Bridge Arrangement Research Articles

Energetics of the adsorption and diffusion of hydrogen molecules in a (001) plate of nanocrystalline aluminum

The energetics of the adsorption and diffusion of hydrogen molecules in the surface of an Al plate bounded by the (001) atomic plane has been investigated by the density-functional method. The spatial configuration of hydrogen atoms and surface layers of the plate corresponding to the most stable structures was determined. It has been established that the energies of the physical and chemical adsorptions of hydrogen are equal to −0.049 eV and −0.080 eV respectively. It is shown that, as a result of the chemisorption, two stable spatial configurations with energies differing by 0.33 eV are formed. A bridge arrangement of hydrogen atoms between the apical atoms of aluminum is characterized by a minimum energy. The energy barrier of transformation of a hydrogen molecule from the physical into the chemisorbed state comprises 0.438 eV. An H2 molecule does not dissociate completely in the process of adsorption and diffusion. On the surface and in the bulk of the metal being considered there arise diagonal-chain structures, in which the distance between the hydrogen atoms does not exceed 2.86 A.

Chemical synthesis and 1H‐NMR 3D structure determination of AgTx2‐MTX chimera, a new potential blocker for Kv1.2 channel, derived from MTX and AgTx2 scorpion toxins

Agitoxin 2 (AgTx2) is a 38-residue scorpion toxin, cross-linked by three disulfide bridges, which acts on voltage-gated K(+) (Kv) channels. Maurotoxin (MTX) is a 34-residue scorpion toxin with an uncommon four-disulfide bridge reticulation, acting on both Ca(2+)-activated and Kv channels. A 39-mer chimeric peptide, named AgTx2-MTX, was designed from the sequence of the two toxins and chemically synthesized. It encompasses residues 1-5 of AgTx2, followed by the complete sequence of MTX. As established by enzyme cleavage, the new AgTx2-MTX molecule displays half-cystine pairings of the type C1-C5, C2-C6, C3-C7, and C4-C8, which is different from that of MTX. The 3D structure of AgTx2-MTX solved by (1)H-NMR, revealed both alpha-helical and beta-sheet structures, consistent with a common alpha/beta scaffold of scorpion toxins. Pharmacological assays of AgTx2-MTX revealed that this new molecule is more potent than both original toxins in blocking rat Kv1.2 channel. Docking simulations, performed with the 3D structure of AgTx2-MTX, confirmed this result and demonstrated the participation of the N-terminal domain of AgTx2 in its increased affinity for Kv1.2 through additional molecular contacts. Altogether, the data indicated that replacement of the N-terminal domain of MTX by the one of AgTx2 in the AgTx2-MTX chimera results in a reorganization of the disulfide bridge arrangement and an increase of affinity to the Kv1.2 channel.

Structure of the Human Lung Cytochrome P450 2A13

The human lung cytochrome P450 2A13 (CYP2A13) activates the nicotine-derived procarcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) into DNA-altering compounds that cause lung cancer. Another cytochrome P450, CYP2A6, is also present in human lung, but at much lower levels. Although these two enzymes are 93.5% identical, CYP2A13 metabolizes NNK with much lower K(m) values than does CYP2A6. To investigate the structural differences between these two enzymes the structure of CYP2A13 was determined to 2.35A by x-ray crystallography and compared with structures of CYP2A6. As expected, the overall CYP2A13 and CYP2A6 structures are very similar with an average root mean square deviation of 0.5A for the Calpha atoms. Like CYP2A6, the CYP2A13 active site cavity is small and highly hydrophobic with a cluster of Phe residues composing the active site roof. Active site residue Asn(297) is positioned to hydrogen bond with an adventitious ligand, identified as indole. Amino acid differences between CYP2A6 and CYP2A13 at positions 117, 300, 301, and 208 relate to different orientations of the ligand plane in the two protein structures and may underlie the significant variations observed in binding and catalysis of many CYP2A ligands. In addition, docking studies suggest that residues 365 and 366 may also contribute to differences in NNK metabolism.

Four disulfide-bridged scorpion beta neurotoxin CssII: Heterologous expression and proper folding in vitro

The gene of the four disulfide-bridged Centruroides suffusus suffusus toxin II was cloned into the expression vector pQE30 containing a 6His-tag and a FXa proteolytic cleavage region. This recombinant vector was transfected into Escherichia coli BL21 cells and expressed under induction with isopropyl thiogalactoside (IPTG). The level of expression was 24.6 mg/l of culture medium, and the His tagged recombinant toxin (HisrCssII) was found exclusively in inclusion bodies. After solubilization the HisrCssII peptide was purified by affinity and hydrophobic interaction chromatography. The reverse-phase HPLC profile of the HisrCssII product obtained from the affinity chromatography step showed several peptide fractions having the same molecular mass of 9392.6 Da, indicating that HisrCssII was oxidized forming several distinct disulfide bridge arrangements. The multiple forms of HisrCssII after reduction eluted from the column as a single protein component of 9400.6 Da. Similarly, an in vitro folding of the reduced HisrCssII generated a single oxidized component of HisrCssII, which was cleaved by the proteolytic enzyme FXa to the recombinant CssII (rCssII). The molecular mass of rCssII was 7538.6 Da as expected. Since native CssII (nCssII) is amidated at the C-terminal residue whereas the rCssII is heterologously expressed in the format of free carboxyl end, there is a difference of 1 Da, when comparing both peptides (native versus heterologously expressed). Nevertheless, they show similar toxicity when injected intracranially into mice, and both nCssII and rCssII show the typical electrophysiological properties of beta-toxins in Na v1.6 channels, which is for the first time demonstrated here. Binding and displacement experiments conducted with radiolabelled CssII confirms the electrophysiological results. Several problems associated with the heterologously expressed toxins containing four disulfide bridges are discussed.

Rare Earth Arenedisulfonate Metal−Organic Frameworks: An Approach toward Polyhedral Diversity and Variety of Functional Compounds

Eight 2D and 3D metal-organic framework (MOF) rare earth naphthalenedisulfonates have been obtained. The different geometry of the naphthalenedisulfonic acids used as connectors [(1,5-NDS) and (2,6-NDS)] gives rise to the three new structure types. In Ln(OH)(1,5-NDS)H2O, LnPF-1 (lanthanide polymeric framework; Ln=La, Nd, Pr, Sm and Eu), the lanthanide ion is octacoordinated. Its 3D structure is formed by (Ln2O14)-S-(Ln2O14) infinite chains, connected through complete NDS connectors. LnPF-2 (Ln=Nd), with the same empirical formula as the former, and the lanthanide in octa- and nonacoordination, owns an arrangement of sulfonate bridges and neodymium polyhedra that gives rise to a 2D structure. [Ln5(2,6-NDS)3(OH)9(H2O)4](H2O)2, LnPF-3 (Ln=Nd, Eu), demonstrates that it is possible to obtain a 3D structure with (2,6-NDS), when a greater Ln/connector ratio is employed. It is worth pointing out the existence, in this latter family of compounds, of a mu5-OH group, whose hydrogen atom is very close to one-sixth Ln atom (distance Ln...H=2.09 A). The materials, with high thermal stability, act as active and selective bifunctional heterogeneous catalysts in oxidation of linalool yielding cyclic hydroxy ethers. The absence of any 3D Nd-Nd magnetic interaction is explained due to the inner nature of 4f orbitals of Nd3+, which do not favor the magnetic exchange. The influence of the polymeric frame matrix results in a better photoluminescence efficiency for NdPF-1.

Development and Evaluation of a Contacting Force Measurement System in a High Temperature and Pressure Water Condition for Fretting Wear Experiments

New system (load cell) for measuring a contacting force in a high temperature and pressure water condition (i.e. 320, 15 MPa) has been developed. The primary purpose is to apply it to the experiments of a nuclear fuel fretting wear, which occurs on the contacts between the fuel rods and the spacer grid spring/dimples due to a flow-induced vibration of the rods. A bi-axial load cell of a cylindrical shape is specially designed. Strain gages for a special use in a high temperature condition were attached on to the sensitive region of it to accommodate the bi-axial loading condition in fretting. The full scale of this load cell is ±50 N in axial force and ±50 N in bending force, respectively. In order to increase the sensitivity and to compensate for an ambient temperature effect, the load cell consists of two Wheatstone full bridge arrangements. The calibration results of the load cell show that the coupling effects of each force were almost negligible. This paper mainly presents the experimental techniques used during the development of the new load cell system. The techniques are characterized by a design optimization of a jig of a cylindrical type, the application of a metallic sealant for a waterproofing, a free welding fabrication process and a temperature compensation circuit. Details on the development procedure, calibration method and application results are also given in this paper.

Developments of the electrochemical noise method (ENM) for more practical assessment of anti-corrosion coatings

The electrochemical noise method (ENM) has particular attractions because of its non-intrusive nature, quickness in gathering data and ease of interpretation. The electrode arrangement for the standard (“Bridge”) method of conducting ENM requires two separate working electrodes, e.g. two painted Q panels and a reference electrode. Although satisfactory for laboratory use, it is not so suitable for monitoring or quality control. An improved experimental configuration is the single substrate (SS) method but this still requires the metal to be connected to the measuring instrument. This is avoided in the most recent development which needs no connection to substrate (NOCS). Results will be given for immersed low VOC samples monitored using the ENM NOCS arrangement and compared with the standard (“Bridge”) method and DC resistance. Results will also be presented for work done using several different electrodes (platinum, calomel and silver/silver chloride). It is accepted that, because of the very small voltages and currents involved, ENM data can sometimes be affected by extraneous signals (although normally the results are changed by only a factor of two or less) and it may be that NOCS is more sensitive to interference of this type than the standard bridge arrangement. A simple data analysis package checking on the Gaussian nature of data enables the operator to have confidence in the R n value. This has been applied to NOCS data. Further work is required to make ENM attractive enough to be employed as the electrochemical method of choice by users, specifiers and producers of organic anti-corrosive paints.

Mononuclear, Dinuclear, and Pentanuclear [{N,S(thiolate)}Iron(II)] Complexes: Nuclearity Control, Incorporation of Hydroxide Bridging Ligands, and Magnetic Behavior

The mixed N3S(thiolate) ligand 1-[bis[2-(pyridin-2-yl)ethyl]amino]-2-methylpropane-2-thiol (Py2SH) was used in the synthesis of four iron(II) complexes: [(Py2S)FeCl] (1), [(Py2S)FeBr] (2), [(Py2S)4Fe5II(mu-OH)2](BF4)4 (3), and [(Py2S)2Fe2II(mu-OH)]BF4 (4). The X-ray structures of 1 and 2 revealed monomeric iron(II)-alkylthiolate complexes with distorted trigonal-bipyramidal geometries. The paramagnetic 1H NMR spectra of 1 and 2 display resonances from delta = -25 ppm to +100 ppm, consistent with a high-spin iron(II) ion (S = 2). Spectral assignments were made on the basis of chemical shift information and T1 measurements and show the monomeric structures are intact in solution. To provide entry into hydroxide-containing complexes, a novel synthetic method was developed involving strict aprotic conditions and limiting amounts of H2O. Reaction of Py2SH with NaH and Fe(BF4)2.6 H2O under aprotic conditions led to the isolation of the pentanuclear, mu-OH complex 3, which has a novel dimer-of-dimers type structure connected by a central iron atom. Conductivity data on 3 show this structure is retained in CH2Cl2. Rational modification of the ligand-to-metal ratio allows control over the nuclearity of the product, yielding the dinuclear complex 4. The X-ray structure of 4 reveals an unprecedented face-sharing, biooctahedral complex with an [S2O] bridging arrangement. The magnetic properties of 3 and 4 in the range 1.9-300 K were successfully modeled. Dinuclear 4 is antiferromagnetically coupled [J = -18.8(2) cm(-1)]. Pentanuclear 3 exhibits ferrimagnetic behavior, with a high-spin ground state of S(T) = 6, and was best modeled with three different exchange parameters [J = -15.3(2), J' = -24.7(3), and J'' = -5.36(7) cm(-1)]. DFT calculations provided good support for the interpretation of the magnetic properties.

A microfabricated Pirani pressure sensor operating near atmospheric pressure

A micromachined pressure sensor based on the measurement of the pressure-dependent thermal conductivity of gaseous media (Pirani principle) is presented. The sensor consists of a freestanding microbridge with a defined distance to the substrate which acts as a heat sink. The theory to compute the necessary geometrical dimensions of the sensor element is presented. Due to a novel fabrication process, based on a sacrificial layer of ZnO, the sensor can readily be adapted to operate in different pressure ranges, even exceeding atmospheric pressure. A fully integrated bridge arrangement was used to compensate for ambient temperature changes. Experimental results for different sensor geometries are presented which correspond well with the theory. Basic design rules for micromachined Pirani pressure sensors are derived from the measurements.

The synaptic acetylcholinesterase tetramer assembles around a polyproline II helix

Functional localization of acetylcholinesterase (AChE) in vertebrate muscle and brain depends on interaction of the tryptophan amphiphilic tetramerization (WAT) sequence, at the C-terminus of its major splice variant (T), with a proline-rich attachment domain (PRAD), of the anchoring proteins, collagenous (ColQ) and proline-rich membrane anchor. The crystal structure of the WAT/PRAD complex reveals a novel supercoil structure in which four parallel WAT chains form a left-handed superhelix around an antiparallel left-handed PRAD helix resembling polyproline II. The WAT coiled coils possess a WWW motif making repetitive hydrophobic stacking and hydrogen-bond interactions with the PRAD. The WAT chains are related by an approximately 4-fold screw axis around the PRAD. Each WAT makes similar but unique interactions, consistent with an asymmetric pattern of disulfide linkages between the AChE tetramer subunits and ColQ. The P59Q mutation in ColQ, which causes congenital endplate AChE deficiency, and is located within the PRAD, disrupts crucial WAT-WAT and WAT-PRAD interactions. A model is proposed for the synaptic AChE(T) tetramer.

Anhydrous tetranuclear, dinuclear, and dimeric lanthanide complexes bearing tetradentate Schiff bases

This contribution describes the synthesis and complete characterization of dinuclear (Ln2(SB)3) and rigorously dimeric ([(SB)Ln(N(TMS)2)]2) lanthanide complexes bearing “saturated” tetradentate Schiff bases (SB). The molecular connectivities, nuclearity, and metal ion coordination geometries in these compounds are governed by the interplay of Schiff-base steric requirements, metal ion radius, and reaction conditions (i.e., solvent choice, concentration, and temperature); the impact of these factors will be discussed and emphasized in the context of structural similarities and notable intramolecular interactions within these related compounds. Also presented is a tetranuclear Nd3+ complex exhibiting a ladder arrangement of metal ions and μ-oxo bridges; its formation occurs by the destruction of THF at elevated temperatures. All the compounds presented can be reproducibly synthesized in high yields and analytical purity.

Different propensity to form amyloid fibrils by two homologous proteins-Human stefins A and B: searching for an explanation.

By using ThT fluorescence, X-ray diffraction, and atomic force microscopy (AFM), it has been shown that human stefins A and B (subfamily A of cystatins) form amyloid fibrils. Both protein fibrils show the 4.7 A and 10 A reflections characteristic for cross beta-structure. Similar height of approximately 3 nm and longitudinal repeat of 25-27 nm were observed by AFM for both protein fibrils. Fibrils with a double height of 5.6 nm were only observed with stefin A. The fibril's width for stefin A fibrils, as observed by transmission electron microscopy (TEM), was in the same range as previously reported for stefin B (Zerovnik et al., Biochem Biophys Acta 2002;1594:1-5). The conditions needed to undergo fibrillation differ, though. The amyloid fibrils start to form at pH 5 for stefin B, whereas in stefin A, preheated sample has to be acidified to pH < 2.5. In both cases, adding TFE, seeding, and alignment in a strong magnetic field accelerate the fibril growth. Visual analysis of the three-dimensional structures of monomers and domain-swapped dimers suggests that major differences in stability of both homologues stem from arrangement of specific salt bridges, which fix alpha-helix (and the alpha-loop) to beta-sheet in stefin A monomeric and dimeric forms.

Octanuclear and nonanuclear supramolecular copper(II) complexes with linear "tritopic" ligands: structural and magnetic studies.

The structures and magnetic properties of self-assembled copper(II) clusters and grids with the "tritopic" ligands 2poap (a), Cl2poap (b), m2poap (c), Cl2pomp (d), and 2pomp (e) are described [ligands derived by reaction of 4-R-2,6-pyridinedicarboxylic hydrazide (R = H, Cl, MeO) with 2-pyridinemethylimidate (a-c, respectively) or 2-acetylpyridine (d, R = Cl; e, R = H)]. Cl2poap and Cl2pomp self-assemble with Cu(NO(3))(2) to form octanuclear "pinwheel" cluster complexes [Cu(8)(Cl2poap-2H)(4)(NO(3))(8)].20H(2)O (1) and [Cu(8)(Cl2pomp-2H)(4)(NO(3))(8)].15H(2)O (2), built on a square [2 x 2] grid with four pendant copper arms, using "mild" reaction conditions. Similar reactions of Cl2pomp and 2pomp with Cu(ClO(4))(2) produce pinwheel clusters [Cu(8)(Cl2pomp-2H)(4)(H(2)O)(8)](ClO(4))(8).7H(2)O (3) and [Cu(8)(2pomp-2H)(4)(H(2)O)(8)](ClO(4))(8) (4), respectively. Heating a solution of 1 in MeOH/H(2)O produces a [3 x 3] nonanuclear square grid complex, [Cu(9)(Cl2poap-H)(3)(Cl2poap-2H)(3)](NO(3))(9).18H(2)O (5), which is also produced by direct reaction of the ligand and metal salt under similar conditions. Reaction of m2poap with Cu(NO(3))(2) produces only the [3 x 3] grid [Cu(9)(m2poap-H)(2)(m2poap-2H)(4)](NO(3))(8).17H(2)O (6) under similar conditions. Mixing the tritopic ligand 2poap with pyridine-2,6-dicarboxylic acid (picd) in the presence of Cu(NO(3))(2) produces a remarkable mixed ligand, nonanuclear grid complex [Cu(9)(2poap-H)(4)(picd-H)(3)(picd-2H)](NO(3))(9).9H(2)O (7), in which aromatic pi-stacking interactions are important in stabilizing the structure. Complexes 1-3 and 5-7 involve single oxygen atom (alkoxide) bridging connections between adjacent copper centers, while complex 4 has an unprecedented mixed micro-(N-N) and micro-O metal ion connectivity. Compound 1 (C(76)H(92)N(44)Cu(8)O(50)Cl(4)) crystallizes in the tetragonal system, space group I, with a = 21.645(1) A, c = 12.950(1) A, and Z = 2. Compound 2 (C(84)H(88)N(36)O(44)Cl(4)Cu(8)) crystallizes in the tetragonal system, space group I, with a = 21.2562(8) A, c = 12.7583(9) A, and Z = 2. Compound 4 (C(84)H(120)N(28)O(66)Cl(8)Cu(8)) crystallizes in the tetragonal system, space group I4(1)/a, with a = 20.7790(4) A, c = 32.561(1) A, and Z = 4. Compound 7(C(104)H(104)N(46)O(56)Cu(9)) crystallizes in the triclinic system, space group P, with a = 15.473(1) A, b = 19.869(2) A, c = 23.083(2) A, alpha = 88.890(2) degrees, beta = 81.511(2) degrees, gamma = 68.607(1) degrees, and Z = 2. All complexes exhibit dominant intramolecular ferromagnetic exchange coupling, resulting from an orthogonal bridging arrangement within each polynuclear structure.

Solvothermal syntheses of high-nuclearity vanadium(III) clusters.

Superheating alcohol solutions of simple trimetallic vanadium(III) precursors gives the octa- and decametallic vanadium(III) clusters [V(8)(OEt)(8)(OH)(4)(O(2)CPh)(12)] (1) and [V(10)(OMe)(20)(O(2)CMe)(10)] (2). Cluster 2 is the largest vanadium(III) cluster synthesised to date. Thus solvothermal synthetic techniques are an excellent route to high-nuclearity vanadium(III) clusters. Both 1 and 2 consist of a planar or near-planar array of V(III) ions. The metal ions in 1 are bridged by either a micro(2)-hydroxide and two micro(2)-benzoate groups or two micro(2)-ethoxides and a micro(2)-benzoate groups, the two bridging arrangements alternating around the ring. In 2 each pair of neighbouring metal ions is bridged by two micro(2)-methoxides and a micro(2)-acetate, and this molecule is the V(III) analogue of Lippard's famous "ferric wheel". Preliminary magnetic susceptibility studies show the exchange coupling in both complexes to be antiferromagnetic in nature, with the coupling stronger in 1 than in 2.

Molecular Mechanics Study of the Steric Structure of the Dipeptides Vilon and Thymogen

Molecular dynamics trajectories of two dipeptides, vilon (Lys-Glu) and thymogen (Glu-Trp), were traced up to 700 ps. Both structures comprise intramolecular salt bridges which make them less conformationally flexibile. Vilon is comparatively more flexible due to the aliphatic side chain of Lys, whereas thymogen is quite a rigid structure, which is defined by the mutual arrangement of salt bridges and aromatic rings in the side chain of Trp. A probable mechanism of ligand-receptor binding of dipeptides with excitable membrane was proposed. The binding can be effected via the nitrogen and oxygen atoms forming the salt bridges.

The dual nature of human extracellular superoxide dismutase: one sequence and two structures.

Human extracellular superoxide dismutase (EC-SOD; EC 1.15.1.1) is a scavenger of superoxide anions in the extracellular space. The amino acid sequence is homologous to the intracellular counterpart, Cu/Zn superoxide dismutase (Cu/Zn-SOD), apart from N- and C-terminal extensions. Cu/Zn-SOD is a homodimer containing four cysteine residues within each subunit, and EC-SOD is a tetramer composed of two disulfide-bonded dimers in which each subunit contains six cysteines. The amino acid sequences of all EC-SOD subunits are identical. It is known that Cys-219 is involved in an interchain disulfide. To account for the remaining five cysteine residues we purified human EC-SOD and determined the disulfide bridge pattern. The results show that human EC-SOD exists in two forms, each with a unique disulfide bridge pattern. One form (active EC-SOD) is enzymatically active and contains a disulfide bridge pattern similar to Cu/Zn-SOD. The other form (inactive EC-SOD) has a different disulfide bridge pattern and is enzymatically inactive. The EC-SOD polypeptide chain apparently folds in two different ways, most likely resulting in different three-dimensional structures. Our study shows that one gene may produce proteins with different disulfide bridge arrangements and, thus, by definition, different primary structures. This observation adds another dimension to the functional annotation of the proteome.

High-resolution Structures of Retinol-binding Protein in Complex with Retinol: pH-induced Protein Structural Changes in the Crystal State

The targeted delivery of non-polar ligands by binding proteins to membranes or membrane receptors involves the release of these ligands on or near the plasma membrane of target cells. Because these hydrophobic ligands are often bound inside a deep cavity of binding proteins, as shown previously for plasma retinol-binding protein (RBP), their release from these proteins might require the destabilization of the protein structure by partially denaturing conditions, such as those possibly present near plasma membranes. RBP is a plasma transport protein which delivers specifically retinol from its store sites to target cells. Here, we report the high-resolution (1.1–1.4Å) crystal structures of bovine holo-RBP at five different pH values, ranging from 9 to 2. While unraveling details of the native protein structure and of the interactions with retinol at nearly atomic resolution at neutral pH, this study provides evidence for definite pH-induced modifications of several structural features of RBP. The structure most representative of the changes that holo-RBP undergoes at different pH values is that of its flexible state at pH 2. At this pH, most significant are the alteration of the arrangement of salt bridges and of the network of water molecules/H-bonds that participates in the retinol–RBP interaction, an appreciable increase of the volume of the β-barrel cavity, a considerably higher degree of mobility of the RBP-bound ligand and of several protein regions and the disorder of a large number of solvent molecules that are ordered at neutral pH. These changes are likely to be accompanied by a modification of the pattern of charge distribution on the protein surface. All these changes, which reveal a substantially lowered conformational stability of RBP, presumably occur at the initial stages of the acidic denaturation of RBP and are possibly associated with a facilitated release of the retinol molecule from its carrier protein.

A Potential Of Reducing The Risk Of Ship Casualties By 50

From statistics we can read that about 50% of all ship casualties that occurred over a ten-year period were due to collision, grounding and heavy weather, not only when going by the number of ships or by tonnage, but also when referring to lives lost, oil spilled and claims paid. These plain statistics cannot be denied, and they do not leave much doubt that bridge operation is the area where we find the greatest potential for increasing safety and efficiency in waterborne transport at large. By evaluation of the casualties, DNV has found it appropriate to differentiate between Human Errors and Errors, where the Human Errors stands for approximately 80% of the accidents. If the bridge arrangement is based on functional requirements, very few casualties of the type Human Errors should occur, unless negligence or irresponsibility causes them. Situation-caused errors arise from working conditions affecting the ability of the watch officer or a bridge team to carry out the bridge functions and tasks in an efficient and safe manner. Such conditions may make the bridge personnel lose control of operational situations and thereby become the initial cause of a nautical casualty. If the human operator errs, despite optimum operating conditions, this may be due to the lack of professional qualifications or less than adequate quality of personnel personality and attitude). Appropriate knowledge requirements may ensure that qualifications meet the needs, but operator quality is inevitably connected to the selection of personnel. Transactions on the Built Environment vol 68, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509

Reactions between Cu(II) halide complexes of 2,2′-dipyridylamine and nitrophenolates. X-ray diffraction structure of the [bis(2,2′-dipyridylamine)di( p-nitrophenolato)di(μ-methoxo)dicopper(II)], [Cu(dipyam)(pnp)(μ-OCH 3)] 2, a dimeric complex containing axial phenoxo ligands in square-pyramidal arrangements

The reaction between the [Cu(dipyam)Br 2] complex (dipyam=2,2′-dipyridylamine) and nitrophenolates gave products of exchange reactions between halides and phenolates. Starting from [Cu 2(dipyam) 2(OH) 2(H 2O)]Cl 2·2H 2O, o- and p-nitrophenolate complexes with the same rough formula Cu(dipyam)(OR)(OCH 3) (OR= o- or p-nitrophenolate) but a different chemical behaviour have been obtained. The X-ray diffraction analysis shows that the derivative of the p-nitrophenolate is an interesting dimer [Cu(dipyam)(pnp)(μ-OCH 3)] 2 (pnp= p-nitrophenolate), with two centrosymmetrically related five-coordinated copper(II) ions, having a distorted square-pyramidal geometry defined by axial phenolate ligands and by equatorial dipyam nitrogens and μ-methoxo oxygen bridges. The bridging arrangement forms a Cu 2O 2 core, with a Cu⋯Cu separation of 3.062(2) Å. The crystal structures of the [Cu(dipyam)(pnp) 2]·2H 2O and [Cu(dipyam)(onp) 2] complexes (onp= o-nitrophenolate) are also reported.

Dominant structural motifs of NO−⋅(H2O)n complexes: Infrared spectroscopic and ab initio studies

Argon predissociation spectroscopy is used together with ab initio electronic structure calculations to characterize the NO−⋅(H2O)n=1–3 clusters. In all cases, the water molecules bind to the ion through single ionic H bonds. Two isomeric forms are assigned for the n=1 species that differ according to whether the H bond occurs to the N or O atom of the core ion. While the spectra of the dihydrate indicate formation of an H-bonded water dimer subcluster consistent with all four predicted isomers, their calculated vibrational spectra are too similar to establish which of these forms is created in the ion source. Three classes of isomers are predicted for the NO−⋅(H2O)3 clusters, and in this case a comparison of the experimental and theoretical infrared spectra indicates the formation of a bridging arrangement in which two of the water molecules are attached to one atom and the third to the other atom of NO−. This distorted water trimer motif is intermediate between the symmetrical trimer found in the X−⋅(H2O)3 [X=Cl,Br,I] clusters and the open structure displayed by the O2−⋅(H2O)3 system. The structural differences between the complexes of water with O2− and NO− are discussed in the context of the relative proton affinities and the electronic structures of their ground states (i.e., ∑−3NO− versus ∏g2O2−). Spectra of the NO−⋅(H2O)4,5 clusters are also presented together with a qualitative discussion of the likely morphologies at play in these higher hydrates.