscous forces.