Pseudocontact chemical shift

From NMR Wiki

The pseudocontact is a contribution to chemical shift that is caused by the presence of heavy atoms paramagnetic centers, it display also a thermal sensibility, 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 shift 2 Application 3 Disadvantages 4 References

Source of the shift

The frequency shift is caused by the dipolar coupling from of an electron and a nucleus. The consequence for the electron will be a very small coupling, but for the nucleus the coupling will be very strong. Like for J-coupling the spectra should present a doublet, that's not the case for the nucleus, because the relaxation of the electron, is very fast in comparison to the frequency separation, so the two expected peak will be only one peak. But this peak will be slightly shifted because the population of the nucleus will not be the same for the higher peak of the doublet, that for the lower one.

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 ions causes also relaxation, this relaxation make the measurement of NOE more difficult in the proximity of the paramagnetic center.

- 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]