Direct carbon detection in paramagnetic metalloproteins to further exploit pseudocontact shift restraints

Direct detection heteronuclear NMR allows us to drastically reduce paramagnetic contributions to the line width as compared to 1H detection. As an example, a calcium binding protein (human oncomodulin), in which one of the calcium ions was selectively substituted with Tb3+, is used. Through a variety of 13C direct detection NMR experiments, resonances were detected as close as 5.5 A from the metal ion. Pseudocontact shifts measured through 13C direct detection experiments provide structural constraints in regions of the protein where 1H resonances are broadened beyond detection through Curie relaxation (up to 16 A from the paramagnetic center).