The Temperature Dependence of the Electron Paramagnetic Resonance Spectrum of Ferricytochrome c Solutions Between 4.2 °K and 77 °K

Electron paramagnetic resonance (E.P.R.) signals from tuna ferricytochrome c solutions were obtained in the temperature range 4.2–77 °K. The microwave power and temperature dependence of the spectra obtained fit the theory for a Kramer's doublet split by a magnetic field. The g-values obtained are 1.25, 2.24, and 3.05 in agreement with other workers (Salmeen, I., and Palmer, G.: J. Chem. Phys. 48, 2049 (1968)). The g = 3.05 line changes from 380 G wide at 20–40 °K with Gaussian shape to 700 G wide at 77 °K with Lorentzian shape. Analysis shows that the Gaussian shape can be explained by variation in the rhombic symmetry of the heme iron environment, a ± 6% spread in rhombic potential being required to give a 380 G line width. At higher temperatures the line appears to be determined by the electron spin-lattice relaxation time. Below 20 °K the E.P.R. absorption signal is saturated, and a large E.P.R. dispersion signal appears, due to fast passage effects, which can be used to give further information on electron spin relaxation times.