Cadmium-113 magnetic resonance spectroscopy

Reactions of Cadmium(II) Halides and Di-2-Pyridyl Ketone Oxime: One-Dimensional Coordination Polymers

The coordination chemistry of 2-pyridyl ketoximes continues to attract the interest of many inorganic chemistry groups around the world for a variety of reasons. Cadmium(II) complexes of such ligands have provided models of solvent extraction of this toxic metal ion from aqueous environments using 2-pyridyl ketoxime extractants. Di-2-pyridyl ketone oxime (dpkoxH) is a unique member of this family of ligands because its substituent on the oxime carbon bears another potential donor site, i.e., a second 2-pyridyl group. The goal of this study was to investigate the reactions of cadmium(II) halides and dpkoxH in order to assess the structural role (if any) of the halogeno ligand and compare the products with their zinc(II) analogs. The synthetic studies provided access to complexes {[CdCl2(dpkoxH)∙2H2O]}n (1∙2H2O), {[CdBr2(dpkoxH)]}n (2) and {[CdI2(dpkoxH)]}n (3) in 50-60% yields. The structures of the complexes were determined by single-crystal X-ray crystallography. The compounds consist of structurally similar 1D zigzag chains, but only 2 and 3 are strictly isomorphous. Neighboring CdII atoms are alternately doubly bridged by halogeno and dpkoxH ligands, the latter adopting the η1:η1:η1:μ (or 2.0111 using Harris notation) coordination mode. A terminal halogeno group completes distorted octahedral coordination at each metal ion, and the coordination sphere of the CdII atoms is {CdII(η1 - X)(μ - X)2(Npyridyl)2(Noxime)} (X = Cl, Br, I). The trans-donor-atom pairs in 1∙2H2O are Clterminal/Noxime and two Clbridging/Npyridyl; on the contrary, these donor-atom pairs are Xterminal/Npyridyl, Xbridging/Noxime, and Xbridging/Npyridyl (X = Br, I). There are intrachain H-bonding interactions in the structures. The packing of the chains in 1∙2H2O is achieved via π-π stacking interactions, while the 3D architecture of the isomorphous 2 and 3 is built via C-H∙∙∙Cg (Cg is the centroid of one pyridyl ring) and π-π overlaps. The molecular structures of 1∙2H2O and 2 are different compared with their [ZnX2(dpkoxH)] (X = Cl, Br) analogs. The polymeric compounds were characterized by IR and Raman spectroscopies in the solid state, and the data were interpreted in terms of the known molecular structures. The solid-state structures of the complexes are not retained in DMSO, as proven via NMR (1H, 13C, and 113Cd NMR) spectroscopy and molar conductivity data. The complexes completely release the coordinated dpkoxH molecule, and the dominant species in solution seem to be [Cd(DMSO)6]2+ in the case of the chloro and bromo complexes and [CdI2(DMSO)4].

Zinc finger structure determination by NMR: Why zinc fingers can be a handful

Zinc fingers can be loosely defined as protein domains containing one or more tetrahedrally-co-ordinated zinc ions whose role is to stabilise the structure rather than to be involved in enzymatic chemistry; such zinc ions are often referred to as "structural zincs". Although structural zincs can occur in proteins of any size, they assume particular significance for very small protein domains, where they are often essential for maintaining a folded state. Such small structures, that sometimes have only marginal stability, can present particular difficulties in terms of sample preparation, handling and structure determination, and early on they gained a reputation for being resistant to crystallisation. As a result, NMR has played a more prominent role in structural studies of zinc finger proteins than it has for many other types of proteins. This review will present an overview of the particular issues that arise for structure determination of zinc fingers by NMR, and ways in which these may be addressed.

113Cd as a Probe in NMR Studies of Allosteric Host-Guest-Ligand Complexes of Porphyrin Cage Compounds

Cadmium porphyrin cage compounds Cd1 and 113 Cd1 have been synthesized from the free base porphyrin cage derivative H21 and Cd(OAc)2 ⋅ 2 H2O or 113Cd(OAc)2 ⋅ 2 H2O, respectively. The compounds form allosteric complexes with the positively charged guests N,N'-dimethylimidazolium hexafluorophosphate (DMI) and N,N'-dimethylviologen dihexafluorophosphate (Me2V), which bind in the cavity of the cage, and tbupy, which coordinates as an axial ligand to the outside of the cage. In the presence of tbupy, the binding of DMI in Cd1 is enhanced by a factor of ∼31, while the presence of DMI or Me2V in the cavity of Cd1 enhances the binding of tbupy by factors of 55 and 85, respectively. The X-ray structures of the coordination complexes of Cd1 with acetone, acetonitrile, and pyridine, the host-guest complex of Cd1 with a bound viologen guest, and the ternary allosteric complex of Cd1 with a bound DMI guest and a coordinated tbupy ligand, were solved. These structures revealed relocations of the cadmium center in and out of the porphyrin plane, depending on whether a guest or a ligand is present. 113Cd NMR could be employed as a tool to quantify the binding of guests and ligands to 113 Cd1. 1D EXSY experiments on the ternary allosteric system Cd1-tbupy-Me2V revealed that the coordination of tbupy significantly slowed down the dissociation of the Me2V guest. Eyring plots of the dissociation process revealed that this kinetic allosteric effect is entropic in nature.

Single-crystal NMR spectroscopy

Single-crystal (SC) NMR spectroscopy is a solid-state NMR method that has been used since the early days of NMR to study the magnitude and orientation of tensorial nuclear spin interactions in solids. This review first presents the field of SC NMR instrumentation, then provides a survey of software for analysis of SC NMR data, and finally it highlights selected applications of SC NMR in various fields of research. The aim of the last part is not to provide a complete review of all SC NMR literature but to provide examples that demonstrate interesting applications of SC NMR.

A DFT/ZORA Study of Cadmium Magnetic Shielding Tensors: Analysis of Relativistic Effects and Electronic-State Approximations

Theoretical considerations are discussed for the accurate prediction of cadmium magnetic shielding tensors using relativistic density functional theory (DFT). Comparison is made between calculations that model the extended lattice of the cadmium-containing solids using periodic boundary conditions and pseudopotentials with calculations that use clusters of atoms. The all-electron cluster-based calculations afford an opportunity to examine the importance of (i) relativistic effects on cadmium magnetic shielding tensors, as introduced through the ZORA Hamiltonian at either the scalar (SC) or spin-orbit (SO) levels and (ii) variation in the class of the DFT approximation. Twenty-three combinations of pseudopotentials or all-electron methods, DFT functionals, and relativistic treatments are assessed for the prediction of the principal components of the magnetic shielding tensors of 30 cadmium sites. We find that the inclusion of SO coupling can increase the cadmium magnetic shielding by as much as ca. 1100 ppm for a certain principal values; these effects are most pronounced for cadmium sites featuring bonds to other heavy atoms such as cadmium, iodine, or selenium. The best agreement with experimental values is found at the ZORA SO level in combination with a hybrid DFT method featuring a large admixture of Hartree-Fock exchange such as BH&HLYP. Finally, a theoretical examination is presented of the magnetic shielding tensor of the Cd(I) site in Cd₂(AlCl₄)₂.

Strain-Induced Reactivity in the Dynamic Covalent Chemistry of Macrocyclic Imines

The displacement of molecular structures from their thermodynamically most stable state by imposition of various types of electronic and conformational constraints generates highly strained entities that tend to release the accumulated strain energy by undergoing either structural changes or chemical reactions. The latter case amounts to strain-induced reactivity (SIR) that may enforce specific chemical transformations. A particular case concerns dynamic covalent chemistry which may present SIR, whereby reversible reactions are activated by coupling to a high-energy state. We herewith describe such a dynamic covalent chemical (DCC) system involving the reversible imine formation reaction. It is based on the formation of strained macrocyclic bis-imine metal complexes in which the macrocyclic ligand is in a high energy form enforced by the coordination of the metal cation. Subsequent demetallation generates a highly strained free macrocycle that releases its accumulated strain energy by hydrolysis and reassembly into a resting state. Specifically, the metal-templated condensation of a dialdehyde with a linear diamine leads to a bis-imine [1+1]-macrocyclic complex in which the macrocyclic ligand is in a coordination-enforced strained conformation. Removal of the metal cation by a competing ligand yields a highly reactive [1+1]-macrocycle, which then undergoes hydrolysis to transient non-cyclic aminoaldehyde species, which then recondense to a strain-free [2+2]-macrocyclic resting state. The process can be monitored by (1) H NMR spectroscopy. Energy differences between different conformational states have been evaluated by Hartree-Fock (HF) computations. One may note that the stabilisation of high-energy molecular forms by metal ion coordination followed by removal of the latter, offers a general procedure for producing out-of-equilibrium molecular states, the fate of which may then be examined, in particular when coupled to dynamic covalent chemical processes.

Challenging conventional wisdom: single domain metallothioneins

Metallation studies of human metallothioneins support the role of single metal-binding-domains as commonplace with the typical two-domain-cluster structure as exceptional.

Cadmium(II) complex formation with selenourea and thiourea in solution: an XAS and 113Cd NMR study

The complexes formed in methanol solutions of Cd(CF(3)SO(3))(2) with selenourea (SeU) or thiourea (TU), for thiourea also in aqueous solution, were studied by combining (113)Cd NMR and X-ray absorption spectroscopy. At low temperature (~200 K), distinct (113)Cd NMR signals were observed, corresponding to CdL(n)(2+) species (n = 0-4, L = TU or SeU) in slow ligand exchange. Peak integrals were used to obtain the speciation in the methanol solutions, allowing stability constants to be estimated. For cadmium(II) complexes with thione (C═S) or selone (C═Se) groups coordinated in Cd(S/Se)O(5) or Cd(S/Se)(2)O(4) (O from MeOH or CF(3)SO(3)(-)) environments, the (113)Cd chemical shifts were quite similar, within 93-97 ppm and 189-193 ppm, respectively. However, the difference in the chemical shift for the Cd(SeU)(4)(2+) (578 pm) and Cd(TU)(4)(2+) (526 ppm) species, with CdSe(4) and CdS(4) coordination, respectively, shows less chemical shielding for the coordinated Se atoms than for S, in contrast to the common trend with increasing shielding in the following order: O > N > Se > S. In solutions dominated by mono- and tetra-thiourea/selenourea complexes, their coordination and bond distances could be evaluated by Cd K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. At ~200 K and high excess of thiourea, a minor amount (up to ~30%) of [Cd(TU)(5-6)](2+) species was detected by an upfield shift of the (113)Cd NMR signal (up to 423 ppm) and an amplitude reduction of the EXAFS oscillation. The amount was estimated by fitting linear combinations of simulated EXAFS spectra for [Cd(TU)(4)](2+) and [Cd(TU)(6)](2+) complexes. At room temperature, [Cd(TU)(4)](2+) was the highest complex formed, also in aqueous solution. Cd L(3)-edge X-ray absorption near edge structure (XANES) spectra of cadmium(II) thiourea solutions in methanol were used to follow changes in the CdS(x)O(y) coordination. The correlations found from the current and previous studies between (113)Cd NMR chemical shifts and different Cd(II) coordination environments are generally useful for evaluating cadmium coordination to thione-containing or Se-donor ligands in biochemical systems or for monitoring speciation in solution.

Lithol Red: a systematic structural study on salts of a sulfonated azo pigment

The first systematic series of single-crystal diffraction structures of azo lake pigments is presented (Lithol Red with cations=Mg(II), Ca(II), Sr(II), Ba(II), Na(I) and Cd(II)) and includes the only known structures of non-Ca examples of these pigments. It is shown that these commercially and culturally important species show structural behaviour that can be predicted from a database of structures of related sulfonated azo dyes, a database that was specifically constructed for this purpose. Examples of the successful structural predictions from the prior understanding of the model compounds are that 1) the Mg salt is a solvent-separated ion pair, whereas the heavier alkaline-earth elements Ca, Sr and Ba form contact ion pairs, namely, low-dimensional coordination complexes; 2) all of the Lithol Red anions exist as the hydrazone tautomer and have planar geometries; and 3) the commonly observed packing mode of alternating inorganic layers and organic bilayers is as expected for an ortho-sulfonated azo species with a planar anion geometry. However, the literature database of dye structures has no predictive use for organic solvate structures, such as that of the observed Na Lithol Red DMF solvate. Interestingly, the Cd salt is isostructural with the Mg salt and not with the Ca salt. It is also observed that linked eight-membered [MOSO](2) rings are the basic coordination motif for all of the known structures of Ca, Sr and Ba salts of sulfonated azo pigments in which competing carboxylate groups are absent.

CADMIUM AND PROTON NMR STUDY ON [Cd2(BIS(2-PYRIDYL)FORMAMINE)3]

The structure of a bi - Cd organic complex [ Cd 2(bis(2-pyridyl)formamine)3 or 1], is analyzed thoroughly by 1 H and 113/111 Cd NMR in solution. Only one representative set of pyridyl proton resonance of a total of six pyridines in 1 was observed. The resonance of protons in pyridine ring can be assigned by both 2D COSY and 2D NOESY. Similarly, only one Cd (either 113 Cd or 111 Cd NMR signal was observed too. This indicates that the two Cd atoms in 1 are chemically equivalent. The Cd signal is split by its three neighboring equivalent protons on the bridged carbon atoms ( C a H )3, resulting in a quartet. The 3 J 113 Cd –1 H and 3 J 111 Cd –1 H are 44 and 42 Hz, respectively, measured directly from the spectral pattern. The proton NMR signals of C a H composed from three sources: (1) superimposed doublets arising from Cd – H –113 Cd and Cd – H –111 Cd ; (2) a pseudo triplet from 113 Cd – H –113 Cd , 111 Cd – H –111 Cd and 113 Cd – H –111 Cd and (3) a singlet from those H's surrounded by all non-magnetic Cd atoms. They superimpose and appear as a quintet with intensity ratio of 1:12:38:12:1, explainable by the above constituents and their associated Cd – H splitting analyses. We deduced that the three bis(2-pyridyl)formamidine groups have a three-fold symmetry with respect to the Cd – Cd axis, i.e., the three bis(2-pyridyl)formamidine units are co-planar and spaced 120 degrees from each other based on above observations. This implies the key C a atom exists as a hybrid of two canonical forms (– N = C a H – N – ↔ – N – C a H = N –). As a consequence, 1 does not exhibit chirality. We exemplified that Cd NMR study is well suited for investigating those metalloproteins if the calcium and/or zinc ions contained can be replaced by 113 Cd /111 Cd ions of similar size.

Cadmium(II) complex formation with glutathione

Complex formation between heavy metal ions and glutathione (GSH) is considered as the initial step in many detoxification processes in living organisms. In this study the structure and coordination between the cadmium(II) ion and GSH were investigated in aqueous solutions (pH 7.5 and 11.0) and in the solid state, using a combination of spectroscopic techniques. The similarity of the Cd K-edge and L(3)-edge X-ray absorption spectra of the solid compound [Cd(GS)(GSH)]ClO(4).3H(2)O, precipitating at pH 3.0, with the previously studied cysteine compound {Cd(HCys)(2).H(2)O}(2).H(3)O(+).ClO(4) (-) corresponds to Cd(S-GS)(3)O (dominating) and Cd(S-GS)(4) four-coordination within oligomeric complexes with mean bond distances of 2.51 +/- 0.02 A for Cd-S and 2.24 +/- 0.04 A for Cd-O. For cadmium(II) solutions (C (Cd(II)) approximately 0.05 M) at pH 7.5 with moderate excess of GSH (C (GSH)/C (Cd(II)) = 3.0-5.0), a mix of Cd(S-GS)(3)O (dominating) and Cd(S-GS)(4) species is consistent with the broad (113)Cd NMR resonances in the range 632-658 ppm. In alkaline solutions (pH 11.0 and C (GSH)/C (Cd(II)) = 2.0 or 3.0), two distinct peaks at 322 and 674 ppm are obtained. The first peak indicates six-coordinated mononuclear and dinuclear complexes with CdS(2)N(2)(N/O)(2) and CdSN(3)O(2) coordination in fast exchange, whereas the second corresponds to Cd(S-GS)(4) sites. At high ligand excess the tetrathiolate complex, Cd(S-GS)(4), characterized by a sharp delta((113)Cd) NMR signal at 677 ppm, predominates. The average Cd-S distance, obtained from the X-ray absorption spectra, varied within a narrow range, 2.49-2.53 A, for all solutions (pH 7.5 and 11.0) regardless of the coordination geometry.

Cadmium-113 NMR studies on homoleptic complexes containing thioether ligands: the crystal structures of [Cd([12]aneS4)2](ClO4)2, [Cd([18]aneS4N2)](PF6)2 and [Cd([9]aneS3)2](PF6)2

We report the measurement of 113Cd NMR chemical shift data for homoleptic thioether and related aza and mixed aza/thiacrown complexes. In a series of Cd(II) complexes containing trithioether to hexathioether ligands, we observe solution 113Cd NMR chemical shifts in the range of 225 to 731 ppm. Upfield chemical shifts in these NMR spectra are seen whenever: (a) the number of thioether sulfur donors in the complex is decreased, (b) a thioether sulfur donor is replaced by a secondary nitrogen donor, or (c) the size of the macrocycle ring increases without a change in the nature or number of the donor atoms. Changes in the identity of non-coordinating anions such as perchlorate or hexafluorophosphate have little effect upon the 113Cd NMR chemical shift in solution. We report the X-ray structure of the complex [Cd([12]aneS4)2](ClO4)2 ([12]aneS4 = 1,4,7,10-tetrathiacyclododecane) (1) which shows the first example of octakis(thioether) coordination of a metal ion, forming an unusual eight-coordinate square antiprismatic structure. We report the X-ray structure of the complex [Cd([9]aneS3)2](PF6)2 ([9]aneS3 = 1,4,7-trithiacyclononane) (3a) which shows hexakis(thioether) coordination to form a distorted octahedral structure. We have also prepared and characterized the Cd(II) complex of a mixed azathiacrown, [Cd([18]aneS4N2)](PF6)2 ([18]aneS4N2 = 1,4,10,13-tetrathia-7,16-diazacyclooctadecane) (6). Its X-ray structure shows a distorted octahedral S4N2 environment around the Cd(II) with the ligand coordinated in the rac fashion. We observe a solvent- and temperature-dependent 14N-1H coupling in the 1H NMR spectrum of the complex which is not present in analogous complexes with this ligand.

Mercury-199 NMR Studies of Thiacrown and Related Macrocyclic Complexes: The Crystal Structures of [Hg(18S6)](PF6)2 and [Hg(9N3)2](ClO4)2

We wish to report the first measurements of (199)Hg NMR chemical shift data for a series of homoleptic Hg(II) complexes with thiacrown ligands and related aza and mixed thia/aza macrocycles. In mercury(II) complexes containing trithiacrown through hexathiacrown ligands, we observed (199)Hg NMR chemical shifts in the range of -298 to -1400 ppm. Upfield chemical shifts in these NMR spectra are seen whenever (a) the number of thioether sulfur donors in the complex is decreased, (b) a thioether sulfur donor is replaced by a secondary nitrogen donor, and (c) the size of the macrocycle ring increases without a change in the nature or number of the donor atoms. Changes in noncoordinating anions, such as hexafluorophosphate and perchlorate, have little effect on the (199)Hg chemical shift. For several complexes, we observed (3)J((199)Hg-(1)H) coupling in the range of 50-100 Hz, the first example of proton-mercury coupling through a C-S thioether bond. Also, we obtained unusual upfield (13)C NMR chemical shifts for methylene resonances in several of the thiacrown complexes which correspond to distortions within the five- and six-membered chelate rings bound to the mercury ion. We report the X-ray crystal structure of the complex [Hg(18S6)](PF(6))(2) (18S6 = 1,4,7,10,13,16-hexathiacyclooctadecane). The molecule crystallizes in the rare trigonal space group Pm1 with hexakis(thioether) coordination around the Hg(II) center confirming previous X-ray photoemission spectroscopic data on the compound. The lack of an observable (199)Hg NMR signal for the complex is the result of the identical length (2.689(2) Angstroms) of all six Hg-S bonds. We additionally report the X-ray structure of the complex [Hg(9N3)(2)](ClO(4))(2) (9N3 = 1,4,7-triazacyclononane) which shows hexakis(amine) coordination of the 9N3 to form a distorted trigonal prismatic structure. Solution dissociation of the one of the 9N3 ligands from the mercury ion is confirmed by multinuclear NMR experiments. For six-coordinate macrocyclic Hg(II) complexes, N6 donor sets have a preference for trigonal prisms while S6 donor sets favor octahedral structures.

First principle calculations of 113Cd chemical shifts for proteins and model systems

113Cd isotropic NMR shieldings are calculated for a number of metal ion binding sites in proteins, using the GIAO-B3LYP and GIAO-HF methods with the uncontracted (19s15p9d4f) polarized basis set of Kellö and Sadlej on cadmium and 6-31G(d) on the ligands. The results compare favorably with experimental data, indicating that first principle calculations are a useful tool for structural interpretation of (113)Cd chemical shift data from metal ion containing proteins. The effect of different ligand types (thiolate, imidazole, water, and monodentate carboxylate), coordination number, and deviations of the coordination geometry from ideal structures is evaluated. In particular, the ligand type and coordination number are important factors, but also changes in cadmium-ligand bond lengths may cause significant changes of the chemical shift.

Interaction of Cd and Zn with biologically important ligands characterized using solid-state NMR and ab initio calculations

For the first time, coordination geometry and structure of metal binding sites in biologically relevant systems are studied using chemical shift parameters obtained from solid-state NMR experiments and quantum chemical calculations. It is also the first extensive report looking at metal-imidazole interaction in the solid state. The principal values of the (113)Cd chemical shift anisotropy (CSA) tensor in crystalline cadmium histidinate and two different cadmium formates (hydrate and anhydrate) were experimentally measured to understand the effect of coordination number and geometry on (113)Cd CSA. Further, (13)C and (15)N chemical shifts have also been experimentally determined to examine the influence of cadmium on the chemical shifts of (15)N and (13)C nuclei present near the metal site in the cadmium-histidine complex. These values were then compared with the chemical shift values obtained from the isostructural bis(histidinato)zinc(II) complex as well as from the unbound histidine. The results show that the isotropic chemical shift values of the carboxyl carbons shift downfield and those of amino and imidazolic nitrogens shift upfield in the metal (Zn,Cd)-histidine complexes relative to the values of the unbound histidine sample. These shifts are in correspondence with the anticipated values based on the crystal structure. Ab initio calculations on the cadmium histidinate molecule show good agreement with the (113)Cd CSA tensors determined from solid-state NMR experiments on powder samples. (15)N chemical shifts for other model complexes, namely, zinc glycinate and zinc hexaimidazole chloride, are also considered to comprehend the effect of zinc binding on (15)N chemical shifts.

A general strategy for the NMR observation of half-integer quadrupolar nuclei in dilute environments

A general strategy for the observation of low gamma half-integer quadrupolar nuclides in biological systems is presented. The methodology combines low-temperature (4-100 K) techniques with cross-polarization (CP) experiments while employing a so-called Carr-Purcell-Meiboom-Gill spin-echo sequence (CPMG). This combined approach is termed CP/QCPMG. Also discussed are data processing issues that are unique to the induced signals that result from the QCPMG pulse sequence. Central to this strategy is the development of a stable low-temperature (4 to 250 K) NMR double-resonance probe. The probe is robust enough to handle multiple contact experiments and long acquisition periods with 1H decoupling. This approach is illustrated with low-temperature solid-state 67Zn and 25Mg NMR CP/QCPMG experiments on model compounds. The conclusion reached is that the strategy affords sufficient sensitivity to examine Zn2+ and/or Mg2+ binding sites in metalloproteins.

113Cd Shielding Tensors of Monomeric Cadmium Compounds Containing Nitrogen Donor Atoms. 3. CP/MAS Studies on Five-Coordinate Cadmium Complexes Having N(3)X(2) (X = H, N, O, and S) Donor Atoms

The principal elements of the (113)Cd shielding tensor for a set of five- coordinate compounds having mixed donor atoms coordinating to the cadmium were determined via CP/MAS NMR experiments. The first complex, [HB(3,5-Me(2)pz)(3)]CdBH(4) (where pz = pyrazolyl), has a CdN(3)H(2) inner coordination sphere. The isotropic chemical shift in the solid state is 355.1 ppm, and its chemical shift anisotropy (CSA, Deltasigma) is -596 ppm with an asymmetry parameter (eta) of 0.64. The second complex, [HB(3,5-Me(2)pz)(3)]Cd[H(2)B(pz)(2)], has five nitrogen donor atoms bonded to the cadmium. This N(5) or N(3)N(2) compound was the only material of this study to manifest dipolar splitting of the cadmium resonance from the quadrupolar (14)N. The isotropic chemical shift, CSA, and the value of eta for this material were therefore determined at higher field where the dipolar splitting was less than the linewidth, yielding values of 226.6 ppm, -247 ppm, and 0.32, respectively. A second N(5) material, [HB(3-Phpz)(3)]Cd[H(2)B(3,5-Me(2)pz)(2)], was also investigated and has an isotropic shift of 190.2 ppm, a CSA of 254 ppm, and an eta of 0.86. Also studied was [HB(3-Phpz)(3)]Cd[(Bu(t)CO)(2)CH], which has an CdN(3)O(2) inner core. The isotropic chemical shift of this complex is 173.6 ppm, and the values of Deltasigma and eta were determined to be -258 ppm and 0.38, respectively. The final compound, [HB(3,5-Me(2)pz)(3)]Cd[S(2)CNEt(2)], with N(3)S(2) donor atoms, has an isotropic shift of 275.8 ppm, an eta of 0.51, and a CSA of +375 ppm. Utilizing previous assignments, the most shielded tensor element was determined to be oriented normal to the plane of the tridentate ligand. The shielding tensor information is used to speculate on the coordination geometry of the CdN(3)O(2) inner core complex.

Probe of cadmium(II) binding on soil fulvic acid investigated by113Cd NMR spectroscopy

Binding of cadmium(II) on soil fulvic acid (FA) was investigated over a range of fulvate-to-cadmium concentration ratios (8 – 59 equiv. mol−1) using113Cd NMR spectroscopy. The113Cd chemical shift of cadmium bound on fulvate was observed in a more downfield region (δ −20.4 to −15.6) than that bound on synthetic polymers, poly(acrylic acid) (PAA: δ −36.6 to −38.2), poly(methacrylic acid) (PMAA: δ −34.0 to −25.4), and poly(vinyl benzoic acid) (PVBA: δ −34.7 to −31.2). The calculated values of individual chemical shifts for the species CdL+and CdL2(L: carboxylate) formed in Cd(II)–carboxylate systems (e.g., acetate, benzoate) are δ −22 to −24 and δ −39 to −40, respectively. The relative downfield shift of cadmium(II)–fulvate suggests that functional groups (e.g., hydroxyl and neutral N donor) other than carboxylates may be involved in cadmium coordination. The chemical shifts of cadmium complexes of hydroxycarboxylates (e.g., glycolate) or carboxylates containing neutral N donor (e.g., picolinate) were generally observed in more downfield regions than their carboxylate counterparts. Key words: fulvic acid, polyfunctionality, binding sites, chemical shift,113Cd NMR.

Mercury-199 NMR of the metal receptor site in MerR and its protein-DNA complex

Structural insights have been provided by mercury-199 nuclear magnetic resonance (NMR) into the metal receptor site of the MerR metalloregulatory protein alone and in a complex with the regulatory target, DNA. The one- and two-dimensional NMR data are consistent with a trigonal planar Hg-thiolate coordination environment consisting only of Cys side chains and resolve structural aspects of both metal ion recognition and the allosteric mechanism. These studies establish 199Hg NMR techniques as useful probes of the metal coordination environment of regulatory proteins, copper enzymes, and zinc transcription factor complexes as large as 50 kilodaltons.

Cadmium-113 and magnesium-25 NMR study of the divalent metal binding sites of isocitrate dehydrogenases from pig heart

The metal activator sites of NAD(+)-dependent and NADP(+)-dependent isocitrate dehydrogenases from pig heart have been probed using 113Cd- and 25Mg-NMR. In the presence of isocitrate and ADP, a broad resonance for cadmium bound to NAD-dependent isocitrate dehydrogenase was observed (-8 ppm) arising from exchange with isocitrate (-20 ppm) and/or ADP (27 ppm) complexes. The Cd shift with ADP suggests interaction of the metal with the nucleotide ring nitrogen. Increasing shifts with excess ADP are indicative of macrochelate formation. 25Mg-NMR demonstrates that, unlike manganese, magnesium has a similar dissociation constant (1.8 mM) from NADP-dependent isocitrate dehydrogenase as from the enzyme-isocitrate complex (1.1 mM). The extrapolated line width of bound magnesium increases from 674 Hz in the binary complex to 10,200 Hz in the ternary complex. The quadrupole coupling constant, calculated from relaxation rates, is larger in the ternary complex, indicative of greater distortion in the magnesium coordination sphere. The line widths of magnesium complexed to NAD-dependent isocitrate dehydrogenase are broader, as expected for the larger octamer. 113Cd- and 25Mg-NMR both show that the metal sites have anisotropic octahedral symmetry. 25Mg relaxation rates yield correlation times corresponding to motions of a domain with motion independent of the enzyme multimers.