Biomedical subjects

X Chen

Publications and source records attributed to X Chen.

At least 199 records · Page 11Linked to original sources

PubMed2001

Rhenium(I) carbonyl complexes of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPT). Rhenium(I)-promoted methoxylation of the triazine ring carbon atom in dinuclear rhenium complexes.

2,4,6-Tris(2-pyridyl)-1,3,5-triazine (TPT) bridged dinuclear rhenium(I) tricarbonyl halide complexes with the composition (mu-TPT)[ReX(CO)(3)](2) (3, X = Cl; 4, X = Br) can be made either by one-pot reaction of TPT with 2 equiv of [ReX(CO)(5)] (X = Cl and Br) in chloroform or by reacting mononuclear [ReX(CO)(3)(TPT)] (2) (1, X = Cl; 2, X = Br) with an excess amount of [ReX(CO)(5)]. Crystal data are as follows. 1: monoclinic, P2(1)/c, a = 11.751(1) A, b = 11.376(1) A, c = 15.562(2) A, beta = 103.584(2) degrees, V = 2022.0(4) A(3), Z = 4. 2: monoclinic, P2(1)/c, a = 11.896(1) A, b = 11.396(1) A, c = 15.655(1) A, beta = 104.474(2) degrees, V = 2054.9(3) A(3), Z = 4. 3: triclinic, P1, a = 11.541(2) A, b = 12.119(2) A, c = 13.199(2) A, alpha = 80.377(2) degrees, beta = 76.204(3) degrees, gamma = 66.826(2) degrees, V = 1642.5(4) A(3), Z = 2. Crystals of 4 crystallized from acetone: triclinic, P1, a = 11.586(5) A, b = 12.144(5) A, c = 13.364(6) A, alpha = 80.599(7) degrees, beta = 76.271(8) degrees, gamma = 67.158(8) degrees, V = 1678.0(12) A(3), Z = 2. Crystals of 4' are obtained from CH(2)Cl(2)-pentane solution: monoclinic, C2/c, a = 17.555(4) A, b = 15.277(3) A, c = 13.093(3) A, beta = 111.179(3) degrees, V = 3274.0(12) A(3), Z = 4. By contrast, similar reactions in the presence of methanol yielded complexes with the composition [mu-C(3)N(3)(OMe)(py)(2)(pyH)][ReX(CO)(3)](2) (5, X = Cl; 6, X = Br). Crystal data for 5: monoclinic, C2/c, a = 26.952(2) A, b = 16.602(1) A, c = 14.641(1) A, beta = 116.147(1) degrees, V = 5880.5(8) A(3), Z = 8. 6: monoclinic, C2/c, a = 27.513(3) A, b = 16.740(2) A, c = 14.837(2) A, beta = 116.925(2) degrees, V = 6092.8(10) A(3), Z = 8. An unusual metal-induced methoxylation at the carbon atom of the triazine ring of the bridging TPT ligand was observed. The nucleophilic attack of MeO(-) on C(3) results in a tetrahedral geometry around the carbon atom. Concomitantly, the uncoordinated pyridyl ring is protonated and rotated into a perpendicular orientation relative to the central C(3)N(3) ring. Reaction of TPT with [NEt(4)](2)[ReBr(3)(CO)(3)] in benzene-methanol resulted in an unexpected dinuclear complex 7, with formulation [mu-C(3)N(3)(OMe)(py)(3)][Re(CO)(3)][ReBr(CO)(3)]. The methoxylated TPT ligand functions simultaneously as a tridentate and bidentate ligand with two fac-Re(CO)(3)(+) cores. Crystal data for 7: monoclinic, P2(1)/n, a = 12.114(1) A, b = 14.878(1) A, c = 15.807(1) A, beta = 104.601(1) degrees, V = 2756.9(3) A(3), Z = 4.

Journal Article↗

PubMed2001

Krüppel-like factor 4 (gut-enriched Krüppel-like factor) inhibits cell proliferation by blocking G1/S progression of the cell cycle.

Krüppel-like factor 4 (KLF4) is an epithelial cell-enriched, zinc finger-containing transcription factor, the expression of which is associated with growth arrest. Previous studies show that constitutive expression of KLF4 inhibits DNA synthesis but the manner by which KLF4 exerts this effect is unclear. In the present study, we developed a system in which expression of KLF4 is controlled by a promoter that is induced upon treatment of cells containing the receptors for the insect hormone, ecdysone, with ponasterone A, an ecdysone analogue. The rate of proliferation of a stably transfected colon cancer cell line, RKO, was significantly decreased following addition of ponasterone A when compared with untreated cells. Flow cytometric analyses indicated that the inducible expression of KLF4 caused a block in the G(1)/S phase of the cell cycle. A similar block was observed when ecdysone receptor-containing RKO cells were infected with a replication-defective recombinant adenovirus containing an inducible KLF4 and treated with ponasterone A. Results of these studies provide evidence that the inhibitory effect of KLF4 on cell proliferation is mainly exerted at the G(1)/S boundary of the cell cycle.

Adenoviridae↗

PubMed2001

p63alpha and DeltaNp63alpha can induce cell cycle arrest and apoptosis and differentially regulate p53 target genes.

The p53 tumor suppressor protein plays a critical role in the regulation of the cell cycle and apoptosis. The importance of p53's functions is underscored by the high incidence of p53 mutations in human cancers. Recently, two p53-related proteins, p73 and p63, were identified as members of the p53 gene family. Multiple isoforms of p73 have been found, including DeltaN variants in which the N-termini are truncated. p63 is expressed as three major forms, p63alpha, p63beta and p63gamma, each of which differ in their C-termini. All three forms can be alternatively transcribed from a cryptic promoter located within intron 3, producing DeltaNp63alpha, DeltaNp63beta and DeltaNp63gamma. The high degree of similarity of p73 and p63 to evolutionarily conserved regions of p53 suggests that these proteins play an important and potentially redundant role in regulating cell cycle arrest and apoptosis. Here we describe the characterization of cell lines generated to inducibly express p63alpha and DeltaNp63alpha. We have found that p63alpha and DeltaNp63alpha can differentially regulate endogenous p53 target genes and induce cell cycle arrest and apoptosis. Deletion of the N-terminal 26 amino acids of DeltaNp63alpha abolished its ability to transactivate p53 target genes and induce cell cycle arrest and apoptosis. This indicates that a putative transactivation domain exists within the N-terminus of the DeltaN variants of p63. Furthermore, the differential regulation of p53 target genes by p63alpha and DeltaNp63alpha suggests that p63 and p53 utilize both similar and different signaling pathways to execute their cellular functions.

14-3-3 Proteins↗

PubMed2001

Expression of a dominant negative inhibitor of NF-kappaB protects MIN6 beta-cells from cytokine-induced apoptosis.

BACKGROUND: Islet graft injury by cytokines released from inflammatory cells (macrophages) that infiltrate the transplant site is an important mechanism of early islet transplant dysfunction. This detrimental "cytokine effect" is thought to be mediated by NF-kappaB-dependent up-regulation of iNOS gene expression and increased nitric oxide (NO) production by the islet. We attempted to make a beta-cell resistant to cytokine-induced apoptosis by transfecting the parent line with a dominant negative inhibitor of NF-kappaB. METHODS: A flag-tagged IkappaBalphaM cDNA subcloned into an SFFV-neo vector was used to transfect parent beta-Cell line MIN6. MIN6 and the resultant mutant (2Bm) were cultured for 24 h in a cytokine mixture including IL-1beta (50 units/mL), TNF-alpha (1000 units/mL), and IFN-gamma (750 units/mL) and cotreated with either the iNOS inhibitor L-NMMA (1 mM) or the caspase inhibitor Z-VAD (0.1 mM). NF-kappaB translocation was determined by gel shift. Nitrite production was determined by the Griess reaction. Apoptosis was determined by flow cytometry. RESULTS: When treated with cytokine 2Bm demonstrated significantly less NF-kappaB nuclear translocation, nitrite production, and apoptosis than parent MIN6. The rate of apoptosis in cytokine-treated 2Bm was a third less than that for cytokine-treated MIN6 and was similar to MIN6 cotreated with L-NMMA. Z-VAD cotreatment completely eliminated apoptosis in both MIN6 and 2Bm. CONCLUSIONS: Cytokine-induced cell death in the MIN6 beta-cell line involves mechanisms that are, in part, NF-kappaB and NO dependent. Inhibition of NF-kappaB and NO production by the dominant negative inhibitor of NF-kappaB is cytoprotective. This type of genetic modification may prove to be one avenue for improving efficacy of islet transplantation.

Animals↗

PubMed2001

Investigation of pharmaceutical high-performance liquid chromatography assay bias using experimental design.

This article presents a systematic approach to investigate, document, and eliminate pharmaceutical HPLC assay bias using experimental design. This is the first article to describe the application of experimental design in the area of assay bias. It is found that both formulation and analytical variables can contribute to pharmaceutical HPLC assay bias using model compounds and formulations.

Chromatography, High Pressure Liquid↗

PubMed2001

[Mycophenolate mofetil in the treatment of primary nephrotic syndrome].

OBJECTIVE: To investigate the efficacy and safety of mycophenolate mofetil (MMF) on treating refractory primary nephrotic syndrome. METHODS: Forty-one patients with refractory nephrotic syndrome confirmed by renal biopsy, 19 with minor lesion (minimal lesion nephropathy and mesangial proliferative glomerulonephritis), 18 with membranous nephropathy (MN), 3 with focal segmental glomerulosclerosis (FSGS), and one with mesangioproliferative glomerulonephritis (MPGN), were treated by MMF combined with prednisone. The initial dosage of MMF was 1.0 - 2.0 g/d for three months and then the dosage was tapered gradually. The duration of MMF treatment was at least six months. Prednisone at the dose of 20 - 60 g/d was used at the beginning of the combined treatment and then the dosage was tapered gradually. Follow-up interview was conducted regularly. Four patients were rebiopsyed by the end of observation. RESULTS: The combined treatment of MMF/prednisone decreased the urine protein and elevated the serum albumin significantly among patients with minor lesion and MN (P < 0.001). All patients with minor lesion achieved clinical remission. Eleven of the nineteen cases responded within four weeks, and twelve of them obtained complete clinical remission. The dosage of prednisone could be tapered smoothly among the steroid dependent patients. Thirteen of the eighteen patients with MN achieved remission, however only six responded within four weeks and only three of them achieved complete clinical remission. During the treatment, four patients experienced transient increase of urine protein due to infection and recovered spontaneously without alteration of treatment. Side effects were tolerable except one case was withdrawn due to the decrease of hemoglobin. Renal function remained stable during the treatment. No obvious alteration could be found in renal biopsy by the end of treatment among four patients. CONCLUSION: MMF is an effective and safe immunosuppressive agent for refractory nephrotic syndrome.

Adolescent↗

PubMed2001

Simultaneous determination of flupirtine and its major active metabolite in human plasma by liquid chromatography-tandem mass spectrometry.

A rapid, selective and sensitive HPLC-tandem mass spectrometry method was developed and validated for simultaneous determination of flupirtine and its active metabolite D-13223 in human plasma. The analytes and internal standard diphenhydramine were extracted from plasma samples by liquid-liquid extraction, and chromatographed on a C18 column. The mobile phase consisted of acetonitrile-water-formic acid (60:40:1, v/v/v), at a flow rate of 0.5 ml/min. Detection was performed on a triple quadrupole tandem mass spectrometer by selected reaction monitoring (SRM) mode via atmospheric pressure chemical ionization (APCI). The method has a limit of quantitation of 10 ng/ml for flupirtine and 2 ng/ml for D-13223, using 0.5-ml plasma sample. The linear calibration curves were obtained in the concentration range of 10.0-1500.0 ng/ml for flupirtine and 2.0-300.0 ng/ml for D-13223. The intra- and inter-run precision (RSD), calculated from quality control (QC) samples was less than 7.2% for flupirtine and D-13223. The accuracy as determined from QC samples was less than 5% for the analytes. The overall extraction recoveries of flupirtine and D-13223 were determined to be about 66% and 78% on average, respectively. The method was applied for the evaluation of the pharmacokinetics of flupirtine and active metabolite D-13223 in volunteers following peroral administration.

Aminopyridines↗

PubMed2001

[Urinary RBP and glucocorticoid hormone therapy sensibility in adult primary nephrotic syndrome].

OBJECTIVE: To clarify the relationship between the renal tubular function and the efficacy of glucocorticoid, in adult patients with primary nephrotic syndrome. METHODS: Pro- and post-therapy urinary RBP and NAG were determined double antibodies sandwich ELISA and color comparimetry with p-nitrate reductase, respectively, in eighty adult patients with primary nephritic syndrome, according to the concentrations of urinary protein, these patients were divided into three no remission group, partial remission group and fully remission group, fifty-one normal persons as control group. Renal tubular function parameters among groups were compared before and after therapy, and the results were analysed when those parameters were used to predict the sensibility of glucocorticoid therapy. RESULTS: 1. There were no significant differences in urinary RBP, NAG and protein levels before treatment among patient groups (P > 0.05); 2. There were significant decreases in urinary RBP and NAG following glucocorticoid therapy in those response to glucocorticoid (P < 0.01); 3. There were significant differences in urinary protein, RBP and NAG among three groups of patients with primary nephrotic syndrome after treatment. CONCLUSION: Determination of urinary RBP may predict the sensibility of the therapy in the adult patients with primary nephrotic syndrome and its diagnostic efficiency is better than urinary NAG.

Acetylglucosaminidase↗

PubMed2001

Ribozyme-mediated inhibition of caspase-3 activity reduces apoptosis induced by 6-hydroxydopamine in PC12 cells.

6-Hydroxydopamine (6-OHDA) is a neurotoxin used in the induction of experimental Parkinson's disease in both animals and PC12 cells, which are derived from rat pheochromocytoma tumors and have many properties similar to dopamine neurons. Biochemical and molecular approaches have shown that low doses of 6-OHDA induce apoptosis in PC12 cells and, in the processing of apoptosis, caspases are crucial mediators, and caspase inhibition is sufficient to rescue PC12 cells from apoptosis induced by 6-OHDA. However, because this caspase inhibition targets multiple caspases, it is not known whether a single caspase is primarily responsible for effecting cell death in this model. To assess the particular member (caspase-3) of the ced-3 family relevant to cell death and to position their activation within the apoptotic pathway, we constructed a hammerhead ribozyme directed against rat caspase-3, which could downregulate the expression of caspase-3 in vitro and in vivo, and transfer to PC12 cells. The results show that the ribozymes against caspase-3 could protect PC12 cells from apoptosis induced by low doses of 6-OHDA. The PC12 cell transfected with the ribozymes shows a significant decrease in caspase-3 activity compared with control cells at various time points. Parallel to the reduced caspase-3 protease activity, similar decreased levels of apoptotic cells and DNA fragmentation were also assessed by staining with Hoechst 33258 and ELISA, respectively. Overexpression of p35, a general caspase inhibitor, also protected PC12 cells from apoptosis. These results confirm that caspases play an important role in 6-OHDA-induced PC12 cell apoptosis and indicate that caspase-3 itself is one of the crucial mediators of neurotoxin-induced PC12 cell apoptosis.

Adrenergic Agents↗

PubMed2001

Health effects of fluoride pollution caused by coal burning.

Recently a huge amount of fluoride in coal has been released into indoor environments by the combustion of coal and fluoride pollution seems to be increasing in some rural areas in China. Combustion of coal and coal bricks is the primary source of gaseous and aerosol fluoride and these forms of fluoride can easily enter exposed food products and the human respiratory tract. Major human fluoride exposure was caused by consumption of fluoride contaminated food, such as corn, chilies and potatoes. For each diagnostic syndrome of dental fluorosis, a log-normal distribution was observed on the logarithm of urinary fluoride concentration in students in China. Urinary fluoride content was found to be a primary health indicator of the prevalence of dental fluorosis in the community. In the fluorosis areas, osteosclerosis in skeletal fluorosis patients was observed with a high prevalence. A biochemical marker of bone resorption, urinary deoxypyridinoline content was much higher in residents in China than in residents in Japan. It was suggested that bone resorption was stimulated to a greater extent in residents in China and fluoride may stimulate both bone resorption and bone formation. Renal function especially glomerular filtration rate was very sensitive to fluoride exposure. Inorganic phosphate concentrations in urine were significantly lower in the residents in fluorosis areas in China than in non-fluorosis area in China and Japan. Since airborne fluoride from the combustion of coal pollutes extensively both the living environment and food, it is necessary to reduce fluoride pollution caused by coal burning.

Adult↗

PubMed2001

Transient dynamics and food-web complexity in the Lotka-Volterra cascade model.

How does the long-term behaviour near equilibrium of model food webs correlate with their short-term transient dynamics? Here, simulations of the Lotka -Volterra cascade model of food webs provide the first evidence to answer this question. Transient behaviour is measured by resilience, reactivity, the maximum amplification of a perturbation and the time at which the maximum amplification occurs. Model food webs with a higher probability of local asymptotic stability may be less resilient and may have a larger transient growth of perturbations. Given a fixed connectance, the sizes and durations of transient responses to perturbations increase with the number of species. Given a fixed number of species, as connectance increases, the sizes and durations of transient responses to perturbations may increase or decrease depending on the type of link that is varied. Reactivity is more sensitive to changes in the number of donor-controlled links than to changes in the number of recipient-controlled links, while resilience is more sensitive to changes in the number of recipient-controlled links than to changes in the number of donor-controlled links. Transient behaviour is likely to be one of the important factors affecting the persistence of ecological communities.

Computer Simulation↗

PubMed2001

Human ITCH is a coregulator of the hematopoietic transcription factor NF-E2.

We have cloned a new protein that interacts with the hematopoietic DNA-binding transcription factor, p45/NF-E2, by screening a human erythroleukemia cell cDNA library with the yeast two-hybrid approach. Predicted peptide sequence and chromosomal mapping identified the cloned molecule to be the product of the human ortholog of the mouse Itch gene, which has been implicated previously in the regulation of growth and differentiation of erythroid and lymphoid cells. Transfection experiments indicate that this human ITCH protein can act as a transcriptional corepressor of p45/NF-E2. Our data provide novel insights into the functional roles of the mammalian ITCH proteins in the development of hematopoietic cell lineages.

Animals↗

PubMed2001

In vivo restoration of laminin 5 beta 3 expression and function in junctional epidermolysis bullosa.

The blistering disorder, lethal junctional epidermolysis bullosa (JEB), can result from mutations in the LAMB3 gene, which encodes laminin 5 beta3 (beta3). Appropriate expression of LAMbeta3 in JEB skin tissue could potentially ameliorate the symptoms of the underlying disease. To explore the utility of this therapeutic approach, primary keratinocytes from six unrelated JEB patients were transduced with a retroviral vector encoding beta3 and used to regenerate human skin on severe combined immunodeficient (SCID) mice. Tissue regenerated from beta3-transduced JEB keratinocytes produced phenotypically normal skin characterized by sustained beta3 expression and the formation of hemidesmosomes. Additionally, beta3 gene transfer corrected the distribution of a number of important basement membrane zone proteins including BPAG2, integrins beta4/beta1, and laminins alpha3/gamma2. Skin produced from beta3-negative (beta3[-]) JEB cells mimicked the hallmarks of the disease state and did not exhibit any of the aforementioned traits. Therefore, by effecting therapeutic gene transfer to beta3-deficient primary keratinocytes, it is possible to produce healthy, normal skin tissue in vivo. These data support the utility of gene therapy for JEB and highlight the potential for gene delivery in the treatment of human genetic skin disease.

Basement Membrane↗

PubMed2001

[Effects of sense and antisense tissue inhibitor metalloproteinase-1 on apoptosis of rat glomerular mesangial cells and its related-gene expressions].

OBJECTIVE: To investigate the role of tissue inhibitor of metalloproteinase-1 (TIMP-1) in apoptosis of rat glomerular mesangial cells (RMC) in vitro. METHODS: TIMP-1 full length cDNA cloned in previous work was used to construct two recombinant plasmids, eukaryotic expression rector, pcDNA3-TIMP-1 (PTs) encoding sense TIMP-1 and pcDNA3-ATIMP-1 (PTas) encoding antisense TIMP-1. These two vectors were then transfected into RMC with lipofectin DOTAP. Northern blotting was used to detect the expression of sense and antisense TIMP-1. Apoptosis was induced in normal RMC, RMC transfected with pCDNA3, PTs and PTas by serum deprivation. Those cells were monitored for apoptosis over extended periods of time (12 h approximately 4 d) by TUNEL and DNA electrophoressis. Furthermore, the expression of apoptosis related-genes was also investigated by RT-PCR. RESULTS: Northern blot analysis and RT-PCR demonstrated that cells transfected with exogenous genes effectively expressed sense or antisense TIMP-1 mRNA. RMC transfected with PTas underwent apoptosis after 12 h of serum deprivation, normal RMC and RMC transfected with pcDNA3 underwent apoptosis after 48 hours in serum-free medium, while RMC transfected with PTs did not undergo apoptosis until 4 days after serum deprivation. Furthermore, TIMP-1 upregulation inhibited expression of bax but had no effect on the expression of bcl-2. CONCLUSION: TIMP-1 suppressed apoptosis of RMC, which suggests a novel activity of TIMP-1 in kidney. Antisense TIMP-1 promotes apoptosis in RMC. The effects of TIMP-1 on RMC apoptosis perhaps correlate with inhibition of bax expression.

Animals↗

PubMed2001

The triple threat to nascent apolipoprotein B. Evidence for multiple, distinct degradative pathways.

We previously showed that Omega-3 fatty acids reduce secretion of apolipoprotein B (apoB) from cultured hepatocytes by stimulating post-translational degradation. In this report, we now characterize this process, particularly in regard to the two known processes that degrade newly synthesized apoB, endoplasmic reticulum (ER)-associated degradation and re-uptake from the cell surface. First, we found that Omega-3-induced degradation preferentially reduces the secretion of large, assembled apoB-lipoprotein particles, and apoB polypeptide length is not a determinant. Second, based on several experimental approaches, ER-associated degradation is not involved. Third, re-uptake, the only process known to destroy fully assembled nascent lipoproteins, was clearly active in primary hepatocytes, but Omega-3-induced degradation of apoB continued even when re-uptake was blocked. Cell fractionation showed that Omega-3 fatty acids induced a striking loss of apoB100 from the Golgi, while sparing apoB100 in the ER, indicating a post-ER process. To determine the signaling involved, we used wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, which blocked most, if not all, of the Omega-3 fatty acid effect. Therefore, nascent apoB is subject to ER-associated degradation, re-uptake, and a third distinct degradative pathway that appears to target lipoproteins after considerable assembly and involves a post-ER compartment and PI3K signaling. Physiologic, pathophysiologic, and pharmacologic regulation of net apoB secretion may involve alterations in any of these three degradative steps.

Androstadienes↗

PubMed2001

Soluble electroluminescent poly(phenylene vinylene)s with balanced electron- and hole injections.

We report a new route for the design of efficient soluble electroluminescent PPV-based copolymers bearing electron-deficient oxadiazole (OXD) moieties on side chains. The introduction of OXD through a long alkylene spacer with PPV backbone provides a molecular dispersion of OXD in the film; both the side chain OXD and the main chain PPV do retain their own electron-transport and emissive properties, respectively. The use of phenylene vinylene derivatives with asymmetric and branched substituents and a long spacer provides solubility for ease of device fabrication as well as amorphous structure to allow a well-mixing of OXD groups with the main chains. By properly adjusting the OXD content through copolymerization, we can tailor the chemical structure of electroluminescent material to give a balance of hole- and electron injections for various metal cathodes, such that the quantum efficiency is significantly improved and the turn-on voltage is reduced for the devices with aluminum and calcium. For the device with calcium fabricated in open air, a maximum brightness of 15000 cd/m(2) at 15 V/100 nm and a maximum luminance efficiency of 2.27 cd/A can be obtained, respectively, about 30 times brighter and 9.4 times more efficient than those with the corresponding homopolymer, poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV). The use of physical blends to simulate the copolymers provides no significant improvement, since phase-separation structures appear, causing an inefficient utilization of OXD and sometimes voltage-dependent emission spectra. The present route permits a fabrication of single layer PLED with high brightness, high efficiency, and low turn-on voltage.

Journal Article↗

PubMed2001

Unraveling structure-reactivity relationships in SNV reactions: kinetics of the reactions of methoxybenzylidenemalononitrile, 2-(methylthiobenzylidene)-1,3-indandione, 2-(benzylthiobenzylidene)-1,3-indandione, and methyl beta-methylthio-alpha-nitrocinnamate with OH- and thiolate ions in aqueous DMSO.

The kinetics of the title reactions were determined in 50% DMSO-50% water (v/v) at 20 degrees C; n-BuS-, HOCH2CH2S-, and MeO2CCH2S- were used as thiolate ions. The reactions with the thiolate ions gave rise to two separate kinetic processes. The first refers to rapid, reversible attachment of RS- to the substrate leading to a tetrahedral intermediate (k1RS), k(-1)RS, the second to the conversion of the intermediate to products (k2RS). In most cases all of the rate constants (k1RS, k(-1)RS and k2RS could be determined. In combination with results from previous studies, a detailed discussion regarding the effects of activating substituents and leaving groups on rate and equilibrium constants as well as on intrinsic rate constants is presented. The reaction with OH- only allowed a determination of k1OH for nucleophilic attack on the substrate; in this case the tetrahedral intermediate remains at steady-state levels under all conditions.

Journal Article↗

PubMed2001

Rb(4)Hg(5)(Te(2))(2)(Te(3))(2)Te(3), [Zn(en)3](4)In(16)(Te2)4(Te3)Te22, and K2Cu2(Te2)(Te3): novel metal polytellurides with unusual metal-tellurium coordination.

Three novel metal polytellurides Rb(4)Hg(5)(Te(2))(2)(Te(3))(2)Te(3) (I), [Zn(en)(3)](4)In(16)(Te(2))(4)(Te(3))Te(22) (II), and K(2)Cu(2)(Te(2))(Te(3)) (III) have been prepared by solvothermal reactions in superheated ethylenediamine at 160 degrees C. Their crystal structures have been determined by single-crystal X-ray diffraction techniques. Crystal data for I: space group Pnma, a = 9.803(2) A, b = 9.124(2) A, c = 34.714(7) A, Z = 4. Crystal data for II: space group C2/c, a = 36.814(7) A, b = 16.908(3) A, c = 25.302(5) A, beta = 128.46(3) degrees, Z = 4. Crystal data for III: space group Cmcm, a = 11.386(2) A, b = 7.756(2) A, c = 11.985(2) A, Z = 4. The crystal structure of I consists of 1D infinite ribbons of [Hg(5)(Te(2))(2)(Te(3))(2)Te(3)](4-), which are composed of tetrahedral HgTe(4) and trigonal HgTe(3) units connected through the bridging Te(2-), (Te(2))(2-), and (Te(3))(2-) ligands. II is a layered compound containing InTe(4) tetrahedra that share corners and edges via Te, Te(2), and Te(3) units to form a 2D slab that contains relatively large voids. The [Zn(en)(3)](2+) template cations are filled in these voids and between the slabs. The primary building blocks of III are CuTe(4) tetrahedra that are linked by intralayer (Te(3))(2-) and interlayer (Te(2))(2-) units to form a 3D network with open channels that are occupied by the K(+) cations. All three compounds are rare polytelluride products of solvothermal reactions that contain both Te(2) and Te(3) fragments with unusual metal-tellurium coordination.

Journal Article↗