Calcium binding environments probed by (43)Ca NMR spectroscopy.

Calcium is an important component of materials, metalloproteins, minerals, glasses, and small inorganic and organic complexes. However, NMR spectroscopy of the quadrupolar (43)Ca nuclide remains difficult primarily due to its low natural abundance and low resonance frequency. In this Perspective, experimental challenges and recent successes in the field are highlighted, with a focus on solid-state (43)Ca NMR spectroscopy. Solution (43)Ca NMR studies of calcium-binding biomolecules are also presented. The structural insights afforded from quadrupolar and chemical shift parameters are examined. For example: isotropic chemical shifts have been shown to correlate with the mean Ca-O distance and also with calcium coordination number; quadrupolar coupling constants and chemical shift tensor spans have been shown to be useful probes of polymorphism; and, distance measurements involving (43)Ca have been recently demonstrated. Lastly, challenges and opportunities for the future are considered.