Magnetic cross-relaxation among protons in protein solutions

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Abstract
The magnetic spin-lattice relaxation rates of solvent water nuclei are known to increase upon addition of diamagnetic solute protein. This enhancement of the relaxation rate is a function of magnetic field, and the orientational relaxation time of the protein molecules can be deduced from analysis of the field-dependent relaxation rates. Although the nature of the interactions that convey information about the dynamics of protein motion to the solvent molecules is not established, there is a contribution to the relaxation rates of solvent protons that plays no role in the relaxation of solvent deuterons and 17O nuclei. The additional interaction arises from a cross-relaxation process between solvent and solute protons. A heuristic 3-parameter model in which protein protons and solvent protons are considered as 2 separate thermodynamic systems that interact across the protein-solvent interface are introduced. The 3 parameters are the intrinsic relaxation rates of each system and a cross-relaxation term. The sign of the latter term must always be positive for all values of magnetic field, for magnetization energy to flow from the hotter to the cooler system. The magnetic field-dependence of the cross-relaxation contribution is much like that of the remaining solvent proton relaxation, i.e., about the same as the deuteron relaxation field dependence. This finding is not compatible with the predictions of expressions for the cross-relaxation that have been used by other authors, but not applied to data over a wide range of magnetic field strength. The model predicts that the relaxation behavior of the protein protons and the solvent protons is the sum of 2 exponentials, the relative contributions of which would vary with protein concentration and solvent isotopic composition. This suggests the presence of 2 classes of protein protons, when there is in reality only 1. This finding has immediate implications for the interpretation of published proton relaxation rates in complex systems such as tissues; these data should be reexamined with cross-relaxation taken into account.