Pseudocontact chemical shift

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The pseudocontact chemical shift is a contribution to chemical shift that is caused by the presence of paramagnetic centers, when the magnitude of the magnetic moment of the unpaired electron depends on the molecular orientation with respect to the magnetic field.

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. This 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 (true?), 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.

The observed value of this type of chemical shift perturbation equals a one half of the electron-nucleus dipolar coupling averaged over all the orientations that the molecule adopts in solution with respect to the magnetic field orientation. Therefore, for this kind of coupling/apparent peak shift to be observable, the magnitute of the electron magnetic moment must depend on the orientation, i.e. - must be anisotropic.

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. McConnell, HM and Robertson, RE. Isotropic nuclear resonance shifts. The Journal of Chemical Physics 29:1361, 1958. BibTeX [mcconnell]

2. 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]

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

4. 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]