Core Interface Research Articles

Mode-area scaling of helical-core, dual-clad fiber lasers and amplifiers using an improved bend-loss model

For small-bend radii, the waveguide condition for total internal reflection is violated in a large angular spread of incident angles at the interface of the fiber core. To account for this, we derived an improved semianalytic bend-loss model that allows for the propagation of radiated fields outside the plane of the fiber bend. This new model is applied to large-mode-area helical-core fibers (which require small-bend radii) for use as high-power fiber lasers and amplifiers. In particular, the limits of scaling the mode area while maintaining good beam quality are explored. Single-mode operation is expected for a 40 μm core with a 0.10 numerical aperture, and scaling to a 100 μm core diameter is shown to be possible. Additionally, helical fibers are shown to readily perform where conventional coiled fibers are not appropriate to operate long term.

Environmentally Modulated Phosphoproteome of Photosynthetic Membranes in the Green Alga Chlamydomonas reinhardtii

Mapping of in vivo protein phosphorylation sites in photosynthetic membranes of the green alga Chlamydomonas reinhardtii revealed that the major environmentally dependent changes in phosphorylation are clustered at the interface between the photosystem II (PSII) core and its light-harvesting antennae (LHCII). The photosynthetic membranes that were isolated form the algal cells exposed to four distinct environmental conditions affecting photosynthesis: (i) dark aerobic, corresponding to photosynthetic State 1; (ii) dark under nitrogen atmosphere, corresponding to photosynthetic State 2; (iii) moderate light; and (iv) high light. The surface-exposed phosphorylated peptides were cleaved from the membrane by trypsin, methyl-esterified, enriched by immobilized metal affinity chromatography, and sequenced by nanospray-quadrupole time-of-flight mass spectrometry. A total of 19 in vivo phosphorylation sites were mapped in the proteins corresponding to 15 genes in C. reinhardtii. Amino-terminal acetylation of seven proteins was concomitantly determined. Sequenced amino termini of six mature LHCII proteins differed from the predicted ones. The State 1-to-State 2 transition induced phosphorylation of the PSII core components D2 and PsbR and quadruple phosphorylation of a minor LHCII antennae subunit, CP29, as well as phosphorylation of constituents of a major LHCII complex, Lhcbm1 and Lhcbm10. Exposure of the algal cells to either moderate or high light caused additional phosphorylation of the D1 and CP43 proteins of the PSII core. The high light treatment led to specific hyperphosphorylation of CP29 at seven distinct residues, phosphorylation of another minor LHCII constituent, CP26, at a single threonine, and double phosphorylation of additional subunits of a major LHCII complex including Lhcbm4, Lhcbm6, Lhcbm9, and Lhcbm11. Environmentally induced protein phosphorylation at the interface of PSII core and the associated antenna proteins, particularly multiple differential phosphorylations of CP29 linker protein, suggests the mechanisms for control of photosynthetic state transitions and for LHCII uncoupling from PSII under high light stress to allow thermal energy dissipation.

Effect of the interface stiffness and skin–core adhesion efficiency on the interfacial stress distribution of sandwich structures

Abstract This paper investigates the load distribution along the skin–core interface length of FRP-metal laminates subjected to in-plane tension, using an improved shear lag model. The model elucidates the significance of the interface stiffness concept on the skin–core interface load distribution in FRP-metal laminate beams and introduces a basic hypothesis according which, the interface stiffness depends on the difference in shear moduli of the constituent materials as well as on the degree of adhesion which, in turn, depends on the abrupt jump in shear moduli at the skin–core boundary. Details of an experimental procedure carried out and of experimental results obtained are also presented. From the combined experimental and theoretical investigation it can be concluded that for a given FRP-metal laminate structure with known core and facing properties, it is possible to evaluate the skin–core degree of adhesion as well as the interface stiffness and predict the interfacial stress distribution by simply conducting a single tensile test. Thus, the proposed method can be considered as a tool for the structural performance and reliability control of FRP-metal laminate structures.

A Trade between Similar but Nonequivalent Intrasubunit and Intersubunit Contacts in Cro Dimer Evolution,

The homodimeric lambda Cro protein has a "ball-and-socket" interface that includes insertion of an aromatic side chain, Phe 58, from each subunit into a cavity in the hydrophobic core of the other subunit. This overlap between the subunit core and dimer interface hypothetically explains the strong dimerization and weak monomer stability of lambda Cro in comparison to homologues. According to a model developed here and in a previous study [LeFevre, K. R., and Cordes, M. H. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 2345-2350], the socket cavity evolved in part by replacement of a buried tryptophan in an ancestral stable monomer with a smaller side chain (Ala 33 in lambda Cro). The resulting core defect was in effect repaired by insertion of a different side chain (Phe 58) from a second subunit, generating the ball and socket. Consistent with such an evolutionary trade between intrasubunit and intersubunit interactions, we showed in the previous study that restoration of the ancestral Trp 33 in lambda Cro stabilized the monomer and reduced the extent of dimerization. Here, we report the solution structure of a stable lambda Cro monomer containing the Ala33Trp mutation, which confirms that the restored tryptophan fulfills its ancestral role as a core side chain, filling part of the socket cavity occupied by Phe 58 in the wild-type dimer. The structure also reveals, however, that the cavity is not completely filled by Trp 33, suggesting that its formation could have involved multiple mutations that reduced side chain volume. We offer suggestive evidence of a role of mutations at a second position.

A procedure to embed fibre Bragg grating strain sensors into GFRP sandwich structures

Abstract Embedding FBG strain sensors within a GFRP sandwich composite material allows early detection of internal defects. However, the sensors need to survive the manufacturing process to provide this capability. Vacuum infusion is commonly used to manufacture GFRP sandwich composite materials but, it needs to be modified to accommodate the embedding process. A stage by stage procedure is demonstrated here to embed FBG strain sensors between the skin–core interface of a GFRP sandwich beam specimen using the vacuum infusion method. Practical issues relating to sensor placement, fibre alignment, specimen lay-up and resin infusion are discussed. Also, the post cure effects of the resin on the FBG strain sensors are investigated. Static and dynamic load analyses are then performed to verify the repeatability and accuracy of the FBG strain sensors.

Transport of rarefied gases inside a vortex tube induced by a surface wave

We derive the entrainment of rarefied gases induced by a surface wave (SW) propagating along the wall or interface of the vortex core in a rather long straight vortex-tube (vortex or vorticity filament) by using the perturbation method. The values of the critical reflux pressure-gradient for time-averaged macroscopic flows of rarefied gases decrease as the Knudsen number increases from zero. Nonlinear coupling due to the boundary effect and the streaming flow of the rarefied gases is observed as the Reynolds number (based on the wave speed) increases to 50 and the wave number of the surface wave is larger. We also address the zero-flux states of our results.

Transport coefficients for solar and stellar dynamos

The large-scale magnetic field of the Sun and solar-type stars is probably generated near the interface of the radiative core and the convection zone. One of the well-known difficulties of interface-type and other dynamo models is to explain the latitudinal distribution of magnetic fields as observed on the Sun. In this contribution new results of numerical MHD simulations of transport coefficients of magnetic fields in the rapid rotation regime, relevant for the base of the solar convection zone, are presented that may contribute to explaining the latitudinal distribution of magnetic fields observed on the Sun. A brief outlook on further numerical simulations of transport coefficients and their relevance for mean-field dynamo theory is given.

Modeling of dissolved oxygen levels in the bottom waters of the Lower St. Lawrence Estuary: Coupling of benthic and pelagic processes

Recent measurements of dissolved oxygen (DO) along the Laurentian Channel in Eastern Canada revealed the presence of hypoxic waters in the bottom 50 m of the water column. At hypoxic oxygen levels, many fish species cannot survive or reproduce, and the microbial life community undergoes significant modifications. The cumulative effect of a substantial sediment oxygen demand along the Lower St. Lawrence Estuary (LSLE) is proposed as the possible cause of this DO depletion. To verify this hypothesis, a laterally integrated, two-dimensional model of the DO distribution was implemented for the bottom waters of the Laurentian Channel along a transect of stations sampled in July 2003. The fluid transport was parameterized in a simple advection–diffusion finite element grid where sedimentation of organic matter (OM) feeds the processes that lead to O 2 depletion in the deep waters. Two major types of OM are considered in this study: a fast-reacting marine component that originates from autochthonous material produced in surface waters, and a more refractory terrestrial component originating from continental river discharges. To counterbalance the OM oxygen sink, the deep, landward mean circulation continuously brings O 2-rich waters from the Atlantic. Both the DO and the early diagenesis model parameters were calibrated using field data collected between 1985 and 2003. Our physical parameter sensitivity study reveals that vertical diffusion from the oxygenated upper water column has the greatest impact on deep DO concentrations. The diagenetic model reproduces the oxygen penetration depths and fluxes very well along the Gulf of St. Lawrence portion of the Channel but overestimates the sediment oxygen demand in the LSLE. We propose that the sediment oxygen demands calculated from DO gradients, measured by voltammetric micro-electrodes, across the sediment–water interface of cores retrieved in the Lower St. Lawrence Estuary are underestimated.

Rates and Mechanisms of Conversion of Ice Nanocrystals to Ether Clathrate Hydrates: Guest-Molecule Catalytic Effects at ∼120 K

A Fourier transform infrared investigation of the rates and energetics of conversion of ice nanocrystals within 3-D arrays to ether clathrate-hydrate (CH) particles at approximately 120 K is reported. After an induction period, apparently necessitated by relatively slow nucleation of the CH phase, the well-established shrinking-core model of particle-adsorbate reaction applies to these conversions in the presence of an abundance of adsorbed ether. This implies that the transport of the ether adsorbate through the product crust encasing a reacting particle core (a necessary aspect of a particle reaction mechanism) is the rate-controlling factor. Diffusion moves adsorbed reactant molecules to the reaction zone at the interface of the ice core with the product (CH) crust. The results indicate that ether hydrate formation rates near 120 K resemble rates for gas hydrates measured near 260 K, implying rates greater by many orders of magnitude for comparable temperatures. A surprising secondary enhancement of ether CH-formation rates by the simultaneous incorporation of simple small gas molecules (N2, CO2, CH4, CO, and N2O) has also been quantified in this study. The rapid CH formation at low temperatures is conjectured to derive from defect-facilitated transport of reactants to an interfacial reaction zone, with the defect populations enhanced through transient H bonding of guest-ether proton-acceptor groups with O-H groups of the hydrate cage walls.

Fluorescence studies on the microenvironments of proteins in CO 2-expanded reverse micellar solutions

The effect of compressed CO 2 on the microenvironment of the two proteins (cytochrome c and RNase A) in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles was investigated using high-pressure steady-state fluorescence and time-resolved fluorescence. It is found that RNase A can shift from the water core to the interface of the reverse micellar cores as CO 2 is dissolved, while cytochrome c maintains at the interface of the micellar core. As the pressure reaches to a high enough value, the proteins can be precipitated from the reverse micelle, which has been detected by high-pressure UV–vis technique. The results show that cytochrome c was easier to be precipitated from the reverse micellar solution by CO 2 than RNase A. The possible reasons for these behaviors were discussed based on the changes of the microenvironment of the two proteins, which could be tuned by the dissolution of compressed CO 2.

Passive transport of atomic gases within a vortex tube induced by a noise wave

Derivations of the entrainment of atomic gases induced by a surface wave (SW) propagating along the wall or interface of the vortex core in a rather long straight vortex-tube (vortex or vorticity filament) are conducted. The values of the critical reflux pressure-gradient for time-averaged macroscopic flows of molecular gases decrease as the Knudsen number increases from zero. Stronger nonlinear coupling due to the boundary effect and the streaming flow of the atomic gases is observed as the Reynolds number increases to 50 and the wave number of the surface wave is larger.

Dynamic Response of Sandwich Plates to Medium-velocity Impact

Overall and local deflections of a single span of a continuous sandwich plate are evaluated under local contacts with a rigid indenter, at relative velocities of 10 and 20 m/s (20-40 knots), to simulate the effect of impacts that can be applied to ship sandwich structures by floating objects or moorings. Laminated carbon-vinyl ester face sheets and a H100 PVC foam core, possibly separated from the impacted face sheet by a stiff and ductile polyurethane (PUR) interlayer, comprise the sandwich plate. As in our previous study of the effect of quasi-static contacts (Suvorov, A.P. and Dvorak, G.J. (2005). Protection of Sandwich Plates from Low Velocity Impact, Journal of Composite Materials (to appear)), the focus is on the evaluation of contact forces and deflections of the face sheet and foam core interface, and on the membrane or in-plane strains in the face sheet at the contact point. These quantities are well-known potential sources of damage, causing core crushing, and face sheet delamination and/or penetration. As expected, the results show that the magnitudes of these quantities depend on both the distance of the contact point from the nearest support, and on the initial velocity of the indenter of a given mass. Both local face sheet deflections and core interface indentations are high if the impact point is within a distance from a support that is equal to 2-3 times the total sandwich plate thickness; significantly lower values are found at more distant contact points. These deflections are much reduced by replacing the foam core with an aluminum honeycomb in the supported region of the plate. However, this reinforcement elevates both the contact force and the in-plane strain in the face sheet. Therefore, thickening of the exposed face sheets should be considered around supports. Most of the kinetic energy is absorbed by the core, hence addition of the stiff and ductile PUR interlayer does not have as much effect on the results as it had in quasi-static contact.

Shape-Controlled Synthesis of CdS Nanocrystals in Mixed Solvents

The formation of CdS nanotetrahedrons, pencil-shaped nanorods, tetrapods, prickly spheres, and high aspect-ratio hexagonal nanoprisms has been, respectively, achieved by adjusting the ratio of two solvents ethylenediamine and ethylene glycol under the solvothermal condition. None of the surfactants or other templates was needed in the process. The reaction time and temperature can be used as the additional means to control the size and morphology. By closely inspecting the growing process of the tetrapod structure and the crystallographic analysis of the products, we proposed that the lattice space match and the location match of ions at the interface of the zinc blende core and the wurtzite arms are two important structural factors to guide this furcate growth. Also, the anisotropic adsorption ability of ethylenediamine at the different surfaces of wurtzite CdS crystals results in the 1D growth of the arms. The possible growing mechanisms of CdS with other shapes were also discussed.

Tet repressor mutants with altered effector binding and allostery

To learn about the correlation between allostery and ligand binding of the Tet repressor (TetR) we analyzed the effect of mutations in the DNA reading head-core interface on the effector specific TetR(i2) variant. The same mutations in these subdomains can lead to completely different activities, e.g. the V99G exchange in the wild-type leads to corepression by 4-ddma-atc without altering DNA binding. However, in TetR(i2) it leads to 4-ddma-atc dependent repression in combination with reduced DNA binding in the absence of effector. The thermodynamic analysis of effector binding revealed decreased affinities and positive cooperativity. Thus, mutations in this interface can influence DNA binding as well as effector binding, albeit both ligand binding sites are not in direct contact to these altered residues. This finding represents a novel communication mode of TetR. Thus, allostery may not only operate by the structural change proposed on the basis of the crystal structures.

PH-induced structural change in a sodium/proton antiporter from Methanococcus jannaschii

Na+/H+ antiporters are pH-dependent membrane transport proteins that maintain the homeostasis of H+ and Na+ in living cells. MjNhaP1 from Methanococcus jannaschii, a hyperthermophilic archaeon that grows optimally at 85 degrees C, was cloned and expressed in Escherichia coli. Two-dimensional crystals were obtained from purified protein at pH 4. Electron cryomicroscopy yielded an 8 A projection map. Like the related E. coli antiporter NhaA, MjNhaP1 is a dimer, but otherwise the structures of the two antiporters differ significantly. The map of MjNhaP1 shows elongated densities in the centre of the dimer and a cluster of density peaks on either side of the dimer core, indicative of a bundle of 4-6 membrane-spanning helices. The effect of pH on the structure of MjNhaP1 was studied in situ. A major change in density distribution within the helix bundle, and an approximately 2 A shift in the position of the helix bundle relative to the dimer core occurred at pH 6 and above. The two conformations at low and high pH most likely represent the closed and open states of the antiporter.

Diffusion-controlled melting and re-solidification of metal micro layers on a reactive substrate

The paper offers a theoretical approach to a prediction of residue formation inherent to melting and subsequent solidification of micro layers of molten aluminum alloys. The residue formation follows a reactive flow of a portion of the melt that is removed by a surface tension action. The residue portion solidifies in situ. The phenomenon studied is associated with materials’ processing during controlled atmosphere brazing of aluminum. The model assumes that diffusion of Silicon, present in an Al+Si clad of a brazing sheet, has a twofold role. First, a solid state Si diffusion prior to melting and across the clad–core interface of a composite brazing sheet takes place and modifies alloys’ composition on both sides of the interface. Subsequently, Si diffusion within clad controls the melting process. Both processes are essential for clad residue formation. The approach advocated in this paper leads to a prediction of the residue formation through a modeling of the non-equilibrium diffusion-controlled melting. A heuristic interpretation of physical mechanisms was discussed and a related mathematical model devised. The model was solved numerically in terms of Si concentration distributions for a moving boundary problem and corroborated with empirical data. Empirical data were gathered using an experimental controlled atmosphere brazing facility. The results of the modeling and their corroboration with the experimental data indicate a strong dependence of residue formations on the pre-melting state of the clad, in particular on the grain size within Al-clad matrix. A good agreement between numerically predicted residue mass and experimental findings is documented in detail.

The Relationship between the Flexibility of Proteins and their Conformational States on Forming Protein–Protein Complexes with an Application to Protein–Protein Docking

We investigate the extent to which the conformational fluctuations of proteins in solution reflect the conformational changes that they undergo when they form binary protein-protein complexes. To do this, we study a set of 41 proteins that form such complexes and whose three-dimensional structures are known, both bound in the complex and unbound. We carry out molecular dynamics simulations of each protein, starting from the unbound structure, and analyze the resulting conformational fluctuations in trajectories of 5 ns in length, comparing with the structure in the complex. It is found that fluctuations take some parts of the molecules into regions of conformational space close to the bound state (or give information about it), but at no point in the simulation does each protein as whole sample the complete bound state. Subsequent use of conformations from a clustered MD ensemble in rigid-body docking is nevertheless partially successful when compared to docking the unbound conformations, as long as the unbound conformations are themselves included with the MD conformations and the whole globally rescored. For one key example where sub-domain motion is present, a ribonuclease inhibitor, principal components analysis of the MD was applied and was also able to produce conformations for docking that gave enhanced results compared to the unbound. The most significant finding is that core interface residues show a tendency to be less mobile (by size of fluctuation or entropy) than the rest of the surface even when the other binding partner is absent, and conversely the peripheral interface residues are more mobile. This surprising result, consistent across up to 40 of the 41 proteins, suggests different roles for these regions in protein recognition and binding, and suggests ways that docking algorithms could be improved by treating these regions differently in the docking process.

Realizing low loss air core photonic crystal fibers by exploiting an antiresonant core surround

The modal properties of an air core photonic crystal fiber which incorporates an anti-resonant feature within the region that marks the transition between the air core and the crystal cladding are numerically calculated. The field intensity at the glass/air interfaces is shown to be reduced by a factor of approximately three compared to a fiber with more conventional core surround geometry. The reduced interface field intensity comes at the expense of an increased number of unwanted core interface modes within the band gap. When the interface field intensity is associated with modal propagation loss, the findings are in accord with recent measurements on fabricated fibers which incorporate a similar antiresonant feature.

Molecular Determinants Responsible for Differences in Desensitization Kinetics of AMPA Receptor Splice Variants

Flip (i) and flop (o) alternatively spliced variants of the four glutamate AMPA receptor subunits (GluR1-4) are differentially expressed in the CNS and can display distinct rates of desensitization that contribute to the heterogeneity of native AMPA receptor-dependent synaptic responses. In the present study, we initially compared the kinetics of desensitization in response to fast application of glutamate (1 mm) for the eight different homomeric recombinant human AMPA receptors (hGluR1-4i and o) heterologously expressed in mammalian cells. Consistent with previous reports on recombinant rat AMPA receptors, the time constants of desensitization between human GluR1i and GluR1o receptors were the same, whereas the flip isoforms for GluR2-4 receptors exhibited significantly slower rates of desensitization compared with the flop isoforms. To identify the molecular determinants responsible for these functional differences, the effects of exchanging amino acid residues in the flip-flop cassette of GluR2i and GluR2o were investigated. Three amino acid residues in the flip-flop region (Thr765, Pro766, and Ser775 in flip and Asn765, Ala766, and Asn775 in flop) were identified that contribute to splice-variant differences in the rate of desensitization. Recent structural data show that these three residues are located on helix J, which forms part of the intradimer interface of AMPA receptor ligand-binding cores, and that the stability of this interface may regulate desensitization. The present results suggest that these three residues may confer differences in flip and flop receptor desensitization rates by directly and/or indirectly influencing the stability of the interface between adjacent subunits.

An evaluation of electrode insertion techniques for measurement of redox potential in estuarine sediments

E h measurements by electrodes are commonly used to characterize redox status of sediments in freshwater, marine and estuarine studies, due to the relative ease and rapidity of data collection. In our studies of fine-grained estuarine seabeds, we observed that E h values measured in intact sediment cores were influenced by different electrode insertion techniques. Sediment E h measurements generated via lateral insertion of platinum electrodes through silicone-filled ports in acrylic cores were systematically more positive (on the order of 10–100 mV) than profiles generated via vertical insertion of platinum electrodes downward through the sediment–water interface of the same cores. A review of the literature indicated that while researchers routinely use both insertion techniques to measure E h, no discrepancy in output has previously been reported. We discuss the results of three experiments conducted to determine if the discrepancy in output was caused by electrode poisoning by sulfides during the stepwise vertical insertion technique, or was caused by contact of the electrode with the silicone plug during the lateral insertion technique. We conclude that contact between the platinum surface of the electrode and the silicone plug biases the E h measurements, resulting in erroneously positive E h values. Insertion of electrodes into sediment through silicone plugs produced E h values that were an average of 105.6 mV (±10.4 SE) more positive than values generated upon electrode insertion directly into sediment. Thus, we recommend against using an insertion technique where the platinum electrode remains in contact with the silicone plug, as this method results in misclassification of sediment redox state and estimated depth of the redoxcline.