SEARCH · PubMed Health

Results for “Transition Elements”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 847 records · Page 47Linked to original sources

PubMed2005

Quantum chemical calculations show that the uranium molecule U2 has a quintuple bond.

Covalent bonding is commonly described by Lewis's theory, with an electron pair shared between two atoms constituting one full bond. Beginning with the valence bond description for the hydrogen molecule, quantum chemists have further explored the fundamental nature of the chemical bond for atoms throughout the periodic table, confirming that most molecules are indeed held together by one electron pair for each bond. But more complex binding may occur when large numbers of atomic orbitals can participate in bond formation. Such behaviour is common with transition metals. When involving heavy actinide elements, metal-metal bonds might prove particularly complicated. To date, evidence for actinide-actinide bonds is restricted to the matrix-isolation of uranium hydrides, including H2U-UH2, and the gas-phase detection and preliminary theoretical study of the uranium molecule, U2. Here we report quantum chemical calculations on U2, showing that, although the strength of the U2 bond is comparable to that of other multiple bonds between transition metals, the bonding pattern is unique. We find that the molecule contains three electron-pair bonds and four one-electron bonds (that is, 10 bonding electrons, corresponding to a quintuple bond), and two ferromagnetically coupled electrons localized on one U atom each-so all known covalent bonding types are contributing.

Journal Article↗

PubMed1999

Identification of discriminator base atomic groups that modulate the alanine aminoacylation reaction.

Specific aminoacylation of tRNAs involves activation of an amino acid with ATP followed by amino acid transfer to the tRNA. Previous work showed that the transfer of alanine from Escherichia coli alanyl-tRNA synthetase to a cognate RNA minihelix involves a transition state sensitive to changes in the tRNA acceptor stem. Specifically, the "discriminator" base at position 73 of minihelix(Ala) is a critical determinant of the transfer step of aminoacylation. This single-stranded nucleotide has previously been shown by solution NMR to be stacked predominantly onto G(1) of the first base pair of the alanine acceptor stem helix. In this work, RNA duplex(Ala) variants were prepared to investigate the role of specific discriminator base atomic groups in aminoacylation catalytic efficiency. Results indicate that the purine structure appears to be important for stabilization of the transition state and that major groove elements are more critical than those located in the minor groove. This result is in accordance with the predicted orientation of a class II synthetase at the end of the acceptor helix. In particular, substitution of the exocyclic amino group of A(73) with a keto-oxygen resulted in negative discrimination at this site. Taken together, these new results are consistent with the involvement of major groove atomic groups of the discriminator base in the formation of the transition state for the amino acid transfer step.

Acylation↗

PubMed2004

Isotopic fractionation during diffusion of transition metal ions in solution.

Isotope ratios and elemental concentrations were measured in aqueous solutions sampled at varying distances from sources of Fe or Zn ions. The measurements reveal fractionation of isotopes resulting from pure diffusion in solution. Our data demonstrate that diffusion alone can cause changes in (56)Fe/(54)Fe and (66)Zn/(64)Zn isotope ratios in excess of -0.3 per thousand. These findings thus confirm previous suspicions that transport processes contribute to observed variations in isotopic compositions. Diffusion must therefore be considered when attempting to make inferences from isotope measurements on samples originating from aqueous systems where concentration gradients may develop.

Journal Article↗

PubMed2003

Deletions of any single residues in Glu40-Ser48 loop connecting a domain and the first transmembrane helix of sarcoplasmic reticulum Ca(2+)-ATPase result in almost complete inhibition of conformational transition and hydrolysis of phosphoenzyme intermediate.

Possible roles of the Glu40-Ser48 loop connecting A domain and the first transmembrane helix (M1) in sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1a) were explored by mutagenesis. Deletions of any single residues in this loop caused almost complete loss of Ca(2+)-ATPase activity, while their substitutions had no or only slight effects. Single deletions or substitutions in the adjacent N- and C-terminal regions of the loop (His32-Asn39 and Leu49-Ile54) had no or only slight effects except two specific substitutions of Asn39 found in SERCA2b in Darier's disease pedigrees. All the single deletion mutants for the Glu40-Ser48 loop and the specific Asn39 mutants formed phosphoenzyme intermediate (EP) from ATP, but their isomeric transition from ADP-sensitive EP (E1P) to ADP-insensitive EP (E2P) was almost completely or strongly inhibited. Hydrolysis of E2P formed from Pi was also dramatically slowed in these deletion mutants. On the other hand, the rates of the Ca(2+)-induced enzyme activation and subsequent E1P formation from ATP were not altered by the deletions and substitutions. The results indicate that the Glu40-Ser48 loop, with its appropriate length (but not with specific residues) and with its appropriate junction to A domain, is a critical element for the E1P to E2P transition and formation of the proper structure of E2P, therefore, most likely for the large rotational movement of A domain and resulting in its association with P and N domains. Results further suggest that the loop functions to coordinate this movement of A domain and the unique motion of M1 during the E1P to E2P transition.

Adenosine Triphosphate↗

PubMed1994

Non-Fourier motion analysis.

It has been realized for some time that the visual system performs at least two general sorts of motion processing. First-order motion processing applies some variant of standard motion analysis (i.e. spatiotemporal Fourier energy analysis) directly to stimulus luminance, whereas second-order motion processing applies standard motion analysis to one or another grossly non-linear transformation of stimulus luminance. We have developed a method for disentangling the different sorts of mechanisms that may operate in human vision to detect second-order motion. This method hinges on an empirical condition called transition invariance that may or may not be satisfied by a family psi of textures. Any failure of this condition indicates that more than one mechanism is involved in detecting the motion of stimuli composed of the textures in psi. We have shown that the family of sinusoidal gratings oriented orthogonally to the direction of motion and varying in contrast and spatial frequency is transition invariant. We modelled the results in terms of a single-channel motion computation. We have new results indicating that a specific class of textures differing in texture element density and texture element contrast decisively fails the test of transition invariance. These findings suggest that in addition to the single second-order motion channel required by our earlier results there exists at least one other second-order motion channel. We argue that the preprocessing transformation used by this channel is a pointwise non-linearity that maps stimulus contrasts of absolute value less than some relatively high threshold tau onto 0, but increases with magnitude of c-tau for contrasts. c of absolute value greater than tau.

Animals↗

PubMed1998

Structure of the chromosome VII centromere region in Neurospora crassa: degenerate transposons and simple repeats.

DNA from the centromere region of linkage group (LG) VII of Neurospora crassa was cloned previously from a yeast artificial chromosome library and was found to be atypical of Neurospora DNA in both composition (AT rich) and complexity (repetitive). We have determined the DNA sequence of a small portion (approximately 16.1 kb) of this region and have identified a cluster of three new retrotransposon-like elements as well as degenerate fragments from the 3' end of Tad, a previously identified LINE-like retrotransposon. This region contains a novel full-length but nonmobile copia-like element, designated Tcen, that is only associated with centromere regions. Adjacent DNA contains portions of a gypsy-like element designated Tgl1. A third new element, Tgl2, shows similarity to the Ty3 transposon of Saccharomyces cerevisiae. All three of these elements appear to be degenerate, containing predominantly transition mutations suggestive of the repeat-induced point mutation (RIP) process. Three new simple DNA repeats have also been identified in the LG VII centromere region. While Tcen elements map exclusively to centromere regions by restriction fragment length polymorphism analysis, the defective Tad elements appear to occur most frequently within centromeres but are also found at other loci including telomeres. The characteristics and arrangement of these elements are similar to those seen in the Drosophila centromere, but the relative abundance of each class of repeats, as well as the sequence degeneracy of the transposon-like elements, is unique to Neurospora. These results suggest that the Neurospora centromere is heterochromatic and regional in character, more similar to centromeres of Drosophila than to those of most single-cell yeasts.

Algorithms↗

PubMed2000

Inversion in the change of the refractive index and memory effect near the nematic-isotropic phase transition in a lyotropic liquid crystal.

This work demonstrates the occurrence of dn/dT inversion from negative to positive near the nematic-isotropic phase transition in a lyotropic liquid crystal. It is suggested that this effect can be attributed to a sudden increase of the electronic polarizability due to a change in the micelle shape near this phase transition. Formation of a long lasting lenslike element within the sample when it is irradiated at moderately high laser powers is also reported. This permanent lens is erasable by increasing the temperature above the nematic-isotropic transition temperature.

Crystallization↗

PubMed1984

Binding of N-carboxymethyl dipeptide inhibitors to thermolysin determined by X-ray crystallography: a novel class of transition-state analogues for zinc peptidases.

The mode of binding of the specific thermolysin inhibitor N-(1-carboxy-3-phenylpropyl)-L-leucyl-L-tryptophan (KI approximately 5 X 10(-8) M) [Maycock, A. L., DeSousa, D. M., Payne, L. G., ten Broeke, J., Wu, M. T., & Patchett, A. A. (1981) Biochem. Biophys. Res. Commun. 102, 963-969] has been determined by X-ray crystallography and refined to an R value of 17.1% at 1.9-A resolution. The inhibitor binds to thermolysin with both oxygens of the N-carboxymethyl group liganded to the zinc to give overall pentacoordination of the metal. The bidentate ligation of the inhibitor differs from the monodentate binding seen previously for carboxylate-zinc interactions in thermolysin and is closer to the bidentate geometry observed for the binding of hydroxamates [Holmes, M. A., & Matthews, B. W. (1981) Biochemistry 20, 6912-6920]. The geometry of the inhibitor and its interactions with the protein have a number of elements in common with the presumed transition state formed during peptide hydrolysis. The observed zinc ligation supports the previous suggestion that a pentacoordinate intermediate participates in the mechanism of catalysis. However, the alpha-amino nitrogen of the inhibitor is close to Glu-143, suggesting that this residue might accept a proton from an attacking water molecule (as proposed before) and subsequently donate this proton to the leaving nitrogen. By analogy with thermolysin, it is proposed that a related mechanism should be considered for peptide cleavage by carboxypeptidase A.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding Sites↗

PubMed2001

Biofeedback avoids surgery in patients with slow-transit constipation: report of four cases.

Biofeedback is established treatment for intractable constipation in patients with an element of pelvic floor dysfunction. In those with intractable slow-transit constipation and normal pelvic floor function, colectomy is usually recommended. We report four patients with isolated slow-transit constipation who benefited from biofeedback and avoided surgery. All four patients were extensively investigated for pelvic floor dysfunction before undergoing a standard biofeedback course of four outpatient sessions. All improved in terms of bowel frequency, laxative use, bloating, straining, and lifestyle. Improvement has been maintained for a median of nine (range, 5-12) months without the requirement for further treatment. Biofeedback represents a safe and inexpensive treatment for these patients and may avoid surgery in a significant proportion.

Adult↗

PubMed1998

Retinal ganglion cell axon progression from the optic chiasm to initiate optic tract development requires cell autonomous function of GAP-43.

Pathfinding mechanisms underlying retinal ganglion cell (RGC) axon growth from the optic chiasm into the optic tract are unknown. Previous work has shown that mouse embryos deficient in GAP-43 have an enlarged optic chiasm within which RGC axons were reportedly stalled. Here we have found that the enlarged chiasm of GAP-43 null mouse embryos appears subsequent to a failure of the earliest RGC axons to progress laterally through the chiasm-tract transition zone to form the optic tract. Previous work has shown that ventral diencephalon CD44/stage-specific embryonic antigen (SSEA) neurons provide guidance information for RGC axons during chiasm formation. Here we found that in the chiasm-tract transition zone, axons of CD44/SSEA neurons precede RGC axons into the lateral diencephalic wall and like RGC axons also express GAP-43. However unlike RGC axons, CD44/SSEA axon trajectories are unaffected in GAP-43 null embryos, indicating that GAP-43-dependent guidance at this site is RGC axon specific or occurs only at specific developmental times. To determine whether the phenotype results from loss of GAP-43 in RGCs or in diencephalon components such as CD44/SSEA axons, wild-type, heterozygous, or homozygous GAP-43 null donor retinal tissues were grafted onto host diencephalons of all three genotypes, and graft axon growth into the optic tract region was assessed. Results show that optic tract development requires cell autonomous GAP-43 function in RGC axons and not in cellular elements of the ventral diencephalon or transition zone.

Animals↗

PubMed1999

Origin and racial distribution of glandular tissue in the anterior compartment of the prostate: an autopsy study.

BACKGROUND: We previously reported that African-American men (AAM) have tumors located in the anterior compartment more often than American Caucasian men (ACM) in radical prostatectomy specimens [Tiguert et al.: Prostate 37:230-235, 1998]. In this study, we evaluated the distribution of glandular tissue in the anterior compartment of normal prostate specimens, with specific attention to the anterior fibromuscular area, in order to determine the frequency and origin of glands in this region. METHODS: We analyzed 94 prostatectomy specimens obtained from autopsied men between ages 20-30 years. Men in this age group were chosen because few pathological changes are present in the prostate in this age range. The anterior compartment of the prostate was defined by drawing a horizontal line, anterior to the urethra, through the midpoint of the anterior-posterior diameter parallel to the rectal surface. In each slide, anterior compartment prostatic tissue was identified and characterized as peripheral zone, transitional zone, and fibromuscular stroma. Any glandular elements identified in the anterior prostatic compartment were recorded in terms of zonal origin and number of glands. RESULTS: Prostates from 76 AAM and 18 ACM were examined. Overall, prostatic glands were absent in the anterior compartment in only 2% of cases. Glands were derived from the peripheral zone only in 6 (6.5%) cases, peripheral zone and transitional zone in 53 (56.5%), transitional zone only in 13 (14%), and anterior fibromuscular stroma in 20 (21%). There was no difference between the two races in terms of the number of glands present. The morphology of the peripheral zone was not different between the two races, with glands from the peripheral zone joining in the anterior compartment in 33% of AAM compared to 56% of ACM (P = 0.123). CONCLUSIONS: Anterior prostatic glands can arise from the peripheral zone, transitional zone, or fibromuscular stroma. There are no racial differences in terms of the number of anterior glandular elements, and also in the architecture of the peripheral zone.

Adult↗

PubMed2006

Quartet states of the acetylene cation: electronic structure calculations and spin-orbit coupling terms.

Highly correlated ab initio methods have been used to generate one-dimensional cuts of the six-dimensional potential energy surfaces of the quartet and lowest doublet states for the HCCH(+) ion along the CH, CC, and cis and trans bending coordinates. Transition dipole moments and spin-orbit matrix elements are deduced. For the lowest 1 (4)Sigma(u) (+) state, the calculations predict a possible photon emission through the 1 (4)Pi(g)<--1 (4)Sigma(u) (+) transition competing with internal conversion and predissociation processes. The potential surfaces are used together with spin-orbit matrix elements to discuss the metastability and the predissociation processes forming the C(2)HC(2)H(+)+H(+)H products. Multistep spin-orbit induced predissociation pathways are suggested.

Journal Article↗

PubMed2004

Transition of G1 to early S phase may be required for zinnia mesophyll cells to trans-differentiate to tracheary elements.

We have used the Zinnia elegans mesophyll cell system, in which single isolated leaf mesophyll cells can be induced to trans-differentiate into tracheary elements in vitro, to study the relationship between the cell division cycle and cell differentiation. Almost all cells go through several rounds of division before characteristic features of tracheary element formation are observed. The addition of aphidicolin, a DNA synthesis inhibitor, blocks cell division but not cell differentiation in the zinnia system. Low concentrations of aphidicolin, which possibly delay cells in the early S phase, can significantly enhance levels of tracheary element formation. In contrast, roscovitine, an inhibitor of cyclin-dependent kinase activity, decelerates the cell division cycle and inhibits tracheary element formation with similar dose responses. Cells blocked in S phase and then transferred to roscovitine-containing medium can divide once, indicating that roscovitine may target the G1/S transition, but do not differentiate. Cells inhibited in G1/S in roscovitine-containing medium that are subsequently blocked in S phase by transfer to aphidicolin-containing medium, do not divide but do differentiate. Taken together, our results indicate that cells may be required to transit the G1/S checkpoint and enter early S phase to acquire competence to trans-differentiate to tracheary elements.

Aphidicolin↗

PubMed1999

Mapping the interactions present in the transition state for unfolding/folding of FKBP12.

The structure of the transition state for folding/unfolding of the immunophilin FKBP12 has been characterised using a combination of protein engineering techniques, unfolding kinetics, and molecular dynamics simulations. A total of 34 mutations were made at sites throughout the protein to probe the extent of secondary and tertiary structure in the transition state. The transition state for folding is compact compared with the unfolded state, with an approximately 30 % increase in the native solvent-accessible surface area. All of the interactions are substantially weaker in the transition state, as probed by both experiment and molecular dynamics simulations. In contrast to some other proteins of this size, no element of structure is fully formed in the transition state; instead, the transition state is similar to that found for smaller, single-domain proteins, such as chymotrypsin inhibitor 2 and the SH3 domain from alpha-spectrin. For FKBP12, the central three strands of the beta-sheet, beta-strand 2, beta-strand 4 and beta-strand 5, comprise the most structured region of the transition state. In particular Val101, which is one of the most highly buried residues and located in the middle of the central beta-strand, makes approximately 60 % of its native interactions. The outer beta-strands and the ends of the central beta-strands are formed to a lesser degree. The short alpha-helix is largely unstructured in the transition state, as are the loops. The data are consistent with a nucleation-condensation model of folding, the nucleus of which is formed by side-chains within beta-strands 2, 4 and 5, and the C terminus of the alpha-helix. The precise residues involved in the nucleus differ in the two simulated transition state ensembles, but the interacting regions of the protein are conserved. These residues are distant in the primary sequence, demonstrating the importance of tertiary interactions in the transition state. The two independently derived transition state ensembles are structurally similar, which is consistent with a Bronsted analysis confirming that the transition state is an ensemble of states close in structure.

Computer Simulation↗

PubMed2002

0(+)(gs) --> 2(+)(1) excitations in the mirror nuclei 32Ar and 32Si.

We measured the strength of the 0(+)(gs)-->2(+)(1) excitations in the radioactive mirror nuclei 32Ar and 32Si using the techniques of intermediate-energy Coulomb excitation for 32Ar and inelastic proton scattering in inverse kinematics for 32Si. The 32Ar measurement, taken together with previously existing Coulomb excitation data for 32Si, yields the isoscalar and isovector multipole matrix elements for the 0(+)(1)-->2(+)(1) transition between T = 2 states in the A = 32 system. The proton scattering measurement for 32Si, when combined with the Coulomb excitation data for this nucleus, yields a ratio of neutron and proton matrix elements, M(n)/M(p), for 32Si.

Journal Article↗

PubMed2004

Synthesis of a novel dinucleating aminocarboxylate macrocycle and structures of its dinuclear complexes with di- and trivalent transition and lanthanide metal ions.

The synthesis of the new ligand L1(H4L1= 3,8,16,21-tetrakis(carboxylmethyl)-3,8,16,21,27,28-hexaazatricyclo[21.3.1.1(10,14)]octacosa-1(27),10,12,14(28),23,25-hexaen-5,18-diine) is reported. The solid-state structures of the four homodinuclear chelate complexes, [Zn2(L1)(H2O)2] x 6H2O, [Fe2(L1)(mu-O)] x 4H2O, [La2(L1)(NO3)2(H2O)2] x 6H2O and Na[Eu2(L1)(mu-AcO)3] x 3H2O, were determined by single-crystal X-ray structural analysis.

Crystallography, X-Ray↗

PubMed2005

Synthesis, characterization and biological activity of some transition metals with Schiff base derived from 2-thiophene carboxaldehyde and aminobenzoic acid.

Metal complexes of Schiff base derived from 2-thiophene carboxaldehyde and 2-aminobenzoic acid (HL) are reported and characterized based on elemental analyses, IR, 1H NMR, solid reflectance, magnetic moment, molar conductance and thermal analysis (TGA). The ligand dissociation as well as the metal-ligand stability constants were calculated pH metrically at 25 degrees C and ionic strength mu=0.1 (1M NaCl). The complexes are found to have the formulae [M(HL)2](X)n.yH2O (where M=Fe(III) (X=Cl, n=3, y=3), Co(II) (X=Cl, n=2, y=1.5), Ni(II) (X=Cl, n=2, y=1) and UO2(II) (X=NO3, n=2, y=0)) and [M(L)2] (where M=Cu(II) (X=Cl) and Zn(II) (X=AcO)). The molar conductance data reveal that Fe(III) and Co(II), Ni(II) and UO2(II) chelates are ionic in nature and are of the type 3:1 and 2:1 electrolytes, respectively, while Cu(II) and Zn(II) complexes are non-electrolytes. IR spectra show that HL is coordinated to the metal ions in a terdentate manner with ONS donor sites of the carboxylate O, azomethine N and thiophene S. From the magnetic and solid reflectance spectra, it is found that the geometrical structure of these complexes are octahedral. The thermal behaviour of these chelates shows that the hydrated complexes losses water molecules of hydration in the first step followed immediately by decomposition of the anions and ligand molecules in the subsequent steps. The activation thermodynamic parameters, such as, E*, DeltaH*, DeltaS* and DeltaG* are calculated from the DrTG curves using Coats-Redfern method. The synthesized ligands, in comparison to their metal complexes also were screened for their antibacterial activity against bacterial species, Escherichia coli, Pseudomonas aeruginosa, Staphylococcus pyogones and Fungi (Candida). The activity data show that the metal complexes to be more potent/antibacterial than the parent Schiff base ligand against one or more bacterial species.

4-Aminobenzoic Acid↗

PubMed2006

Auto-assembling of ditopic macrocyclic lanthanide chelates with transition-metal ions. Rigid multimetallic high relaxivity contrast agents for magnetic resonance imaging.

PhenHDO3A is a ditopic ligand featuring a tetraazacyclododecane unit substituted by three acetate arms and one 6-hydroxy-5,6-dihydro-1,10-phenanthroline group (PhenHDO3A = rel-10-[(5R,6R)-5,6-dihydro-6-hydroxy-1,10-phenantholin-5-yl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid). This ligand was specially designed so as to obtain highly stable heteropolymetallic assemblies. PhenHDO3A has been prepared starting from phenanthroline epoxide and either a triprotected tetraazacyclododecane or tert-butyl triester of N,N',N' '-tetraazacyclododecane-triacetic acid. The latter yields PhenHDO3A in a single step. PhenHDO3A forms kinetically stable lanthanide complexes (acid-catalyzed kinetic constant kH = (1.2 +/- 0.2) x 10(-3) s(-1) M(-1)) whose solution structure has been deduced from a quantitative analysis of the paramagnetic shifts and the longitudinal relaxation times of the proton nuclei of YbPhenHDO3A. The alcohol group of the dihydro-phenanthroline unit remains coordinated to the encapsulated metal ion despite the steric crowding brought about by this group. Furthermore, the complexes are monohydrated, as shown by luminescence lifetime measurements on EuPhenHDO3A solutions. Relaxivity titrations at 20 MHz clearly indicate that the phenanthroline unit of GdPhenHDO3A is available for the spontaneous formation of highly stable tris complexes with the Fe2+ and Ni2+ ions. The water-exchange times and the rotational correlation times of GdPhenHDO3A and Fe(GdPhenHDO3A)32+ have been deduced from variable temperature 17O NMR studies and from nuclear relaxation dispersion curves. Despite rather slow water-exchange rates (taum0 = 1.0-1.2 x 10(-6) s), relaxivity gains of 90% have been observed upon the formation of the heterometallic tris complexes. The latter rotate about four times more slowly (taur0= 398 ps) than the monomeric unit (taur0 = 105 ps) and their relaxivity is, accordingly, twice as high. The relaxivity of the tris complexes between 10 and 50 MHz is comparable to relaxivities reported for Gd3+-containing dendrimers of much higher molecular weights. The high relaxivity of the tris-PhenHDO3A lanthanide complexes is attributed to their internal rigidity.

Acids↗