Electron Spin Resonance Spectroscopy of Aqueous Solutions of Concanavalin A

A detailed analysis of the esr spectra of aqueous solutions of concanavalin A at Q-band frequency is presented. Three species were studied: (i) Mn²⁺-concanavalin A; (ii) Mn²⁺, Ca²⁺-concanavalin A, and (iii) the α-methyl glucoside complex of the latter form. The spectra correspond to high spin Mn²⁺at the slow tumbling limit where only the — ½⟷½ fine structure transitions are observable. The other transitions are smeared out by the large anisotropy due to the quadratic zero field splitting interaction. The — ½ ⟷½ transitions exhibit sufficient structure due to second-order effects to allow a detailed analysis of the magnetic and dynamic parameters. It was found that in all three forms they are identical within the experimental accuracy to those found in single cyrstals of Mn²⁺, Ca²⁺-concanavalin A. Theoretical spectra of the —½ ⟷½ transitions were calculated for different values of line width and rotational correlation times and compared with the experimental spectra. From the line shape analysis, information on the correlation times associated with the Mn²⁺ions in the various concanavalin A forms was derived. It was found that for species ii and iii these correlation times could be identified with the diffusion of the molecule. In species i, i.e., Mn²⁺-concanavalin A, the observed correlation times were shorter, indicating some extra mobility of the Mn²⁺ binding site.