Tunable paramagnetic relaxation enhancements by [Gd(DPA)(3)] (3-) for protein structure analysis.

Paramagnetic relaxation enhancements (PRE) present a powerful source of structural information in nuclear magnetic resonance (NMR) studies of proteins and protein-ligand complexes. In contrast to conventional PRE reagents that are covalently attached to the protein, the complex between gadolinium and three dipicolinic acid (DPA) molecules, Gd(DPA)(3), can bind to proteins in a non-covalent yet site-specific manner. This offers straightforward access to PREs that can be scaled by using different ratios of Gd(DPA)(3) to protein, allowing quantitative distance measurements for nuclear spins within about 15 A of the Gd(3+) ion. Such data accurately define the metal position relative to the protein, greatly enhancing the interpretation of pseudocontact shifts induced by Ln(DPA)(3) complexes of paramagnetic lanthanide (Ln(3+)) ions other than gadolinium. As an example we studied the quaternary structure of the homodimeric GCN4 leucine zipper.