Measurement of the spin -lattice relaxation time from the broadening of the EPR spectrum of a randomly oriented system with [formula omitted]: Application to iron-sulfur proteins

Abstract

When the broadening of the EPR line due to the spin-lattice interactions is noticeable, the relaxation time T 1 can be deduced from width measurements. We consider a powder spectrum of spins S = 1 2 with an anisotropic g tensor, and show that in some cases T 1 may be readily obtained by using simple functions to approximate some parts of the spectrum. We test the accuracy of this method by applying it to calculated axial and rhombic spectra. It appears that, at least for spectra similar to those given by the iron-sulfur proteins, a precision better than 10% can be expected. The method is used to measure the spin-lattice relaxation time, in the range 50 to 128K, of the 2Fe2S proteins extracted from the two algae Spirulina maxima and Scenedesmus.