Line shape studies of the electron spin resonance spectra of manganese protein complexes

The ESR spectra of Mn²⁺,Ca²⁺-concanavalin A and Mn²⁺-soybean agglutinin have been interpreted in terms of a spin Hamiltonian with a dominant Zeeman energy, an isotropic hyperfine interaction, and an anisotropic zero field splitting term of general orthorhombic symmetry. Line positions and intensities have been corrected up to third order (perturbation) terms and line shapes of lyophilized powders have been calculated. A rapid and simple analysis of dynamic line shapes of these asymmetric Mn²⁺ sites in the limit of slow motion, as appropriate for macromolecular complexes, is proposed. The method is based on the use of a closed expression and on the knowledge of the static powder parameters and a variable correlation time. We apply it to the 35-GHz ESR spectrum of an aqueous solution of Mn²⁺,Ca²⁺-concanavalin A and find that the correlation rate of the manganese ion is similar to that of the entire protein.