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Nuclear Magnetic Relaxation Dispersion in Protein Solutions

I. APOTRANSFERRIN

From the ¹ IBM Watson Laboratory, Columbia University, New York, New York 10025

We report measurements of the magnetic field dependence of the nuclear magnetic spin lattice relaxation rate T₁^-1 of solvent protons in solutions of the diamagnetic protein apotransferrin, as a function of temperature, concentration, and pH. The bulk of the data were taken over a range of magnetic field (4 Oe to 1000 Oe) much lower than that traditionally used in nuclear magnetic resonance experiments. The major result in that T₁^-1 is highly magnetic field-dependent; it is very large below 10 Oe, and decreases monotonically as the magnetic field is increased. The data are compared with the predictions of several models for the interaction of solvent protons with the protein molecules, among them the model (Daszkiewicz, O. K., Hennel, T. W., Lubas, B., and Szczepkowski, T. W., Nature, 200, 1006 (1963)) in which water molecules bind irrotationally to the protein surface. The theory for all of the models is very similar. The conclusions are that proton exchange between the bulk water and the protein surface occurs via the exchange of water molecules in the pH range 5 to 8. At higher pH values, direct proton exchange with basic amino acid residues may be important. The theory shows that exchanging water molecules need not be irrotationally bound to the protein; it is sufficient that the water molecules be restricted to rotate about an axis that maintains a fixed geometry with respect to the protein molecule. The number of water molecules involved is a small fraction of the number usually considered to be in the first hydration shell. From the temperature dependence of the data, it is shown that the variation of T₁^-1 with magnetic field is related to the rotational diffusion relaxation time of the protein molecules, for all models considered. Additionally, the lifetime on the protein molecules of the exchanging entities, whether entire water molecules or individual protons, is in the range 0.1 to 10 µsec.

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