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PubMed2006

Tissue distribution and subcellular localization of trace metals in the pond snail Lymnaea stagnalis with special reference to the role of lysosomal granules in metal sequestration.

The present study was undertaken to elucidate the cellular mechanisms, which govern metal sequestration and detoxification in gastropods. For this purpose the pond snail, Lymnaea stagnalis was exposed to environmentally relevant concentrations of three species of metals (Al, Zn and Cd) for 30 days and the localization and fate of these metals were followed in different tissues of the snails. The measurement of relative distribution of metals between tissues revealed that the digestive gland and kidney account for most of the accumulated metals. Al and Cd (non-essential metals) were redistributed to the digestive gland, possibly because of the presence of specific binding entities in the digestive glands of the herein species. This study focuses on the role of intracellular metal-containing granules on metal sequestration. Three main types of granules were identified in the digestive gland cells namely small, green and yellow granules. The morphological examination and the progressive accumulation of elements within these granules revealed that they are developmental stages with the yellow granule being the mature one. The total number of these granules was found to be significantly increased upon exposure of the snails to Al only. This increase may be a response to the large amount of Al that is accumulated through feeding route of this grazing snail. X-ray microanalysis (XRMA) revealed that metals were localized in all three types of digestive gland granules. The increased amount of ligands (P and S) in the granules may give evidence for their role in metal sequestration. Levels of Al and P were positively correlated in the digestive gland granules. It is possible that aluminium is bound to phosphorus to render it insoluble and so to both immobilize it within the lysosome and to be excreted in a highly insoluble form. On the other hand, both Zn and Cd induced marked upregulation of S in mature (yellow) granules by 26- and 11-folds, respectively. The lysosomal codeposition of S and either Cd or Zn in the lysosomal granules in addition to the increase in RER cisternae may indicate that the exposure to these metals could induce metallothionein synthesis in the cells. The microscopical examinations in the present study revealed that metal detoxification from the digestive gland cells may occur via faeces or via basal exocytosis towards hemocytes dispersed by the connective tissue in the visceral mass. In the kidney, one type of granules, the excretory concretions, was identified in the nephrocytes. The significant increase in the number of these concretions in the snail L. stagnalis upon exposure to metals may give further evidence for their role in metal excretion.

Aluminum Compounds↗
PubMed2006

Electron capture dissociation of peptides metalated with alkaline-earth metal ions.

The possible use of divalent alkaline-earth metal ions, including Mg2+, Ca2+, Sr2+, and Ba2+, as charge carrier for electron capture dissociation of peptides was investigated. Model peptides of RGGGVGGGR and NGGGWGGGN were used to simplify the interpretation of spectral information. It was demonstrated that useful electron capture dissociation (ECD) tandem mass spectra of these metalated peptides could be generated. Interestingly, peptides metalated with different alkaline-earth metal ions generated very similar ECD tandem mass spectra. Metalated c-ions and z-ions were the predominant fragment ions. Only Mg2+-metalated peptides gave somewhat different results. Some nonmetalated c-ions were observed from ECD of [RGGGVGGGR + Mg]2+ but not from [NGGGWGGGN + Mg]2+. Together with some ab initio calculations, it was established that the bound metal ions might activate the acidity of the amide hydrogen. With the presence of high proton affinity moiety, such as N-terminal amino group and/or side chain of the arginine residues, the metalated peptide ions could exist predominantly in their zwitterion forms, in which one or two backbone amide group(s) was deprotonated and the high proton affinity functional group(s) was protonated. It was believed that electron capture leads primarily to the reduction of the mobile proton rather than the metal ions. With this zwitterion model, the formation of nonmetalated c-fragments and the generation of similar ECD spectra for peptides metalated with various alkaline-earth metal ions could readily to be explained. Another interesting observation in the ECD mass spectra of metalated peptides is related to the enhanced formation of the minor ECD products, i.e., (c - 1)(+*) and (z + 1)+ ions. Together with ab initio calculations using a truncated peptide model, various possible reaction mechanisms for the formation of these minor ECD products were evaluated. It was concluded that hydrogen transfer between the initiated formed c and z(.) species plays an important role in the formation (c - 1)(+*) and (z + 1)+ ions. Although peptides metalated with these metal ions do not have better ECD efficiency compared to the multiply-protonated peptides, it provides practical accessibility of ECD methods to analyze small peptides with no basic amino acid residues.

Electrons↗
PubMed2004

Metal-on-metal total hip arthroplasty: the concerns.

The metal-on-metal bearing couple is having a resurgence in clinical applications seen in total hip and hip resurfacing technologies. The most noteworthy advantage of a metal-on-metal implant is the improved wear characteristics seen in vitro on wear simulators and in vivo with retrieved implants. All bearings have disadvantages, and a metal-on-metal bearing is no exception. Concerns exist regarding the generation of metal ions seen in the blood and urine of patients with metal-on-metal implants. These elevated metal ions have theoretical, although not proven, risks related to carcinogenic and biologic concerns. Additionally, concerns exist regarding hypersensitivity, increased incidence of instability and increased costs. Specific patient selection issues arise with metal-on-metal implants. The current generation of implants has only early and mid-term results available, with no long-term series yet published. Therefore, although a metal-on-metal bearing may be considered a viable alternative to either polyethylene or ceramic implants, outstanding and unresolved issues continue to exist with this bearing, as they do with the alternatives.

Adult↗
PubMed2005

Metal-on-metal bearings and hypersensitivity in patients with artificial hip joints. A clinical and histomorphological study.

BACKGROUND: Some patients who have a total hip replacement with a second-generation metal-on-metal articulation have persistent or early recurrence of preoperative symptoms. Characteristic histological changes in the periprosthetic tissues suggested the development of an immunological response. Therefore, in order to determine the relevance of these symptoms, we performed a study of the clinical data and periprosthetic tissues associated with endoprostheses with a metal-on metal articulation that had been retrieved at revision. METHODS: Periprosthetic tissues as well as the clinical data on the patients were obtained from the first nineteen consecutive revisions performed at the treating hospitals. At the time of the revision, fourteen patients had the metal-on-metal articulation exchanged for either an alumina-ceramic or a metal-on-polyethylene articulation. Five patients received another second-generation metal-on-metal total joint replacement. Five-micrometer sections were prepared from the tissue samples, were stained with routine and immunohistochemical methods, and were examined histologically. Histological specimens from three groups of patients, two of which were treated with non-metal-on-metal implants, served as controls. RESULTS: The majority of patients had persistence of their preoperative pain or early recurrence of the pain after the original total hip replacement, and often a pronounced hip joint effusion had developed after the original replacement. Radiographic follow-up showed the development of radiolucent lines in five hips and of osteolysis in another seven hips. At the revision surgery, both the cup and the stem were found to be well fixed in nine patients. The characteristic histological features were diffuse and perivascular infiltrates of T and B lymphocytes and plasma cells, high endothelial venules, massive fibrin exudation, accumulation of macrophages with droplike inclusions, and infiltrates of eosinophilic granulocytes and necrosis. Only a few metal particles were detected. Immunohistochemical analysis demonstrated that the cellular reaction was still active. The patients who received another second-generation metal-on-metal articulation at the time of the revision had no decrease in symptoms. In the control group of tissues obtained at revisions of endoprostheses without cobalt, chromium, or nickel articulations, there were no similar signs of immune reactions. CONCLUSIONS: These histological findings support the possibility of a lymphocyte-dominated immunological response. Although the prevalence of this reaction is low, the persistence or early reappearance of symptoms, including a marked joint effusion and the development of osteolysis, after primary implantation may suggest the possibility of such a reaction.

Aluminum Oxide↗
PubMed2005

Early osteolysis following second-generation metal-on-metal hip replacement.

BACKGROUND: Total hip arthroplasty with use of metal-on-metal bearings has been reintroduced as an alternative to the use of metal-on-polyethylene bearings because of theoretical advantages such as reduced wear and a lower prevalence of osteolysis. However, we observed early osteolysis in a cohort of patients who had been managed with second-generation metal-on-metal hip replacements and investigated the possible etiologic role of metal hypersensitivity. METHODS: We retrospectively analyzed 165 patients (169 hips) who had undergone primary cementless total hip replacement with a contemporary metal-on-metal total hip design between 2000 and 2002. After a minimum duration of follow-up of twenty-four months, nine patients (ten hips) had an osteolytic lesion localized to the greater trochanter. Skin-patch tests for hypersensitivity to metals were performed on the nine patients and on nine randomly selected patients with total hip replacements who did not have osteolytic changes and who were matched to the study cohort for age and gender. Microbiological cultures, histopathologic examinations, and immunohistochemical analysis were performed on samples of periprosthetic tissue that were collected during revision arthroplasty on two hips with early osteolysis. RESULTS: The patients with early osteolysis had a significantly higher rate of hypersensitivity reaction to cobalt compared with controls (p = 0.031). The retrieved periprosthetic tissues showed no evidence of metallic staining, but histologic analysis revealed a perivascular accumulation of CD3-positive T-cells and CD68-positive macrophages and an absence of both particle-laden macrophages and polymorphonuclear cells. Immunohistochemical analysis demonstrated that bone-resorbing cytokines such as IL-1beta and TNF-alpha were produced mainly by infiltrating lymphocytes and activated macrophages. CONCLUSIONS: These findings raise the possibility that early osteolysis in patients with this second-generation metal-on-metal hip replacement is associated with abnormalities consistent with delayed-type hypersensitivity to metal. A prospective study in which a large group of patients is evaluated with multiple diagnostic methods is needed in order to establish whether there is a causal relationship between metal hypersensitivity and osteolysis.

Adult↗
PubMed2000

Metal transfer as a mechanism for metallothionein-mediated metal detoxification.

High kinetic reactivity and high metal affinity of the metal-binding sites of metallothionein are characteristics that would facilitate involvement of the thionein-zinc metallothionein couple in metal transfer or exchange reactions. Studies demonstrating thionein-metallothionein-mediated activation or inhibition of various zinc metalloenzymes and transcription factors provide support for a potential role for metallothionein in metal transfer reactions with receptor molecules. Although a role in basal zinc regulation is currently a topic of debate, less controversial is a role for metallothionein in the detoxification of metals such as cadmium. The toxicity of metals can, in part, be due to adventitious binding to charged sites of target proteins or the displacement of zinc bound to zinc metalloproteins. Zinc metallothionein has the capability of repairing such structures through abstraction of a toxic metal in the former case or through a reciprocal metal transfer reaction that involves abstraction of the toxic metal and donation of the essential metal zinc in the latter. This would confer on metallothionein an active role in the protective response to metal toxicity, rather than a passive one that is solely dependent on the high metal affinity for binding free metal ions. The efficacy of such a mechanism for metal detoxification has been demonstrated with enzymes, actin and zinc finger proteins. With zinc finger proteins, zinc metallothionein can restore both altered secondary structure and inhibited DNA-binding function to functional states through a zinc for cadmium exchange.

Animals↗
PubMed1996

Electronic Structure of [Pt(2)(&mgr;-O(2)CCH(3))(4)(H(2)O)(2)](2+) Using the Quasi-Relativistic Xalpha-SW Method: Analysis of Metal-Metal Bonding, Assignment of Electronic Spectra, and Comparison with Rh(2)(&mgr;-O(2)CCH(3))(4)(H(2)O)(2).

The electronic structure and metal-metal bonding in the classic d(7)d(7) tetra-bridged lantern dimer [Pt(2)(O(2)CCH(3))(4)(H(2)O)(2)](2+) has been investigated by performing quasi-relativistic Xalpha-SW molecular orbital calculations on the analogous formate-bridged complex. From the calculations, the highest occupied and lowest unoccupied metal-based levels are delta(Pt(2)) and sigma(Pt(2)), respectively, indicating a metal-metal single bond analogous to the isoelectronic Rh(II) complex. The energetic ordering of the main metal-metal bonding levels is, however, quite different from that found for the Rh(II) complex, and the upper metal-metal bonding and antibonding levels have significantly more ligand character. As found for the related complex [W(2)(O(2)CH)(4)], the inclusion of relativistic effects leads to a further strengthening of the metal-metal sigma bond as a result of the increased involvement of the higher-lying platinum 6s orbital. The low-temperature absorption spectrum of [Pt(2)(O(2)CCH(3))(4)(H(2)O)(2)](2+) is assigned on the basis of Xalpha-SW calculated transition energies and oscillator strengths. Unlike the analogous Rh(II) spectrum, the visible and near-UV absorption spectrum is dominated by charge transfer (CT) transitions. The weak, visible bands at 27 500 and 31 500 cm(-)(1) are assigned to Ow --> sigma(Pt(2)) and OAc --> sigma(Pt(2)) CT transitions, respectively, although the donor orbital in the latter transition has around 25% pi(Pt(2)) character. The intense near-UV band around 37 500 cm(-)(1) displays the typical lower energy shift as the axial substituents are changed from H(2)O to Cl and Br, indicative of significant charge transfer character. From the calculated oscillator strengths, a number of transitions, mostly OAc --> sigma(Pt-O) CT in nature, are predicted to contribute to this band, including the metal-based sigma(Pt(2)) --> sigma(Pt(2)) transition. The close similarity in the absorption spectra of the CH(3)COO(-), SO(4)(2)(-), and HPO(4)(2)(-) bridged Pt(III) complexes suggests that analogous spectral assignments should apply to [Pt(2)(SO(4))(4)(H(2)O)(2)](2)(-) and [Pt(2)(HPO(4))(4)(H(2)O)(2)](2)(-). Consequently, the anomalous MCD spectra reported recently for the intense near-UV band in the SO(4)(2)(-) and HPO(4)(2)(-) bridged Pt(III) complexes can be rationalized on the basis of contributions from either SO(4) --> sigma(Pt-O) or HPO(4) --> sigma(Pt-O) CT transitions. The electronic absorption spectrum of [Rh(2)(O(2)CCH(3))(4)(H(2)O)(2)] has been re-examined on the basis of Xalpha-SW calculated transition energies and oscillator strengths. The intense UV band at approximately 45 000 cm(-)(1) is predicted to arise from several excitations, both metal-centered and CT in origin. The lower energy shoulder at approximately 40 000 cm(-)(1) is largely attributed to the metal-based sigma(Rh(2)) --> sigma(Rh(2)) transition.

Journal Article↗
PubMed2006

Metal-binding affinity of the transmembrane site in ZntA: implications for metal selectivity.

ZntA, a P1B-type ATPase, confers resistance specifically to Pb2+, Zn2+, and Cd2 in Escherichia coli. Inductively coupled plasma mass spectrometry measurements show that ZntA binds two metal ions with high affinity, one in the N-terminal domain and another in the transmembrane domain. Both sites can bind monovalent and divalent metal ions. Two proteins, deltaN-ZntA, in which the N-terminal domain is deleted, and C59A/C62A-ZntA, in which the N-terminal metal-binding site is disabled by site-specific mutagenesis, can only bind one metal ion. Because C59A/C62A-ZntA can bind a metal ion at the transmembrane site, the N-terminal domain does not block direct access of metal ions to it from the cytosol. A third mutant protein, C392A/C394A-ZntA, in which cysteines from the conserved CPC motif in transmembrane helix 6 are altered, binds metal ions only at the N-terminal site, indicating that both these cysteines form part of the transmembrane site. The metal affinity of the transmembrane site was determined in deltaN-ZntA and C59A/C62A-ZntA by competition titration using a metal ion indicator and by tryptophan fluorescence quenching. The binding affinity for the physiological substrates, Zn2+, Pb2+, and Cd2+, as well as for the extremely poor substrates, Cu2+, Ni2+, and Co2+, range from 10(6)-10(10) M(-1), and does not correlate with the metal selectivity shown by ZntA. Selectivity in ZntA possibly results from differences in metal-binding geometry that produce different structural responses. The affinity of the transmembrane site for metal ions is of similar magnitude to that of the N-terminal site [Liu J. et al. (2005) Biochemistry 44, 5159-5167]; thus, metal transfer between them would be facile.

Adenosine Triphosphatases↗
PubMed2002

Existence of efficient divalent metal ion-catalyzed and inefficient divalent metal ion-independent channels in reactions catalyzed by a hammerhead ribozyme.

The hammerhead ribozyme is generally accepted as a well characterized metalloenzyme. However, the precise nature of the interactions of the RNA with metal ions remains to be fully defined. Examination of metal ion-catalyzed hammerhead reactions at limited concentrations of metal ions is useful for evaluation of the role of metal ions, as demonstrated in this study. At concentrations of Mn2+ ions from 0.3 to 3 mM, addition of the ribozyme to the reaction mixture under single-turnover conditions enhances the reaction with the product reaching a fixed maximum level. Further addition of the ribozyme inhibits the reaction, demonstrating that a certain number of divalent metal ions is required for proper folding and also for catalysis. At extremely high concentrations, monovalent ions, such as Na+ ions, can also serve as cofactors in hammerhead ribozyme-catalyzed reactions. However, the catalytic efficiency of monovalent ions is extremely low and, thus, high concentrations are required. Furthermore, addition of monovalent ions to divalent metal ion-catalyzed hammerhead reactions inhibits the divalent metal ion-catalyzed reactions, suggesting that the more desirable divalent metal ion-ribozyme complexes are converted to less desirable monovalent metal ion-ribozyme complexes via removal of divalent metal ions, which serve as a structural support in the ribozyme complex. Even though two channels appear to exist, namely an efficient divalent metal ion-catalyzed channel and an inefficient monovalent metal ion-catalyzed channel, it is clear that, under physiological conditions, hammerhead ribozymes are metalloenzymes that act via the significantly more efficient divalent metal ion-dependent channel. Moreover, the observed kinetic data are consistent with Lilley's and DeRose's two-phase folding model that was based on ground state structure analyses.

Base Sequence↗
PubMed2006

Characterization of a novel plant promoter specifically induced by heavy metal and identification of the promoter regions conferring heavy metal responsiveness.

The bean (Phaseolus vulgaris) stress-related gene number 2 (PvSR2) gene responds to heavy metals but not to other forms of environmental stresses. To elucidate its heavy metal-regulatory mechanism at the transcriptional level, we isolated and characterized the promoter region (-1623/+48) of PvSR2. Deletions from the 5' end revealed that a sequence between -222 and -147 relative to the transcriptional start site was sufficient for heavy metal-specific induction of the promoter region of PvSR2. Detailed analysis of this 76-bp fragment indicated that heavy metal-responsive elements were localized in two regions (-222/-188 and -187/-147), each of which could separately confer heavy metal-responsive expression on the beta-glucuronidase gene in the context of a minimal cauliflower mosaic virus 35S promoter. Region I (-222/-188) contains a motif (metal-regulatory element-like sequence) similar to the consensus metal-regulatory element of the animal metallothionein gene, and mutation of this motif eliminated the heavy metal-inducible function of region I. Region II (-187/-147) had no similarity to previously identified cis-acting elements involved in heavy metal induction, suggesting the presence of a novel heavy metal-responsive element. Transformed tobacco (Nicotiana tabacum) seedlings expressing beta-glucuronidase under control of the PvSR2 promoter region (-687/+48) showed heavy metal-specific responsive activity that depended on the type and concentration of the heavy metal and the type of organ. These findings further our understanding of the regulation of PvSR2 expression and provide a new heavy-metal-inducible promoter system in transgenic plants.

Base Sequence↗
PubMed2001

Toxic metals and oxidative stress part I: mechanisms involved in metal-induced oxidative damage.

Toxic metals (lead, cadmium, mercury and arsenic) are widely found in our environment. Humans are exposed to these metals from numerous sources, including contaminated air, water, soil and food. Recent studies indicate that transition metals act as catalysts in the oxidative reactions of biological macromolecules therefore the toxicities associated with these metals might be due to oxidative tissue damage. Redox-active metals, such as iron, copper and chromium, undergo redox cycling whereas redox-inactive metals, such as lead, cadmium, mercury and others deplete cells' major antioxidants, particularly thiol-containing antioxidants and enzymes. Either redox-active or redox-inactive metals may cause an increase in production of reactive oxygen species (ROS) such as hydroxyl radical (HO.), superoxide radical (O2.-) or hydrogen peroxide (H2O2). Enhanced generation of ROS can overwhelm cells' intrinsic antioxidant defenses, and result in a condition known as "oxidative stress". Cells under oxidative stress display various dysfunctions due to lesions caused by ROS to lipids, proteins and DNA. Consequently, it is suggested that metal-induced oxidative stress in cells can be partially responsible for the toxic effects of heavy metals. Several studies are underway to determine the effect of antioxidant supplementation following heavy metal exposure. Data suggest that antioxidants may play an important role in abating some hazards of heavy metals. In order to prove the importance of using antioxidants in heavy metal poisoning, pertinent biochemical mechanisms for metal-induced oxidative stress should be reviewed.

Animals↗
PubMed2001

Influences of metal concentration in phytoplankton and seawater on metal assimilation and elimination in marine copepods.

Radiotracer experiments were conducted to examine the influence of the concentration of Cd, Se, and Zn in ingested phytoplankton (dinoflagellate Prorocentrum minimum and diatom Thalassiosira weissflogii) and in ambient seawater on metal assimilation and elimination efficiencies of three marine copepods, Acartia spinicauda, Paracalanus aculeatus, and Calanus sinicus. The assimilation efficiencies (AEs) decreased by 1.7 to 2.0 times, 1.4 to 4.1 times, and 1.3 to 2.2 times in the copepods with an increase in metal concentration in ingested algae by 16 to 84 times, 14 times, and 45 to 153 times, for Cd, Se, and Zn, respectively. However, the physiologic turnover rate constant was relatively independent of the metal concentration in copepods. No evidence was found of any interaction between Cd and Zn in their assimilation by copepods. Assimilation efficiencies of Cd were higher in copepods feeding on the dinoflagellate P. minimum, whereas the AEs of Zn were higher in copepods feeding on the diatom T. weissflogii. Differences in metal distribution in algal cytoplasm at different ambient metal concentrations may be partially responsible for the observed influence of metal concentration in algal cells on metal assimilation in copepods. However, metal desorption within the gut of the copepod may have little influence on metal assimilation, as a result of the short gut residence time of food particles and the neutral gut pH. Our study also indicated that the ingestion rate of copepods was reduced by a higher concentration of Cd and Se, but was not affected by Zn concentration in the food particles. Consequently, partial regulation of metal trophic transfer in response to increasing metal contamination may be achieved by a change in metal assimilation efficiency and the ingestion activity of the copepod, but not by changes in metal turnover rates from the animals.

Animals↗
PubMed2001

Divalent metal accumulation in freshwater bivalves: an inverse relationship with metal phosphate solubility.

Whole soft tissue concentrations of Mn, Co, Ni, Cu, Zn, Pb, Cd and U were measured in two species of freshwater (unionid) bivalves (Hyridella depressa and Velesunio ambiguus) from a minimally polluted site in the Hawkesbury-Nepean River, south-eastern Australia. Although the mean concentrations of metals in the tissue were similar for each bivalve species, their patterns of accumulation were dissimilar. For each metal, positive linear relationships between tissue concentration and shell length (r2 = 0.37-0.77; P < or = 0.001) and tissue dry weight (r2 = 0.29-0.51; P < or = 0.01) were found in H. depressa, but not in V. ambiguus. However, for both species, positive linear relationships were found between the tissue concentration of each divalent metal and Ca tissue concentration (r2 = 0.59-0.97; P < or = 0.001). For both bivalve species, the normalised rates of accumulation of the metals relative to increasing Ca concentration and/or size, were U approximately = Cd > or = Pb > or = Mn > Co > or = Zn > Cu > Ni. The differential rates of accumulation of divalent metals are interpreted as being predominantly governed by their varying loss rates, which are controlled by the differing solubilities (log Ksp values) of the metals in the phosphatic extracellular granules, the demonstrated major sites of metal deposition in the tissue of H. depressa and V. ambiguus. The rates of accumulation of Mn, Co, Zn, Cu and Ni were linearly and inversely related (r2 = 0.91-0.97; P < or = 0.001) to their solubilities as hydrogen phosphates, a finding consistent with the bioaccumulation model previously developed for the alkaline-earth metals. However, for U, Cd and Pb, this linear inverse relationship did not continue to hold, i.e. their rates of accumulation did not increase with decreasing solubility. However, these results are still consistent with the model if U, Cd and Pb are so insoluble in the granules of H. depressa and V. ambiguus over their lifetime (up to approx. 50 years) that there is effectively no loss of these metals, and hence, no differential between their rates of accumulation. The present results reaffirm the use of Ca tissue concentration to predict the tissue concentrations of other divalent metals by explaining up to 94 and 97% of the variability between individual bivalves of H. depressa and V. ambiguus, respectively. The use of Ca tissue concentration to effectively minimise the inherent variability between individuals in their metal tissue improves the ability of an investigator to discern smaller spatial and/or temporal differences in the metal tissue concentrations of these bivalves, and thus to detect metal pollution.

Animals↗
PubMed1998

Metal-Metal Bonding in d(1)d(1) and d(2)d(2) Bioctahedral Dimer Systems: A Density Functional Study of Face-Shared M(2)X(9)(3)(-) (M = Ti, Zr, Hf, V, Nb, Ta) Complexes.

Density functional theory is used to investigate the electronic and geometric structures and periodic trends in metal-metal bonding of d(1)d(1) and d(2)d(2) face-shared M(2)X(9)(3)(-) dimers of Ti, Zr, Hf (d(1)d(1)) and V, Nb, Ta (d(2)d(2)). For these systems three distinct coupling modes can be recognized, depending on the occupation of the trigonal t(2g)(a(1) + e) single-ion orbitals, which determine the ground-state geometry and extent of metal-metal bonding. For Ti(2)Cl(9)(3)(-), the [a(1) x a(1)] broken-symmetry optimized structure, corresponding to significant delocalization of the metal-based sigma electrons, nicely rationalizes the strong antiferromagnetic coupling reported for Cs(3)Ti(2)Cl(9). The ground-state geometries for Zr(2)Cl(9)(3)(-) and Hf(2)Cl(9)(3)(-) correspond to complete delocalization of the metal-based electrons in a metal-metal sigma bond. For V(2)Cl(9)(3)(-), the global minimum is found to be the ferromagnetic [a(1)e x e(2)] spin-quintet state giving rise to a long V-V separation, consistent with the known structure and reported weak ferromagnetic behavior of Cs(3)V(2)Cl(9). For Nb(2)X(9)(3)(-) (X = Cl, Br, I) and Ta(2)Cl(9)(3)(-), the [a(1)e x a(1)e] spin-triplet state, where complete delocalization of the sigma and delta(pi) electrons occur in a metal-metal double bond, is found to be the global minimum and consequently relatively short internuclear distances result, again, in good agreement with experiment. The periodic trends in metal-metal bonding in these and the isovalent d(3)d(3) complexes can be rationalized in terms of the energetic contributions of orbital overlap (DeltaE(ovlp)) and spin polarization (DeltaE(spe)), the difference DeltaE(spe) - DeltaE(ovlp) determining the tendency of the metal-based electrons to delocalize in the dimer. For d(1)d(1) systems, DeltaE(ovlp) is always greater than DeltaE(spe) and therefore delocalized ground states result for all complexes of the titanium triad. Across the first transition series, the dramatic increase in DeltaE(spe) dominates DeltaE(ovlp) and therefore V(2)Cl(9)(3)(-) and Cr(2)Cl(9)(3)(-) have localized ground states. For the second and third transition series, the much larger DeltaE(ovlp) term ensures that all these complexes remain delocalized.

Journal Article↗
PubMed1999

Metal-Metal Bonding in M(2)Cl(6)(H(2)PCH(2)PH(2))(2), M(2)Cl(6)(PH(3))(4), and M(2)Cl(10)(4-) (M = Cr, Mo, W) Edge-Shared Dimer Systems.

Density functional theory is used to determine the electronic structures, geometries, and periodic trends in metal-metal bonding in the homo- and heterobimetallic d(3)d(3) edge-shared systems M(2)Cl(10)(4-), M(2)Cl(6)(PH(3))(4), and M(2)Cl(6)(H(2)PCH(2)PH(2))(2) (M = Cr, Mo, W). The much shorter metal-metal distances in these complexes relative to M(2)Cl(10)(4-) (M = Mo, W) are shown to arise solely from electronic differences between chlorine and phosphine donors. Due to inversion of the delta and delta orbitals, the complexes M(2)Cl(6)(PH(3))(4) and M(2)Cl(6)(H(2)PCH(2)PH(2))(2) (M = Mo, W) are found to possess formal metal-metal double bonds. The periodic trends in metal-metal bonding in these systems are rationalized in terms of the energetic contributions of orbital overlap (DeltaE(ovlp)) and spin polarization (DeltaE(spe)). The reduction in DeltaE(spe) and increase in DeltaE(ovlp) on replacement of axial chlorides with phosphine both favor stronger metal-metal bonding in the phosphine-based complexes. The strong linear dependence observed between DeltaE(spe) and DeltaE(ovlp) enables the metal-metal bonding in these systems to be predicted simply from single-ion spin-polarization energies. The antiferromagnetic coupling in M(2)Cl(6)(H(2)PCH(2)PH(2))(2) (M = Mo, W) and MoWCl(6)(H(2)PCH(2)PH(2))(2) is shown to be mostly due to coupling of the metal delta electrons, with a smaller contribution from the pi electrons, particularly for the dimolybdenum complex.

Journal Article↗
PubMed2002

Seasonal variations in condition and liver metal concentrations of yellow perch (Perca flavescens) from a metal-contaminated environment.

The relationships between liver metal (Cu, Zn and Ni) concentration and physical condition of yellow perch (Perca flavescens) were compared in fish sampled in five northeastern Ontario Lakes in the spring and seven lakes in the fall of 1997. These lakes represented a wide range of metal contamination, pH, dissolved organic carbon (DOC) and alkalinity. Lakes situated closest to emission point source had lower pH, alkalinity and DOC and higher aqueous metal concentrations than other lakes in this study. Fish from these lakes displayed higher liver metal concentrations. Seasonal variations in liver metal concentrations were observed in the most metal-contaminated lakes, with higher Cu levels in the spring and higher Zn and Ni concentrations in the fall. These fish had lower indicators of physical condition than fish from cleaner lakes. In particular, more metal-contaminated fish were smaller for a similar age, indicating slower growth rate, and had lower relative condition factor and scaling coefficients. The scaling coefficient is a descriptor of the growth pattern of fish, and higher values indicate increased weight gain per unit of growth in length. Negative (P<0.05) relationships are reported between lake alkalinity and liver Cu concentration (in both seasons) and Zn concentration (in spring only). In both seasons mean liver Cu content was negatively associated (P<0.05) with the scaling coefficient of the fish samples. With these data, we propose the use of the scaling coefficient as a simple biomonitoring tool to detect metal effects on the physical condition of yellow perch. Data presented here provide information on seasonal variations in liver metal concentration and condition of yellow perch that should be considered in the design of field studies investigating physiological effects of metals in wild fish. This work also describes a gradient of lake and fish metal contamination that is related to the physical condition of yellow perch. This system provides a unique research opportunity to further our limited knowledge of the effects of metals on the physiology of wild fish.

Animals↗
PubMed2004

Selectivity of metal binding and metal-induced stability of Escherichia coli NikR.

NikR from Escherichia coli is a nickel-responsive transcription factor that regulates the expression of a nickel ion transporter. Metal analysis reveals that NikR can bind a variety of divalent transition metals, including Ni(II), Cu(II), Zn(II), Co(II), and Cd(II). The selectivity of metal binding to NikR was investigated by using electronic absorption spectroscopy and small-molecule competitors. The relative affinities, Mn(II) < Co(II) < Ni(II) < Cu(II) > or = Zn(II), follow the Irving-Williams series of metal-complex stabilities. Similar metal affinities were measured for the isolated metal-binding domain of NikR. To determine if any of these metal ions confer a differential effect on NikR, the stability of the metal-bound complexes was examined. In both thermal and chemical denaturation experiments, nickel binding stabilizes the protein more than any of the other metals tested. Thermal denaturation experiments indicate that metal dissociation occurs after loss of secondary structure, but there was no evidence for metal binding to unfolded protein following reversible chemical denaturation. These experiments demonstrate that, although several different metals can bind to NikR, nickel exerts a selective allosteric effect. The implications of these experiments on the in vivo role of NikR as a nickel metalloregulator are discussed.

Circular Dichroism↗
PubMed1999

Probing the role of metal ions in RNA catalysis: kinetic and thermodynamic characterization of a metal ion interaction with the 2'-moiety of the guanosine nucleophile in the Tetrahymena group I ribozyme.

Deciphering the role of individual metal ions in RNA catalysis is a tremendous challenge, as numerous metal ions coat the charged backbone of a folded RNA. Metal ion specificity switch experiments combined with quantitative analysis may provide a powerful tool for probing specific metal ion-RNA interactions and for delineating the role of individual metal ions among the sea of metal ions bound to RNA. We show herein that Mn(2+) rescues the deleterious effect of replacing the 2'-OH of the guanosine nucleophile (G) by -NH(2) (G(NH)()2) in the reaction catalyzed by the Tetrahymena group I ribozyme (E), and the Mn(2+) concentration dependence suggests that a single metal ion is responsible for rescue. This provides strong evidence for a metal ion interaction with the 2'-moiety of G in this ribozyme (referred to as M(C)), confirming and extending previous results in a bacteriophage group I intron [Sjögren, A.-S., Pettersson, E., Sjöberg, B.-M., and Strömberg, R. (1997) Nucleic Acids Res. 25, 648-654]. Toward understanding the >10(6)-fold catalytic contribution of the 2'-OH of G, we have determined the individual reaction steps affected by M(C) and quantitated these effects. has only a small effect on binding of G(NH)()2 to the free ribozyme or ribozyme.oligonucleotide complexes that lack the reactive phosphoryl group. In contrast, increases the binding of G(NH)()2 to the ribozyme.oligonucleotide substrate (E.S) complex 20-fold and increases the binding of S to the E.G(NH)()2 complex by the same amount. These and other observations suggest that M(C) plays an integral role in the coupled binding of the oligonucleotide substrate and the guanosine nucleophile. This metal ion may be used to align the nucleophile within the active site, thereby facilitating the reaction. Alternatively or in addition, M(C) may act in concert with an additional metal ion to coordinate and activate the 3'-OH of G. Finally, these experiments have also allowed us to probe the properties of this metal ion site and isolate the energetic effects of the interaction of this specific metal ion with the 2'-moiety of G.

Amines↗