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Electron paramagnetic resonance study of cytochrome C solutions and single crystals Mailer, Colin

Abstract

Electron paramagnetic resonance (EPR) signals from tuna ferricytochrome c solutions were obtained between 4.2°K and 77°K, with g-values g(1) = 1.25, g(2) = 2.25, g(3) = 3.05« The g(3) line is 380 gauss wide between 4.2°K and 50°K with Gaussian shape, but has become 700 gauss wide with Lorentzian shape at 77°K. The temperature independent shape and width are best explained by a distribution of rhombic crystal field potentials (r.m.s. deviation = 11%). The Lorentzian shape arises from a short (10(-8) sec.) spin-lattice relaxation time. EPR spectra from horse heart ferricytochrome c single crystals were analysed to obtain the orientation of the g-axes relative to the crystallographic axes. The g(3)-axis was 76° from the crystal c-axis, close to the heme normal (71.5° to c-axis) determined from the 3-dimensional X-ray structure by Dickerson. The other 2 g-axes lay approximately along the N-Fe-N directions in the heme ring. An amended version of Eisenberger and Pershan's theory was used to explain the angular variation of the broad lines (300-2000 gauss) seen in the crystals—best fit was obtained with the distribution of ligand fields from the solution study plus a 1.5° variation in g-axis orientation. The undifferentiated absorption line shapes observed at 4.2°K in both solutions and single crystals were explained by the Portis theory of rapid adiabatic passage in solids. This theory was tested with a model system of charred dextrose, and found to be valid. Using the theory the relaxation time (τ) of the cytochrome c system was found to be, from the phase lag of the EPR signal relative to the magnetic field modulation, 3.8 x 10(-6) sec. at 4.2°K. τ was obtained between 4.2°K and 18°K from the rapid passage signals, and between 50°K and 70°K from the linewidth of the spectra. The temperature dependence of τ below 20°K could arise from a combination of a T(9) Raman spin-lattice relaxation process with a temperature independent spin-spin relaxation time of order 10(-8) seconds (which might arise from dipolar interactions between neighboring iron atoms).

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