Solvent Proton Magnetic Relaxation Dispersion in Solutions of Concanavalin A

Abstract

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 τ_S 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 τ_S 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.