The presence of a paramagnetic ion within a biomolecule (either naturally present or introduced by metal substitution in diamagnetic metalloproteins or by attachment of a paramagnetic tag) induces pseudocontact shifts, residual dipolar couplings and/or paramagnetic relaxation enhancements. The presence and the extent of these effects depend on the specific metal ion, and, among other parameters, on its magnetic susceptibility anisotropy and electron relaxation. We have pioneered the use of pseudocontact shifts, paramagnetic residual dipolar couplings and paramagnetic relaxation enhancements as restraints for protein solution structure determination. These restraints, which contain information on the relative orientation and/or distance with respect ot the magnetic susceptibility anisotropy tensor centered on the paramagnetic metal ion, have been shown to be very useful for the refinement of structural models and for the assembly of multidomain systems. They can also provide information on the maximum occurrence of protein conformations in flexible systems.

Pseudocontact shifts have also been measured in proteins at the solid state through MAS SS-NMR and used to determine the structure of proteins up to 17.6 kDa. At the solid state, attention has to be paid to the fact that the pseudocontact shift of each nucleus depends not only on the metal ion present in the same molecule, but also on the metal ions in close neighbouring molecules.



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