A novel approach to water proton relaxation in paramagnetic ion-macromolecule complexes

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Abstract
The use of the proton relaxation enhancement (PRE) technique in solutions of macromolecules and paramagnetic ions to obtain information on molecular motion and ion-hydration numbers can be hazardous with the possibility of large errors in the derived parameters. A method is described based on solvent proton-deuteron comparative relaxation measurements at a limited number of different magnetic fields for obtaining, reasonably accurately, rapidly, and unambiguously, certain of the parameters normally obtained from a PRE study. In the non-immune rabbit immunoglobulin G(IgG).cntdot.Gd(III) system, values were obtained for the ion hydration number (q), the residence lifetime of H2O molecules in the hydration sphere of the ion (.tau.M) and a correlation time of 5 ns, which can be associated to a large part with a rotational correlation time (.tau.R) for the Fc portion (C-terminal half of heavy-chain dimer) of the IgG molecule. This shows that there is rapid internal motion in the Fc region. The limitations and possible applications of the method are discussed.

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