Enhancement of proton NMR using laser-polarized xenon
In 1996, G. Navon, A. Pines and co-workers (Science, 271, 1996, 1848) have proposed to use laser-polarized xenon to enhance proton spectroscopy thanks to longitudinal dipolar xenon-proton cross-relaxation. In 1999, we were the first group to observe such a polarization transfer to protons of a molecule dissolved in water. We were then, the first to be able to characterize a protein hydrophobic cavity using this procedure. In the following years a complete procedure was developed with the characterization of xenon affinity to the protein, the structure and xenon dynamics inside the cavity and from inside to outside.
The advantage of this approach is its simplicity: one has just to add polarized xenon inside a solution to improve (if the xenon spin temperature is positive) the proton polarization. Nevertheless its efficiency is very low. This results from the smallness of the xenon-proton dipolar cross-relaxation due to (i) the gyromagnetic ratio of xenon (about ¼ of the proton one) (ii) the long xenon-proton distance (due in particular to the large xenon van der Walls diameter equal to 0.42nm) and (iii) the short correlation time resulting from the weak nature of xenon-molecule interactions. In contrast, the efficiency of the proton self-relaxation tends to wash out this enhancement. At the end of the day, this approach appears useful only when the proton longitudinal self-relaxation T1 is very long or when there is an effective affinity between xenon and the host molecule.
In 2005, we have introduced a new procedure to perform polarisation transfer from laser-polarized xenon to proton. This procedure, so-called SPIDER, takes benefit from the large distant dipolar fields created by concentrated and polarized xenon solution to perform a polarization transfer to protons in the Hartmann-Hahn conditions.