Synthesis, structure and spectroscopic studies on hydroxo-bridged heterotrinuclear complexes of cobalt(m) with ethane-l,2-diamine and ammonia as terminal ligands

Hydroxo-bridged homo- and hetero-trinuclear cobalt(III) complexes of the type [MII(H2O)2{(OH)2CoIII(N4)}2]X2·nH2O [MII= a divalent metal ion such as CoII, NiII or ZnII; N4=(en)2(en = ethane-1,2-diamine) or (NH3)4; X = SO4 or (ClO4)2; n= 3 or 5] have been prepared and spectroscopically characterized. The structure of [Cu{(OH)2Co(en)2}2][SO4]2·2H2O 1 has been determined. The geometry around copper atom is a pseudo-square-based pyramid, with the basal sites occupied by four bridging hydroxide oxygens and the apical site is occupied by a weakly co-ordinated sulfate anion [Cu–O 2.516(4)Å]. The hydroxo groups bridge pairs of cobalt(III) ions which are in near-octahedral environments. The ethylenediamine chelate rings have the twist conformation. In the crystal structure of [Cu{(OH)2Co(en)2}2][ClO4]4·2H2O 2 the perchlorate ion is not co-ordinated and the en ligands have envelope conformations. The sulfate ion in [Cu{(OH)2Co(NH3)4}2][SO4]2·4H2O 3 is not co-ordinated to the central copper ion. Electronic, infrared and variable-temperature EPR spectral data are discussed.

Synthesis and Reactions of Binuclear Diolefin Rhodium(I) Compounds bridged by Nitrogen Heterocycles, Crystal Structure of Dichloro-Di-1,5-Cyclooctadiene(Mu-Pyrazine)-Dirhodium(I) [[Rh(1,5-C8h12)Cl]2(Mu-C4h4n2)]

Binuclear complexes of rhodium(I) of the type [(dien)(X)Rh(μ-N-N)Rh(X)(dien)] (dien = 1,5-cyclooctadiene or norbornadiene; N-N = pyrazine, 4,4′-bipyridine or Phenazine and X = Cl or Br) with bridging heterocycles have been isolated and their reactions with carbon monoxide, 2,2′-bipyridine and 1,10-phenanthroline investigated. The crystal structure of [(COD)(Cl)Rh(μ-pyrazine)Rh(Cl)(COD)] has been determined.

Within-Die Gate Delay Variability Measurement Using Reconfigurable Ring Oscillator

We report the design and characterization of a circuit technique to measure the on-chip delay of an individual logic gate (both inverting and noninverting) in its unmodified form. The test circuit comprises of digitally reconfigurable ring oscillator (RO). The gate under test is embedded in each stage of the ring oscillator. A system of linear equations is then formed with different configuration settings of the RO, relating the individual gate delay to the measured period of the RO, whose solution gives the delay of the individual gates. Experimental results from a test chip in 65-nm process node show the feasibility of measuring the delay of an individual inverter to within 1 ps accuracy. Delay measurements of different nominally identicall inverters in close physical proximity show variations of up to 28% indicating the large impact of local variations. As a demonstration of this technique, we have studied delay variation with poly-pitch, length of diffusion (LOD) and different orientations of layout in silicon. The proposed technique is quite suitable for early process characterization, monitoring mature process in manufacturing and correlating model-to-hardware.

Effects of particle size distribution in the response of model granular materials in multi-ring shear

Rail track undergoes complex loading patterns under moving traffic conditions compared to roads due to its continued and discontinued multi-layered structure, including rail, sleepers, ballast layer, sub-ballast layer, and subgrade. Particle size distributions (PSDs) of ballast, subballast, and subgrade layers can be critical in cyclic plastic deformation of rail track under moving traffic on frequent track degradation of rail tracks, especially at bridge transition zones. Conventional test approaches: static shear and cyclic single-point load tests are however unable to replicate actual loading patterns of moving train. Multi-ring shear apparatus; a new type of torsional simple shear apparatus, which can reproduce moving traffic conditions, was used in this study to investigate influence of particle size distribution of rail track layers on cyclic plastic deformation. Three particle size distributions, using glass beads were examined under different loading patterns: cyclic sin-gle-point load, and cyclic moving wheel load to evaluate cyclic plastic deformation of rail track under different loading methods. The results of these tests suggest that particle size distributions of rail track structural layers have significant impacts on cyclic plastic deformation under moving train load. Further, the limitations in con-ventional test methods used in laboratories to estimate the plastic deformation of rail track materials lead to underestimate the plastic deformation of rail tracks.

New initiators for the ring-opening thermal polymerization of hexachlorocyclotriphosphazene: synthesis of linear poly(dichlorophosphazene) in high yields

Ring-opening thermal polymerization of hexachlorocyclotriphosphazene (N3P3C&h)a s been investigated at 250 "C and at 1.333-Pa pressure using chlorocyclotriphosphazenes N3P3C15(N=PPh3) and N3P3Cl,.,(NMe2), (n = 2-4), salt hydrates, triphenylphosphine, and benzoic acid as initiators. The linear poly (dich1orophosphazene) products are phenoxylated, and the phenoxy polymers are characterized by gel permeation chromatography and dilute solution viscometry. Among the various initiators investigated, CaS04.2H20b rings about a high conversion (>60%) of N3P3C&to the linear [NPC12], polymer which possesses a high molecular weight (>5 X lo6). The rationale for the choice of the initiators and possible mechanism(s) of polymerization is discussed. Several mixed substituent polymers, [NP(OPh),(OC6H4Me-p)2,1, and [NP(OPh),(OCHzCF3)2,]nh, ave been prepared and their thermal properties evaluated.

An asparaginyl endopeptidase mediates in vivo protein backbone cyclization

Proteases can catalyze both peptide bond cleavage and formation, yet the hydrolysis reaction dominates in nature. This presents an interesting challenge for the biosynthesis of backbone cyclized (circular) proteins, which are encoded as part of precursor proteins and require post-translational peptide bond formation to reach their mature form. The largest family of circular proteins are the plant-produced cyclotides; extremely stable proteins with applications as bioengineering scaffolds. Little is known about the mechanism by which they are cyclized in vivo but a highly conserved Asn (occasionally Asp) residue at the C terminus of the cyclotide domain suggests that an enzyme with specificity for Asn (asparaginyl endopeptidase; AEP) is involved in the process. Nicotiana benthamiana does not endogenously produce circular proteins but when cDNA encoding the precursor of the cyclotide kalata B1 was transiently expressed in the plants they produced the cyclotide, together with linear forms not commonly observed in cyclotide-containing plants. Observation of these species over time showed that in vivo asparaginyl bond hydrolysis is necessary for cyclization. When AEP activity was suppressed, either by decreasing AEP gene expression or using a specific inhibitor, the amount of cyclic cyclotide in the plants was reduced compared with controls and was accompanied by the accumulation of extended linear species. These results suggest that an AEP is responsible for catalyzing both peptide bond cleavage and ligation of cyclotides in a single processing event.

A Radical Cyclization Route To +/-Andirolactone, A Spiro-Gamma-Butyrolactone

Synthesis of Andirolactone (Image ), starting from 4-methyl cyclohex-3-en-1-one (Image ), via the radical cyclisation of the bromoacetal (Image ), is described.

Cyclic Peptide Disulfides - Solution And Solid-State Conformation Of Boc-Cys-Pro-Aib-Cys-S-S-Bridge-Nhme,A Disulfide-Bridged Peptide Helix

The solution and solid-state conformations of the peptide disulfide Boc-Cys-Pro-Aib-Cys-NHMe have been determined by NMR spectroscopy and X-ray diffraction. The Cys(4) and methylamide NH groups are solvent shielded in CDCI3 and (CD,),SO, suggesting their involvement in intramolecular hydrogen bonding. On the basis of known stereochemical preferences of Pro and Aib residues, a consecutive @-turn structure is favored in solution. X-ray diffraction analysis reveals a highly folded 310 helical conformation for the peptide, with the S-S bridge lying approximately parallel to the helix axis, linking residues 1 and 4. The backbone conformational angles are Cys(1) 4 = -121.1', $ = 65.6"; Pro(2) 4 = -58.9', 4 = -34.0'; Aib(3) 4 = -61.8', $ = -17.9'; Cys(4) 4 = -70.5', $ = -18.6'. Two intramolecular hydrogen bonds are observed between Cys(1) CO--HN Cys(4) and Pro(2) CO--HNMe. The disulfide bond has a right-handed chirality, with a dihedral angle (xss) of 82'.

Tree-ring based hydroclimate reconstruction over northern Vietnam from Fokienia hodginsii: eighteenth century mega-drought and tropical Pacific influence

We present here the first statistically calibrated and verified tree-ring reconstruction of climate from continental Southeast Asia.The reconstructed variable is March-May (MAM) Palmer Drought Severity Index (PDSI) based on ring widths from 22 trees (42 radial cores) of rare and long-lived conifer, Fokienia hodginsii (Po Mu as locally called) from northern Vietnam. This is the first published tree ring chronology from Vietnam as well as the first for this species. Spanning 535 years, this is the longest cross-dated tree-ring series yet produced from continental Southeast Asia. Response analysis revealed that the annual growth of Fokienia at this site was mostly governed by soil moisture in the pre-monsoon season. The reconstruction passed the calibration-verification tests commonly used in dendroclimatology, and revealed two prominent periods of drought in the mid-eighteenth and late-nineteenth enturies. The former lasted nearly 30 years and was concurrent with a similar drought over northwestern Thailand inferred from teak rings, suggesting a ``mega-drought'' extending across Indochina in the eighteenth century. Both of our reconstructed droughts are consistent with the periods of warm sea surface temperature (SST)anomalies in the tropical Pacific. Spatial correlation analyses with global SST indicated that ENSO-like anomalies might play a role in modulating droughts over the region, with El Nio (warm) phases resulting in reduced rainfall. However, significant correlation was also seen with SST over the Indian Ocean and the north Pacific,suggesting that ENSO is not the only factor affecting the climate of the area. Spectral analyses revealed significant peaks in the range of 53.9-78.8 years as well as in the ENSO-variability range of 2.0 to 3.2 years.

Proline ring conformations corresponding to a bistable jump model from 13C spin-lattice relaxation times

A formalism for extracting the conformations of a proline ring based on the bistable jump model of R. E. London [(1978) J. Am. Chem. Soc. 100, 2678-2685] from 13C spin-lattice relaxation times (T1) is given. The method is such that the relaxation data are only partially used to generate the conformations; these conformations are constrained to satisfy the rest of the relaxation data and to yield acceptable ring geometry. An alternate equation for T1 of 13C nuclei to that of London is given. The formalism is illustrated through an example.

Synthesis and characterization of a rare hemiacetalate bridged incomplete View the MathML source dicubane cluster

Reaction of 2-pyridinecarboxaldehyde [(Py)CHO] with Cu(NO3)2·2.5H2O in the presence of 4-aminopyridine and NaN3 in MeOH lead to an incomplete double-cubane [Cu4{PyCH(O)(OMe)}4(N3)4] (1) in 87% isolated yield, representing a rare type of metal cluster containing bridging hemiacetalate ligand [pyCH(O)(OMe)]−1 which was characterized by single crystal structure analysis and variable temperature magnetic behavior.

Flexibility of the furanose ring in nucleic acids: a compilation of crystal data

A compilation of crystal structure data on deoxyribo- and ribonucleosides and their higher derivatives is presented. The aim of this paper is to highlight the flexibility of deoxyribose and ribose rings. So far, the conformational parameters of nucleic acids constituents of ribose and deoxyribose have not been analysed separately. This paper aims to correlate the conformational parameters with the nature and puckering of the sugar. Deoxyribose puckering occurs in the C2′ endo region while ribose puckering is observed both in the C3′ endo and C2′ endo regions. A few endocyclic and exocyclic bond angles depend on the puckering and the nature of the sugar. The majority of structures have an anti conformation about the glycosyl bond. There appears to be a puckering dependence on the torsion angle about the C4′---C5′ bonds. Such stereochemical information is useful in model building studies of polynucleotides and nucleic acids.