Anomalous waveforms observed in laboratory-formed gas hydrate-bearing and ice-bearing sediments

Acoustic transmission measurements of compressional, P, and shear, S, wave velocities rely on correctly identifying the P- and S-body wave arrivals in the measured waveform. In cylindrical samples for which the sample is much longer than the acoustic wavelength, these body waves can be obscured by high-amplitude waveform features arriving just after the relatively small-amplitude P-body wave. In this study, a normal mode approach is used to analyze this type of waveform, observed in sediment containing gas hydrate or ice. This analysis extends an existing normal-mode waveform propagation theory by including the effects of the confining medium surrounding the sample, and provides guidelines for estimating S-wave velocities from waveforms containing multiple large-amplitude arrivals.     

Quantification of solid pressure in the concentration polarization (CP) layer of colloidal particles and its impact on ultrafiltration

Here we describe the nature and implications of the "concentration polarization" (CP) layer that is formed during ultrafiltration of colloidal particles using a new approach in which the solid pressure, which arises from inter-particle interactions, and the inherent osmotic pressure are separately considered. The approach makes use of the particle transport mass balance between the convective and diffusive fluxes. The particle convection rate is hindered when inter-particle interactions take effect by reducing the particle velocities while the particle diffusion is solely controlled by the Brownian motion. An increase in solid pressure accounts for the reduction of the water potential caused by the relative motions of the particles and the surrounding water. A cell model is adopted to relate the local solid pressure with the local solid fraction and inter-particle interactions. The inter-particle interactions critically determine the form of particle accumulation (i.e. CP or gel/cake) on the membrane. The Shirato-Darcy equation is employed to relate the rate of increase in solid pressure, the relative liquid velocity and the solid fraction. Numerical integration approaches are employed to quantify the properties of the CP layer during both the development as well as the steady state phases (with steady state normally being achieved in a few minutes). The solid fractions are always no higher than those obtained when the inter-particle interactions are not considered. The decrease of the water potential caused by CP formation leads to the increase of both the solid pressure and the osmotic pressure. The dependence of the solid pressure on the solid fraction is usually stronger than that of the osmotic pressure. It is thus apparent that the solid pressure would be expected to dominate water potential reduction for solid fractions above a certain value though the solid pressure will be negligible when the solid fraction is relatively low.     

The effect of an electric field on the local stoichiometry of front waves in an ionic chemical system

A model based on an autocatalytic, two-step reaction mechanismincluding two ionic components (of the same charge) and twonon-ionic components, where both reactions are of second orderoverall, is considered when an electric field is applied tothe system. The model is motivated by experimental observationson the iodate-arsenous acid system. The travelling wave equationsare examined first and conditions obtained for the existenceand form of these waves. These conditions are then used to interpretthe results obtained from numerical simulations of the fullsystem. These results display all the main features observedexperimentally, the change in the local stoichiometry and thepossible wave annihilation for sufficiently strong fields. Themodel provides a clear explanation for these features as wellas predicting new features not reported from the experiments.The main one of which is the occurrence of an internal wavepropagating in the reacted part of the system in the directioninduced by the applied field.     

Exploring gradients of halogens and zinc in the surface and subsurface of Nereis jaws

The outstanding mechanical properties of impact-bearing tissues, such as Nereis jaws, make their morphology and chemical composition a subject of particular interest. The complex structure of the jaw was recently reported to exhibit molecular gradients that were closely correlated with stiffness and hardness.(18) Accordingly, we have explored the spatial distribution and bonding chemistries of Zn and the halogens in the surface structure of the jaws. Using secondary ion mass spectrometry (SIMS) and scanning electron microscopy (SEM), we found that Cl, Br, and I distributions are enhanced in surface layers of the basal protected portion of the jaw but are shifted to greater depths toward the exposed jaw tip. There are thus two complementary halogen gradients in the jaw: one on the surface that decreases from the base to the tip, coupled to an increasing one in the subsurface layers. The outer surface coating appeared to have granular morphology, in contrast to the anisotropic, fibrous core that dominates the subarchitecture. Using X-ray photoelectron spectroscopy (XPS), we discovered that Zn, I, and Br in the jaws have single chemical environments whereas chlorine is present in two distinct modes (Cl-Zn and Cl-C). Given the inverse relationship between surface exposure and halogen abundance in the jaws, it is unlikely that the halogens contribute directly to mechanical properties such as wear and hardness.     

Octopamine increases the excitability of neurons in the snail feeding system by modulation of inward sodium current but not outward potassium currents

Background  

Although octopamine has long been known to have major roles as both transmitter and modulator in arthropods, it has only recently been shown to be functionally important in molluscs, playing a role as a neurotransmitter in the feeding network of the snail Lymnaea stagnalis. The synaptic potentials cannot explain all the effects of octopamine-containing neurons on the feeding network, and here we test the hypothesis that octopamine is also a neuromodulator.     

Sequence and structural duality: designing peptides to adopt two stable conformations

To improve our understanding of conformational transitions in proteins, we are attempting the de novo design of peptides that switch structural state. Here, we describe coiled-coil peptides with sequence and structural duality; that is, features compatible with two different coiled-coil motifs superimposed within the same sequence. Specifically, we promoted a parallel leucine-zipper dimer under reducing conditions, and a monomeric helical hairpin in an intramolecularly disulfide bridged state. Using an iterative process, we engineered peptides that formed stable structures consistent with both targets under the different conditions. Finally, for one of the designs, we demonstrated a one-way switch from the helical hairpin to the coiled-coil dimer upon addition of disulfide-reducing agents.