Electron Spin Resonance Spectroscopy of Single Crystals of Concanavalin A

An electron spin resonance (esr) study, at 35 GHz, of single crystals of Mn²⁺,Ca²⁺-concanavalin A is reported. The Mn²⁺spectrum is interpreted in terms of a spin hamiltonian with an isotropic g tensor, nearly isotropic hyperfine tensor, and a quadratic zero field splitting (ZFS) interaction. The ± 5/2 ↔ ±3/2 and ±3/2 ↔ ±½ fine structure transitions are highly broadened by a large distribution in the ZFS parameters, making a quantitative interpretation of these lines impractical. The ½ ↔ — ½ transitions are relatively sharp and show sufficient second-order effects to allow a detailed study of the magnetic parameters of the Mn²⁺ions. These are: g_Mn= 2.0009, A_||= 94.4 G, A_⊥= 91.5 G, D = 232 G, and λ = E/D = 0.185. The distribution in the ZFS parameters is estimated to be ≤80 G and ≤5° in the magnitude and direction of its major component. It is suggested that this spread reflects a distribution in the local ligand structure of the metal binding site.