Paramagnetic ions provide structural restraints in solid-state NMR of proteins

The number of structural restraints that can be obtained to solve 3D structures of proteins in the solid state (SS) is significantly lower than that in solution, and in relatively few cases, their skillful use has actually produced 3D structures. Paramagnetic metal ions, either as substitutes of diamagnetic metals in metalloproteins or inserted in suitably designed tags, are known to provide a wealth of structural restraints in solution, among which pseudocontact shifts are very precious. Here we show that pseudocontact shifts (pcs) generated by paramagnetic metal ions with sufficiently large spin quantum number and magnetic susceptibility anisotropy can be easily measured, and in large numbers, in SS spectra of proteins. The sample was microcrystalline cobalt(II)-substituted matrix metalloproteinase 12 (MMP-12, 17 kDa). About 250 pcs were observed for nuclei up to more than 20 A from the metal and were found in very good agreement with the calculated ones. A limited number of nuclei are also influenced by intermolecular interactions with paramagnetic metals in neighboring molecules within the crystal lattice, and these effects are also quantitatively accounted for. These findings open new perspectives for SS protein structure determinations, including the use of the intermolecular pcs for SS samples with two- or one-dimensional order, such as fibrils.