Electron paramagnetic resonance and magnetic susceptibility studies of dimanganese concanavalin A. Evidence for antiferromagnetic exchange coupling

Abstract

The double Mn2+ complex of concanavalin A with bound saccharide (SMMPL) was examined by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements. A room temperature X-band (9 GHz) EPR spectrum of SMMPL revealed a relatively weak, broad resonance in contrast to the spectrum with a six-line hyperfine-split pattern observed for the mononuclear, high-spin Mn2+ complex found in Ca2+-Mn2+-concanavalin A with saccharide present (SCMPL). The EPR spectrum of SMMPL at 77 K, however, consisted of a series of overlapping patterns of 11 hyperfine-split lines near g = 2.0 with members of each pattern separated by 47 G, half the value of the hyperfine splitting of SCMPL. These 11-line patterns are preserved at Q-band (35 GHz), indicating that the manganese ions in SMMPL form a spin-coupled, binuclear center. As expected for an exchange-coupled system, the EPR signal of SMMPL at 77 K saturates at a higher microwave power than those for SCMPL or Mn2+ aquoion. There is also a marked loss of EPR signal intensity for SMMPL between 4.2 and 1.4 K, which supports the view that the pair of manganese ions is exchanged-coupled. The temperature dependence of both the magnetic susceptibility and the low-temperature EPR spectral intensity can be explained by a model in which the two high-spin Mn2+ ions of SMMPL are antierromagnetically exchanged-coupled with an isotropic coupling constant J = 1.8 cm-1 (for the spin Hamiltonian Hex = JS1.S2). Zero-field splitting D' was estimated to be 375 G from the EPR spectrum.(ABSTRACT TRUNCATED AT 250 WORDS)

The Structural Features of Concanavalin A Governing Non-proline Peptide Isomerization

“…In the anomalous-difference Fourier maps of LMnCa ConA, Ca 2ϩ in S2 appears as the second highest peak with a ratio of Mn 2ϩ over Ca 2ϩ of 2.23 (theoretically 2.18 at 1.5418 Å (23)). This further confirms the presence of Mn 2ϩ in the S2 site of LMnMn pH7.0 ConA, in agreement with nuclear magnetic resonance dispersion experiments (14) and electron paramagnetic resonance and magnetic susceptibility studies (25).…”

Determination of the Metal Ion Separation and Energies of the Three Lowest Electronic States of Dimanganese(II,II) Complexes and Enzymes: Catalase and Liver Arginase

“…The presence in ConA of two p-carboxyl bridges (Asp10 and Aspl9) has been established by X-ray diffraction (Hardman et al, 1982) and is presumed to provide the dominant pathway responsible for the weak antiferromagnetic coupling between the Mn2+ ions in the dimanganese derivative of this protein (J is about -0.9 cm-'; Antanaitis et al, 1987). This example agrees with the correlation between small IDc[, i.e., higher symmetry of the ligand field around Mn(II), and the presence of a second divalent metal coordinated by one or more bridging ligands.…”

Evidence that protein B of the thiosulphate‐oxidizing system of Thiobacillus versutus contains a binuclear manganese cluster

“…Spectra of this type have been observed in model complexes with a pair of Mn(II) ions [17,18] and in dimanganese concanavalin A [19]. In all these cases, the manganese oxidation state is Mn(II).…”