General Considerations and Basic Principles

The preponderance of biological systems whose function depends on the binding of the native metal ions Zn, Mg, and Ca provided the impetus behind our efforts to develop and apply spin I = 1/2 Cd NMR methods to characterize the structural and/or functional role of these native metal ions. While these native metal ions are not strictly spectroscopically silent, they all have isotopes with non-zero spin quantum number I 1 which makes them difficult to use but not totally intractable for studies of macromolecules [7,20]. 

Chemical Exchange A phenomenon that is especially important in Cd NMR is the effect of chemical exchange. The importance of this stems from the extreme sensitivity of the Cd chemical shift ( 950 ppm), which makes it sensitive to exchange rates that are faster by orders of magnitude compared to the conventional exchange rates affecting nuclei like C and H. In the Introduction, this has been alluded to with the analogy between metalloproteins and metalloenzymes. 

In view of the potential role of this exchange affecting the chemical shift, it is important to conduct the appropriate experiments (varying temperature, pH, solvent counterions, etc.) to rule in or out the presence of chemical exchange whenever the shift is being used to report on the nature of the protein ligands. Specific examples are the shift of the A-site Cd in alkaline phosphatase (AP) from 122 ppm in Tris-acetate to 169 ppm in Tris-chloride and in human carbonic anhydrase B (HCAB) where the replacement of the Cd-bound H2O or OH with Cl results in a shift from 145 ppm to 240 ppm. Both of these examples illustrate the deshielding affect of the metal bound Cl.  

Relaxation Properties A knowledge of the spin relaxation properties is of interest for at least two reasons. The first is related to the use of acquisition parameters for optimum signal-to-noise acquisition times and the second is to use the relaxation data to provide important information concerning motional dynamics in and around the metal binding site. Unfortunately, to our knowledge there has been no comprehensive Cd NMR relaxation study on macromolecular systems, which limits our discussion of this topic to early, preUminaiy investigative studies dating back to the mid to late 80s. 

hepta-coordinated with only monodentate ligands , hepta-coordinated with one bidentate and five monodentate ligands. 

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