NMR ( nuclear magnetic resonance ) shift reagents

Paramagnetic shift reagents have the ability to induce chemical shifts and thus simplify complex NMR spectra. The most efficient shift reagents are complexes of paramagnetic lanthanide ions such as europium(III) for down field shifts and praseodymium(III) for upfield shifts.

Over the past several decades, nuclear magnetic resonance spectroscopy has become one of the most important analytical methods of structure elucidation of organic, bioorganic and organometallic compounds. For example, the chemical shifts and coupling patterns provide invaluable information for the determination of stereochemistry. Organic molecules contain mainly carbon and hydrogen, and thus most of the structural information is gained from proton and carbon NMR data. However, the proton NMR signals are not spread over a wide range (0 to 15 ppm) and therefore proton NMR spectra of complex organic and biological molecules consist of featureless clusters that are very difficult to assign. It has been known that paramagnetic transition metal ions perturb the proton NMR spectra of the ligands that coordinate them. Based on this observation, a wide range of paramagnetic shift reagents have been prepared that have the ability to induce chemical shifts and thus simplify complex proton NMR spectra. Theoretical studies revealed that the induced shifts are the consequence of contact and dipolar (pseudo-contact) interactions between the paramagnetic ion and the organic molecule. The requirements for an effective shift reagent include optimal sifting power with minimal line broadening effect and an ability to bind to a large variety of organic molecules. The most efficient shift reagents are complexes of paramagnetic lanthanide ions such as europium (III) for down field shifts and praseodymium(III) for upfield shifts. The most commonly used ligands include dipivaloyl methane (DPM), 2,2,6,6-tetramethyl-3,5-heptanedione (THD), and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione (FOD). Signal overlap is a major problem for determining three dimensonal protein structures. Paramagnetic lanthanide(III) salts and DTPA complexes have been used to resolve overlaps in the NMR spectra of certain proteins.