Cd NMR

The 2 NMR-active nuclei of cadmium are both spin I = /2, and have similar natural abundances (12.75% for Cd and 12.26% for Cd). However, the relative receptivity of Cd is slightly better (see Chapter 1, Table 1.1), making it the nucleus of choice for many Cd NMR studies. Cadmium NMR has proved useful in studies of semiconducting oxide compounds and alloys, and of cadmium exchange in clay minerals and zeolites. The chemical shifts of Cd compounds are normally reported with respect to aqueous Cd(C104)2 solution. 

The Cd MAS NMR spectra have been reported of the cadmium halides (Sakida and Kawamoto 2000) and model halide compounds CdF2 (containing CdF structural 

The effect of exchanging Cd into zeolite LTA has been studied by Cd NMR, enabling the identification of 2 different cadmium coordination states in the hydrated form of the zeolite. The effect of cadmium exchange on the silicon and aluminium environments of this zeolite was also investigated using Si and Al NMR spectroscopy (Eldewick et al. 1999). 

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Raman spectra, indicative of [M-M] + ions, are produced by the yellow glass obtained from the melt of Zn in ZnCl2 and also by the colourless, very moisture sensitive crystals of Cd2Al2Cls obtained from melts of Cd in CdCl2 and AICI3. X-ray studies show that the latter contains ethane-like [Cd2Cl6] groups with Cd-Cd reported as 257.6 pm and 256.1 pm " (cf 302 pm in the metal itself). The Cd nmr... 

D. Tinet, A. M. Faugere, and R. Prost, Cd NMR chemical shift tensor analysis of cadmium-exchanged clays and clay gels, J. Phys. Chem. 95 8804 (1991). 

Structural Studies of Peptoids with Aliphatic Side Chains by CD, NMR, and X-ray Crystallography... 

Molecules which contain a chiral cobalt as well as an asymmetric nitrogen exist in four possible optical isomeric forms. These are represented for Co(sar)(hbg) +, hbg = NH2C( = NH)NHC(=NH)NH2 in Fig. 7.12. All four optically-active isomers have been isolated and characterized by cd, nmr and vis/uv absorption spectroscopy. The kinetics of... 

The ionization is the rds. The spontaneous rate is considered to be accelerated by both inter-molecular (from the buffer) and intramolecular (from His-64) proton transfer. Cadmium can replace the native zinc from metalloenzymes and Cd nmr is being used increasingly to probe metal ligation sites in metalloproteins. The cadmium product often does not show enzyme activity however. 

A series of 14 linear and cyclic peptides have been studied 135 by CD, NMR, and MD simulations. Quantitative NOE measurements and MD were used to determine the fractions of type I and II (3-turns in each peptide. The CD spectra for these same peptides were analyzed by convex constraint analysis 136 to derive a set of CD curves for the two types of 13-turns (Figure 6). These basis spectra gave fractional (3-turn contents in good agreement with NMR measurements. Two basis spectra (components 1 and 4) were obtained for the type I... 

The cyclic hexapeptide c(-Pro-Gly-)3 has been extensively investigated. 181 184 CD, NMR, and molecular mechanics indicate a C3-symmetric conformation with three inverse y-turns in nonpolar solvents. In polar solvents, a nonsymmetrical conformation is dominant, but in the presence of divalent cations, 1 1 and 2 1 complexes with C3 symmetry are formed. 

NMR-active " Cd ions were employed as a surrogate probe for Zn Indeed, " Cd NMR has been widely employed in the spectroscopic study of metalloproteins which bear Zn(II) in their native state. The adaptable ligand coordination, number and geometry of Cd(II) are rather similar to Zn(II) and in many cases where Cd(II) has replaced Zn(II), the catalytic activity of the metaUoenzymes has been retained even to a low extent . 

In contrast, the number of known structures of homoleptic, monomeric two-coordinate amides of cadmium is currently limited to just one example. The structure of the compound [Cd N(SiMe3)2 2] has been determined by electron diffraction and the data showed that it has a linear NCdN skeleton with a Cd—N bond length of 2.03(2) A. It is clear, however, that several other homoleptic cadmium amides (known and others as yet not synthesized) can be predicted to have monomeric structures in the solid state. For example Cd NMR studies of amides such as [Cd N(SiMe3)2 2] or [Cd N(SiMe2Ph)2 2] indicate that they are... 

Metal-substitution studies, especially those in which Co11 replaces Zn11, have proved to be an important tool in the study of zinc metalloenzymes the Con-substituted species often have activities approaching those of the Zn11 enzyme in its in vivo reaction. These modification procedures have been the subject of a recent review,1265 which, however, focusses in particular on the use of substitution-inert metal ions. A recent innovation in metal-substitution techniques is the replacement of Znn in zinc metalloenzymes by 113Cdn, which can then be examined by Cd NMR. The results of a recent such investigation1266 indicate that the u3Cdn can serve as an... 

One of the problems in studying zinc enzymes is that the zinc is silent from both electronic and nuclear points of view, and so provides no information about site symmetry and ligand groups. Accordingly a dominant feature of work on zinc enzymes has been the use of probe metal ions to replace Zn. Co11 has been used with some success, while a more recent innovation is the use of 113Cd2+ as a probe, and the application of Cd NMR techniques. 

O. N. Tolkachev, E. P. Nakova, and R. P. Evstigneeva, Khim. Prir. Soedin., 451 (1977) English translation, Chem. Nat. Compd., 382 (1977) (Chem. Abstr. 87, 201829). A more up-to-date treatment than the first review cited over one-third of 191 references cited are post-1969. Discusses representative bis alkaloids of various types. Includes sections on chemical reactions and spectral methods (UV, IR, ORD, CD, NMR, MS) and a brief treatment of biological activity. 

E. Peggion, A. Bisello, M. Rosenblatt, M. Chorev, D. F. Mierke, Mono- and bicyclic analogs of parathyroid hormone-related protein. 2. Conformational analysis of antagonists by CD, NMR, and distance geometry calculations, Biochemistry 1997, 36, 3300-3307. 

N3CI plane (207). The other halids and the cadmium complexes are isomor-phous with the appropriate forms, and undoubtedly possess related five-coordinate structures. A number of studies of the vibrational spectra of the [M(terpy)X2] species have been described, all of which support the formulation as isomorphous five-coordinate complexes (124, 171, 225, 371). The copper(II) complexes [Cu(terpy)Cl2] are isostructural, and a number of studies of the paramagnetic species doped into a host matrix of [Zn(terpy)Cl2] have been reported (23, 213). The zinc complex [Zn(terpy)Cl2] exhibits absorption maxima at 22,650 and 18,000 cm (23, 348). The mercury(II) halide adducts are not so well characterized, but may be prepared by the direct reaction or HgX2 with terpy (171) or by trans-metallation of [Ph2Sn(terpy)Cl2] with HgCl2 (471). They are thought to possess similar, five-coordinate structures. The structures of the 1 1 adducts of the nitrates M(N03)2 terpy are not known with any certainty (171,328). A Cd NMR study of Cd(N03)2 terpy has been reported (430). 

Support for the above conclusions was also obtained from an analysis of the Cd NMR and solid state MAS spectra of the five, six, and seven coordinate cadmium texaphyrin complexes 116, 156, and 157, respectively [70]. From the solid-state MAS results, a single tensor was observed for the five-coordinate complex 116 with a corresponding isotropic chemical shift of 194 ppm. The MAS spectrum of a complex prepared in the presence of pyridine, however, revealed two tensors, presumed to be due to a mixture of six and seven coordinate species. Based upon the isotropic chemical shifts and the symmetry of the tensors the six coordinate species was assigned to the isotropic shift at 188 ppm and the seven coordinate species assigned to the isotropic shift at 221 ppm. The MAS spectrum of the benzimidazole complex of cadmium texaphyrin consists of only a single... 

Cd and " Cd NMR spectroscopy has been used to study several complexes, both in solid state and in solution, and to investigate the binding site of the metal ion in different metalloproteins. These isotopes have a spin 1/2 and, therefore, the broadening of the NMR signals due to the quadrupolar contribution to NMR relaxation is avoided. The natural abundance is 12.75 and 12.26% for " Cd and " Cd, respectively, but due to the lower receptivity of " Cd respect to " Cd, NMR investigations have mainly been... 

The alkoxides [MeCd(OR)]4 (R = Me, Et, i-Pr, Ph) are tetrameric and are assnmed to have cubic structures (11) by analogy with the related zinc compounds. In contrast, the t-bntoxy derivative [MeCd(0-t-Bu)]2 (12) has been reported to be dimeric in benzene with the proposed structure shown, whereas the zinc analog is tetrameric. However, methylcadmium alkoxides have also been stndied by " Cd NMR spectroscopy, which has suggested that the t-butoxy derivative may in fact also be tetrameric. 

The substitution of other metal ions for copper has played an important role in spectroscopic studies of the type 1 copper proteins. For example, Mn(n), Co(II), and Ni(II) derivatives proved useful in analyzing their electronic absorption spectra and Cd(II) substitution has been used to examine the metal binding sites through Cd NMR spectroscopy. ... 

The first and most direct evidence for the existence of the M 3Sp- and M 4Sn-cores in mammahan MTs came from the homonuclear " Cd NMR decoupling studies on " Cd reconstituted Cd7-MT-2. Evidence for a similar cluster organization involving the tetrahedral M°S4 centers was also obtained for Co - and Fe -containing MT derivatives. For C07-MT such evidence has come indirectly from the temperature dependence of isotropically shifted proton resonances (between -50 and 300 ppm) of the Cys residues of the Co4-cluster in the C-terminal domain. This was further confirmed by MCD, ESR (see Electron... 

The Cd titration studies of apoMT followed by Cd NMR measurements both at pH 7.2 and 8.6 showed that at neutral pH the successive addition of Cd equivalents leads... 

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