Enhancement of proton NMR using laser-polarized xenon
- The SPINOE
approach
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.
Example of a selective SPINOE spectrum acquired on
Tobacco Lipid Transfer Protein (coll. C. Landon, CBS Orléans). The blue spectrum
(respectively green spectrum) is the protein proton NMR spectrum acquired with
positive (respectively negative) xenon spin temperature. The red spectrum is
the difference of the two. Peaks result from xenon-proton dipolar cross
relaxation. They indicate the nearby presence of xenon.
Location of xenon and its dynamics inside Tobacco
Lipid Transfer Protein deduced from SPINOE and relaxation measurement (coll. C.
Landon, CBS Orléans).
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.
- The SPIDER approach
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.