Pseudocontact chemical shift

From NMR Wiki

The pseudocontact chemical shift is a contribution to chemical shift that is caused by the presence of paramagnetic centers. It is also dependent on temperature (???), it has also an anisotropic part that could enhance relaxation, like the dipole N-H dipole in protein it could have correlated motion with the chemical shift, so their is more or less relaxation depending on the relative spin orientation, that's called cross correlated relaxation ( aka CCR )

Contents 1 Source of the pseudocontact shift 2 Application 3 Disadvantages 4 References

Source of the pseudocontact shift

The shift of nuclear larmor frequency is caused by the dipolar coupling between the magnetic moments of nucleus and the unpaired electron. The situation is somewhat similar to the case of j-coupling in a way that we would expect the NMR peak to split into a doublet due to the coupling with the magnetic moment of the unpaired electron. The significant difference however lies in the fact that the magnitude of Zeeman splitting of the electron in the field of the NMR spectrometer magnet is much larger then the average thermal energy, thus only the lower energy spin state of the electron is significantly populated. Therefore only one component of the doublet is observable and the effect is perceived as the perturbation of the chemical shift.

Application

This frequency shift could be used to have a long range distant - angle information on a protein, so it allow to build or refine protein structure. It allow also to locate the paramagnetic center.

Disadvantages

Paramagnetic centers enhance nuclear relaxationrement making NMR signals of atoms in the proximity of the paramagnetic center difficult to record.

We need to do two measurements, one to have only the diamagnetic contribution to chemical shift and one another to have both contributions.

References

1. Ivano Bertini and Claudio Luchinat, XC and Giacomo Parigi. Paramagnetic constraints: an aid for quick solution structure determination of paramagnetic metalloproteins. Conc Magn Res 14(4):259--286, Chichester, UK, UK, 2002. BibTeX [bertini02]

2. Mayo, BC. Lanthanide shift reagents in nuclear magnetic resonance spectroscopy. Chemical Society Reviews 2(1):49--74, 1973. BibTeX [mayo73]

3. Pintacuda, G and John, M and Su, XC and Otting, G. NMR Structure Determination of Protein-Ligand Complexes by Lanthanide Labeling. Acc Chem Res 40(3):206--212, 2007. BibTeX [otting07]