Study Metal Ion Environments in Proteins

X-Ray absorption methods have been extensively used to study atoms in the first-coordination sphere of metals. The method easily distinguishes N/0 from S atoms but unfortunately does not distinguish N from O (15, 16). The limitations of the method extend to the study of metal atoms with a large number of electrons in their outer shell (e.g., Fe, Cu, and Mo) and make it difficult to study lighter atoms (e.g., Na and Mg). The advantage of this technique lies in the precise determination of the number of ligands and the distances between the metal ion and the ligand atoms. 

The measurements of water proton relaxation rates (PRR) as an NMR appli- 

---