Outer Barriers Research Articles

Resonant tunnelling through a symmetric double well

The total resonant tunnel current through a double-well structure is calculated. Remarkably, the current is found to increase as the widths of the outer potential barriers increase until a maximum value is attained when the widths of the outer barriers are half the width of the barrier between the potential wells. The double well is the simplest system in which resonant tunnelling involves coupling between two localised energy levels in addition to couplings between the continuum of energies in the electrodes and discrete energy levels in the potential wells so it serves as a model for tunnelling through superlattices. The results reveal the importance of including the effect of the coupling between the quantum wells and the electrodes when optimising the performance of superlattice tunnelling devices.

Nickel transport in Methanobacterium bryantii.

Methanobacterium bryantii, grown autotrophically on H2-CO2, transported nickel against a concentration gradient by a high-affinity system (Km = 3.1 microM). The system had a pH optimum of 4.9 and a temperature optimum of 49 degrees C with an energy of activation of 7.8 kcal/mol (ca. 32.6 kJ/mol). A headspace of H2-CO2 (4:1, vol/vol) was required for maximum rate of transport. The system was highly specific for nickel and was unaffected by high levels of all monovalent and divalent ions tested (including Mg2+) with the sole exception of Co2+. Kinetic experiments indicated that accumulated nickel became increasingly incorporated into cofactor F430 and protein. Nickel transport was inhibited by nigericin, monensin, and gramicidin but not by carbonyl cyanide-p-trifluoromethoxyphenyl hydrazone, carbonyl cyanide-m-chlorophenyl hydrazone, N,N'-dicyclohexylcarbodiimide, valinomycin plus potassium, or acetylene. The ineffectiveness of carbonyl cyanide-p-trifluoromethoxyphenyl hydrazone, carbonyl cyanide-m-chlorophenyl hydrazone, and N,N'-dicyclohexylcarbodiimide may be related to difficulties in the penetration of these compounds through the outer cell barriers. Nickel uptake was greatly stimulated by an artificially imposed pH gradient (inside alkaline). The data suggest that nickel transport is not dependent on the membrane potential or on intracellular ATP, but is coupled to proton movement.

Late Quaternary Evolution of Coastal Sand Barriers, Port Stephens-Myall Lakes Area, Central New South Wales, Australia

AbstractThe geomorphic and stratigraphic history of six coastal embayments has been studied in the vicinity of Newcastle, New South Wales (N.S.W.), Australia, in order to determine modes of deposition, and the degree to which marine and estuarine deposits can be correlated and dated. Each embayment possesses its own distinctive suite of landforms ranging from those dominated by coastal dunes to those in which beach ridges occur. In four of the bays dual sand barriers, comprising an Inner Barrier and an Outer Barrier, provide the framework for correlation between embayments. Six stages are recognized in the deposition of late Quaternary sediments in this area: (i) Pre-Last Interglacial, involving accumulation of separate composite units consisting of estuarine clays and transgressive dune complexes; (ii) Last Interglacial stage during which Inner Barriers were formed; (iii) Last Glacial reworking of barrier and dune sands by westerly winds; (iv) Postglacial Marine Transgression during which the Outer Barriers were initiated; (v) Mid-Holocene stage following cessation of sea-level rise ca. 6000–6500 yr B.P. on this coast, and involving progradation of Outer Barriers in some embayments; and (vi) Late Holocene episodic eolian reworking of dune complexes and Outer Barriers. The relatively high wave and wind energy as well as the tectonic setting of the central N.S.W. coast results in somewhat different geomorphic histories from barrier-island coasts in North America and Europe.

Experimental inflammatory serous detachment of the retina.

Rhesus monkey eyes were studied after the development of inflammatory serous retinal detachments following internal carotid artery injection of live, yeast-phase Histoplasma capsulatum organisms. Horseradish peroxidase (HRP) was used as a tracer to examine the inner and outer blood-retinal barriers. The HRP was present in a decreasing gradient from the choroid into the subretinal space and was not found to have leaked from retinal vessels. The source of subretinal fluid in this model of inflammatory serous retinal detachment was leakage from the choroid through a defective outer blood-retinal barrier, induced by disruption of the retinal pigment epithelium. This model has relevance to understanding the serous retinal detachments common to diseases affecting the choroid.

Ouabain on active transepithelial sodium transport in frog skin: studies with microelectrodes.

Studies were done with isolated frog skin to determine the effects of 10(-4) M ouabain on the electrophysiological parameters of outer and inner barriers of the Na-transporting cells. Microelectrodes were used to impale the skins from the outer surface to determine the intracellular voltages (Vsco) under conditions of short-circuiting and under conditions where a voltage clamp was used to vary the transepithelial voltage, VT. From this, the electrical resistances of outer (Rfo) and inner (RI) barriers were estimated. In addition, the driving force for active transepithelial Na transport (ENa = E'1) was estimated from the values of VT when the Vo = 0 mV (Helman and Fisher. 1977. J. Gen. Physiol. 69: 571-604). Studies were done with skins bathed with the usual 2.4 meq/liter [K]i in the inner solution as well as with reduced [K]i of 0.5 and 0 meq/liter. Characteristically, the responses to ouabain could be described by an initial rapid phase (5-10 min) during which time the Ri was increased markedly and the E'1 was decreased from control values. Thereafter, during the slow phases of the response, the resistances of both outer and inner barriers increased continuously and markedly with time leading ultimately to essentially complete inhibition of the short-circuit current. Similar studies were done with skins exposed to 10(-4) M amiloride in the outer solution. Although estimates of Ri could not be obtained under these conditions, the effects on the Vsco and E'1 were similar to those observed for the Na-transporting skins. However, the magnitudes of the effects were less and relatively slower than observed for the Na-transporting skins. The results of these studies were analyzed within the context of a proposed electrical model that takes into account the observation that the magnitude of the voltage at the inner barrier appears to exceed the equilibrium potential for K especially when transepithelial Na transport is inhibited at the apical barrier of the cells.

Penetrability of the outer membrane of Neisseria gonorrhoeae in relation to acquired resistance to penicillin and other antibiotics.

Acquired antibiotic resistance in Neisseria gonorrhoeae is principally associated with three genetic markers, penA, mtr, and penB. penA is a specific marker for penicillin resistance, whereas mtr and penB are nonspecific in conferring resistance to penicillin and several other antibiotics as well. It has been suggested that the nonspecific markers may cause a general decrease in the penetrability of the gonococcal outer membrane. To investigate this, antibiotic penetration of the outer membrane was studied in two isogenic strains-FA19 (susceptible parent) and FA140 (containing penA, mtr, and penB)-and also in a clinical isolate with multiple resistance. The method involved brief treatment of exponential cells with ethylenediaminetetraacetic acid at 5 degrees C to disrupt the outer membrane barrier. The 50% inhibitory concentrations of antibiotics for treated and normal cells were measured turbidimetrically, and from their ratios outer membrane penetration barriers were calculated. Small barriers were observed for actinomycin D and benzylpenicillin, and these were very similar in the susceptible and resistant strains. Also, in FA140 no significant barriers for rifampin, erythromycin, and tetracycline were detected. These results suggest that mechanism(s) other than reduced outer membrane penetrability underlie acquired resistance due to penA, mtr, and penB.

R plasmid gene dosage effects in Escherichia coli K-12: Copy mutants of the R plasmid R1 drd-19

Copy mutants of the R plasmid R1 drd-19 were used to study gene dosage effects in Escherichia coli K-12. The specific activity of β-lactamase, chloramphenicol acetyltransferase, and streptomycin adenylylase, as well as ampicillin resistance, increased linearly with the gene dosage up to a level at least tenfold higher than that of the wild-type plasmid. This makes it possible to use ampicillin resistance to determine plasmid copy number and also to select for plasmid copy mutants with defined copy number. Chloramphenicol resistance, despite the increase in enzyme activity, reached a plateau level at a gene dosage less than twice that of the wild-type plasmid, presumably due to the high energy demand on the cells during inactivation of the antibiotic by acetylation with acetyl-coenzyme A. Similarly, resistance to streptomycin plateaued at a gene dosage about three times that of the wild-type plasmid, presumably because of a decreased efficiency of the cells' outer penetration barriers when carrying the R plasmid. The susceptibility of the cells to rifampicin was increased by the presence of plasmid copy mutants.

Sodium washout kinetics across inner and outer barriers of the isolated frog skin epithelium

22Na washout kinetics were measured from isolated frog skin epithelium that had been differentially loaded from the outer, inner, or both bathing solutions in a 7 cm 2 Ussing-type chamber. Regardless of the side from which loading occurred 96% of the exchangeable Na + washed out towards the inner solution. Under the three loading conditions the amount of Na + labelled was: (1) 0.33 μequiv./cm 2 from both solutions, (2) 0.16 μequiv./cm 2 from the outer solution, (3) 0.14 μequiv./cm 2 from the inner solution. Tissue Na + content determined by flame photometry was 0.33μequiv./cm 2. Preincubation with ouabain before loading from the outside reduced the labelled Na + content to 0.05 μequiv./cm 2. However, when loading occurred from the inside solution, ouabain increased the labelled Na + to 0.92 μequiv./cm 2. The Na + transfer coefficient (epithelium-inside) was reduced by ouabain from 0.79 to 0.013 min −1. These results show that: (1) the back leak from the epithelium across the outer barrier is negligible; thus the Na + uptake across this barrier is nearly equal to the net Na + transport; (2) the tissue Na + is almost completely exchangeable with the Na + in both bathing solutions; and (3) ouabain inhibits both Na + uptake across the outer barrier and the pump at the inner barrier, while increasing Na + flux from the inner solution into the epithelium.

Very low energy photofission ofU238

The photofission cross section of $^{238}\mathrm{U}$ was measured in the 2.75- to 5.75-MeV range. Cross sections as small as 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ b were detected. A shelf was observed which had been predicted to occur in the cross section owing to the dominance of radiative decay of levels in the second well. Barrier penetrabilities for both inner and outer barriers were determined from these measurements and together with penetrabilities from other measurements were used to infer a potential shape for $^{238}\mathrm{U}$. The total $\ensuremath{\gamma}$-ray strength function in the second well at 4.25 MeV was found to be 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$. From comparison of expected and measured cross sections at the lowest energies, the density of levels effective in photofission at 3 MeV was found to be greater than 4 per MeV.NUCLEAR REACTIONS, FISSION Measured photofission cross section for $^{238}\mathrm{U}$ in the 2.75-6-MeV interval. Deduced details of fission barrier shape and $\ensuremath{\gamma}$-ray decay properties in both inner and outer wells.

A shelf in the "subthreshold" photofission cross section

The influence of a double-humped fission barrier on the photofission cross section far below the top of the barrier is considered. In the region about 2 MeV below the top of the outer barrier and at a cross section in the region of ${10}^{\ensuremath{-}9}$-${10}^{\ensuremath{-}6}$ b the photofission is expected to become almost entirely isomeric or delayed fission. When this occurs a "shelf" should appear in the photofission cross section where the cross section changes with energy far more slowly than at higher $\ensuremath{\gamma}$-ray energy. The cross section on the shelf can therefore be orders of magnitude higher than otherwise expected. While the angular distribution near the top of the barrier is expected to have a well-defined and nonisotropic angular dependence, the angular distribution on the shelf is expected to be isotropic. The cross sections, although small, appear to be measurable with bremsstrahlung beams in the 3- to 5- MeV range using a several hundred $\ensuremath{\mu}\mathrm{A}$ electron beam. Successful experiments of this type should provide information on the shape of the inner and outer barriers at much lower excitation energies than possible by other techniques.NUCLEAR REACTIONS, FISSION Theory of photofission in the ${E}_{\ensuremath{\gamma}}=2\ensuremath{-}6$ MeV region, a prominent shelf, experiments proposed.

Fission of U, Np, Pu and Am isotopes excited in the (d, p) reaction

Fission cross sections measured by the (d, pf) reaction on 233, 235U, 237Np, 238, 239, 241Pu and 241, 243Am are reported. The fission probability has been calculated assuming a doublehumped fission barrier. Damping of the vibrational states in the second minimum has been treated by means of an imaginary potential. In this way it has been possible to explain the observed resonance structure for the even nuclei and the lack of resonances for the doubly odd nuclei. The heights and transparencies of the inner and outer barriers have been extracted.

Effect of prolonged saline-exposure on sodium transport across frog skin.

1. Differences in Na transport between skins from Rana temporaria and R. esculenta maintained for up to several weeks in water or 0.7% saline (0.7 g NaCl in 100 ml. H(2)O), with and without daily injections of 4% saline (4 g NaCl in 100 ml. H(2)O), were measured, in vitro.2. In saline-treated skins, the following changes were found:(a) An increased Na content.(b) A consistent decrease in short-circuit current (I(sc)).(c) An increased d.c. resistance, R, the consistency of which varied with the anion content of the Ringer solution.(d) A highly significant fall in Na influx, accounting for the reduced I(sc); a small reduction in Na efflux was not significant, statistically.(e) The Pitressin-induced increment in I(sc) was usually considerably lower compared with that in water-exposed skins; considered relative to the pre-Pitressin values, however, there were no clear differences.(f) By calculation from the changes in resistance (R) caused by replacement of outer Na(2)SO(4) Ringer by K(2)SO(4) Ringer solution,I. E(0), the electromotive force of the active sodium transport system, was moderately, but significantly, reduced,II. R(shunt), the shunt path resistance, was moderately, but significantly, increased, andIII. R(ser), the series path resistance, was considerably, and highly significantly, increased.(g) K influx from outer K(2)SO(4) Ringer solution was reduced.3. Differences between skins from water-exposed and saline-treated frogs persisted, in vitro, despite the occurrence of anionic-dependent acute changes after mounting in Ringer solution.4. There were seasonal changes in I(sc), and in the effects of saline treatment.5. The findings are discussed in terms of decreased permeability of outer barriers to ion-diffusion, and reduced activity of a Na pump.