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Publisher Summary Among the naturally occurring isotopes of cadmium there are two, 113 Cd and 111 Cd, that have a nuclear spin of ½ and are useful as nuclear magnetic resonance (NMR) probes for cadmium-containing molecules. Both isotopes are present at a significant percent natural abundance, namely, 12.75 and 12.26% for 111 Cd and 113 Cd, respectively. They are both obtainable in 96% purity. Although there is only one naturally occurring well-characterized protein, metallothionein, that accumulates cadmium, most metalloproteins containing the Zn 2+ ion—5 d 10 electron configuration—can be substituted with the heavier Cd 2+ ion (6 d 10 ). In the case of metalloenzymes, the Cd substitution has often led to a protein that maintains measurable catalytic activity. For a variety of Zn-containing DNA-binding proteins that require Zn to maintain the fold required for DNA recognition and binding, the Cd-substituted proteins have retained DNA binding. Thus, from both structural and functional standpoints 113 Cd ( 111 Cd) NMR can be used as a valuable probe of zinc metalloprotein structure and function. The cadmium(II) ion has an ionic radius of 0.97 A nearly the same as that of Ca 2+ , 0.99 A (Pauling radii). Thus, Cd 2+ can be substituted at most Ca 2+ binding sites found in proteins. The Cd 2+ ion can also fit the sites occupied by the Cu + ion (5 d 10 ) and thus can be used to probe the metal sites in copper electron transfer proteins in which copper alternates between the Cu + and Cu 2+ states. Metal ion replacement employs anaerobic techniques under which the Cu + ion is removed and replaced by 113 Cd 2+ . This chapter describes the application of 113 Cd NMR methods to the study of metalloproteins. |
