Abstract
Cd(II) has been used as a probe of zinc metalloenzymes and proteins because of the spectroscopic silence of Zn(II). One of the most commonly used spectroscopic techniques is (113)Cd NMR; however, in recent years (111m)Cd Perturbed Angular Correlation spectroscopy ((111m)Cd PAC) has also been shown to provide useful structural, speciation and dynamics information on Cd(II) complexes and biomolecules. In this article, we show how the joint use of (113)Cd NMR and (111m)Cd PAC spectroscopies can provide detailed information about the Cd(II) environment in thiolate-rich proteins. Specifically we show that the (113)Cd NMR chemical shifts observed for Cd(II) in the designed TRI series (TRI = Ac-G(LKALEEK)(4)G-NH(2)) of peptides vary depending on the proportion of trigonal planar CdS(3) and pseudotetrahedral CdS(3)O species present in the equilibrium mixture. PAC spectra are able to quantify these mixtures. When one compares the chemical shift range for these peptides (from delta = 570 to 700 ppm), it is observed that CdS(3) species have delta 675-700 ppm, CdS(3)O complexes fall in the range delta 570-600 ppm and mixtures of these forms fall linearly between these extremes. If one then determines the pK(a2) values for Cd(II) complexation [pK(a2) is for the reaction Cd[(peptide-H)(2)(peptide)](+)-->Cd(peptide)(3)(-) + 2H(+)] and compares these to the observed chemical shift for the Cd(peptide)(3)(-) complexes, one finds that there is also a direct linear correlation. Thus, by determining the chemical shift value of these species, one can directly assess the metal-binding affinity of the construct. This illustrates how proteins may be able to fine tune metal-binding affinity by destabilizing one metallospecies with respect to another. More important, these studies demonstrate that one may have a broad (113)Cd NMR chemical shift range for a chemical species (e.g., CdS(3)O) which is not necessarily a reflection of the structural diversity within such a four-coordinate species, but rather a consequence of a fast exchange equilibrium between two related species (e.g., CdS(3)O and CdS(3)). This could lead to reinterpretation of the assignments of cadmium-protein complexes and may impact the application of Cd(II) as a probe of Zn(II) sites in biology.