Magnetic resonance relaxation of ¹H and ¹⁷O in aqueous solutions of concanavalin A

We have studied the nuclear magnetic resonance relaxation times of ¹H and ¹⁷O nuclei in aqueous solutions of Mn²⁺ complexes of concanavalin A. The results indicate that the Mn²⁺ is coordinated to one exchangeable water molecule. The mean residence time, τ_h, of this water molecule at the Mn²⁺ is approximately 1 · 10^-7 s in Mn²⁺-concanavalin A and is ten times as long in Mn²⁺, Ca²⁺-concanavalin A. No changes occur on binding of α-methyl glucoside, or on raising the pH to a value where molecular aggregation occurs. It is possible that on occupation of the Ca²⁺-binding site, the immediate electronic environment of the Mn²⁺ alters and thus affects the mean residence time of the water molecule. It is also possible that the increase in τ_h is brought about by restrictions imposed on the exchange of water molecules between bulk solvent and the Mn²⁺.