Spin fluctuations of paramagnetic iron centers in proteins and model complexes: Mössbauer and EPR results

We have analyzed the Mössbauer spectra of several iron proteins in the regime of intermediate fluctuation rates of the magnetic hyperfine interaction using the dynamic line shape model of Clauser and Blume. All Mössbauer data were recorded in applied fields so that the electron Zeeman interaction is much larger than the hyperfine splittings. We modeled the transitions between the eigenstates of the electronic spin Hamiltonian by an effective spin–phonon coupling V⁽¹⁾=V₀∑′_i,j=x,y,zS_i S_j and described the phonons in the Debye approximation. With V₀ as the only adjustable parameter the model reproduced the major temperature dependence of the spectral shapes for spins from S=1 to S=5/2. We found V₀ to lie in the narrow range 3 K ≤V₀/k≤19 K, which is of the order of the zero‐field splittings, but does not correlate with it. The smallest values of V₀ were observed in the four‐coordinate iron–sulfur proteins. Refinements and implications of the model are discussed.