1H nuclear magnetic resonance studies of ytterbium-substituted porcine intestinal calcium-binding protein

The addition of ytterbium to calcium-saturated porcine intestinal calcium-binding protein resulted in the appearance of broad lanthanide-shifted resonances well outside the normally observed region of the 1H nuclear magnetic resonance spectrum of the calcium form of the protein. Variation of the salt concentration and temperature have led us to conclude that aggregation and chemical exchange do not contribute to the line widths of these resonances. Assuming that the line broadening of these lanthanide-shifted resonances arises from the contribution of the susceptibility line-broadening mechanism for protein residues proximal to the bound Yb3+ ion, we have calculated Yb3+–proton distances for nuclei in the metal-binding site. These lanthanide-shifted resonances provide very sensitive probes of the structure of the protein in solution.