Solvent Proton Magnetic Relaxation Dispersion in Solutions of Concanavalin A

Concanavalin A, a protein isolated from jack beans, exhibits several important biological properties, all of which are related to its ability to bind and precipitate specific polysaccharides. Concanavalin A is a dimer at pH 5.6, and has one transition-metal and one calcium-ion binding site per monomer unit of molecular weight 27,000. Both metal-ion sites must be occupied for the protein to be active. It is of interest to determine the role of the transition metal ion in Concanavalin A and its relationship to the sugar binding activity of the protein. We report the magnetic field and temperature dependences of the spin-lattice magnetic relaxation rates of solvent protons in aqueous solutions of zinc and manganese derivatives of Concanavalin A, and the influence of monosaccharide binding on these rates. The results of a leastsquares fit of the data to the theory, with five adjustable parameters, indicate that there is one rapidly exchanging water molecule ligand on the Mn2+ ion, with a residence lifetime of 2.5 μsec at 25°, and with its protons 0.27 nm (2.7 Å) from the Mn2+ ion. We find that at low magnetic fields (proton Larmor frequencies below about 10 MHz), the correlation time for the dipolar interaction between the Mn2+ electronic spin moment and the protons on the water ligand is the spin-lattice relaxation time τS of the Mn2+ moment, but that at higher magnetic fields the correlation time for the dipolar interaction is determined by the Brownian rotational tumbling of the protein, because of the substantial variation of τS with magnetic field. Monosaccharide binding to manganese Concanavalin A has little effect on the relaxation rates of solvent protons, a result that indicates that the sugars do not bind directly to the transition metal in the protein.