Dynamics of xenon inside hydrophobic cavities as probed by NMR relaxation of dissolved laser-polarized xenon

Longitudinal relaxation times T, of laser-polarized xenon in water solutions containing alpha-cylodextrin and wheat nonspecific lipid transfer protein have been measured. By monitoring the xenon signal decay curves at different solute concentrations and different xenon pressures, we have been able to estimate the binding constant of xenon in the molecular hydrophobic cavities and to extract the xenon longitudinal self-relaxation time of the pure complex form T-lcomplex. By studying the variation of T-lcompex with the static magnetic field, we have estimated the correlation times of the xenon-proton dipolar interactions as well as the contribution of chemical shift anisotropy to the xenon relaxation. It appears that in the two studied cases the correlation time of xenon-proton dipolar interactions is between the overall correlation time of the host molecule and the translational correlation time of xenon diffusing freely around solute protons. In particular in the case of the protein, it is typically one-tenth of the overall rotational correlation time, which explains the success of magnetization transfer from dissolved laser-polarized xenon to protein protons. It also appears that the contribution from chemical shift anisotropy to relaxation is very limited. The influence of these results on the SPINOE experiments and on the energetic mechanisms inducing xenon binding is discussed.