Multiple Occupancy Research Articles

Density scaling and optical properties of semiconductor parabolic and square quantum wells.

Using self-consistent density-functional calculations of the electronic structure of GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As square and parabolic quantum wells as a function of the well width a, curvature \ensuremath{\alpha}, and electron concentration ${\mathit{N}}_{\mathit{s}}$, we examine the transition of the density profile in going from the electric quantum limit to multiple subband occupancy. We show that variation of ${\mathit{N}}_{\mathit{s}}$ produces a similar effect on the electronic structure to variation of a (\ensuremath{\alpha}) for a square (parabolic) well, and we demonstrate a scaling behavior of the density profile (in the square-well case) with respect to these two parameters. We provide a simple formula for ${\mathit{N}}_{\mathit{s}}$ and a values at which the second subband of a square well begins filling. We compute the optical spectrum for the parabolic and square wells. We show that, for a square well, level mixing is small and resonances generally have single-particle character. We show that, for parabolic wells, mode mixing produces a single collective resonance in response to long-wavelength excitations at the bare well frequency. We show that this manifestation of the generalized Kohn theorem is affected by the finite boundaries of a real parabolic well, which allow the electromagnetic wave to access other modes of the interacting system. In particular, we examine this symmetry breaking as the electron concentration is increased and subband filling proceeds.

Crystal structure at 1.5-.ANG. resolution of d(CGCICICG), an octanucleotide containing inosine, and its comparison with d(CGCG) and d(CGCGCG) structures

The octadeoxyribonucleotide d(CGCICICG) has been crystallized in space group P(6)5(22) with unit cell dimensions of a = b = 31.0 A and c = 43.7 A, and X-ray diffraction data have been collected to 1.5-A resolution. Precession photographs and the self-Patterson function indicate that 12 base pairs of Z-conformation DNA stack along the c-axis, and the double helices pack in a hexagonal array similar to that seen in other crystals of Z-DNA. The structure has been solved by both Patterson deconvolution and molecular replacement methods and refined in space group P(6)5 to an R factor of 0.225 using 2503 unique reflections greater than 3.0 sigma (F). Comparison of the molecules within the hexagonal lattice with highly refined crystal structures of other Z-DNA reveals only minor conformational differences, most notably in the pucker of the deoxyribose of the purine residues. The DNA has multiple occupancy of C:I and C:G base pairs, and C:I base pairs adopt a conformation similar to that of C:G base pairs.

Modeling ion permeation through batrachotoxin-modified Na+ channels from rat skeletal muscle with a multi-ion pore

The mechanism of ion permeation through Na+ channels that have been modified by batrachotoxin (BTX) and inserted into planar bilayers has been generally described by models based on single-ion occupancy, with or without an influence of negative surface charge, depending on the tissue source. For native Na+ channels there is evidence suggestive of a multi-ion conduction mechanism. To explore the question of ion occupancy, we have reexamined permeation of Na+, Li+, and K+ through BTX-modified Na+ channels from rat skeletal muscle. Single-channel current-voltage (I-V) behavior was studied in neutral lipid bilayers in the presence of symmetrical Na+ concentrations ranging from 0.5 to 3,000 mM. The dependence of unitary current on the mole fraction of Na+ was also examined in symmetrical mixtures of Na(+)-Li+ and Na(+)-K+ at a constant total ionic strength of 206 and 2,006 mM. The dependence of unitary conductance on symmetrical Na+ concentration does not exhibit Michaelis-Menten behavior characteristic of single-ion occupancy but can be simulated by an Eyring-type model with three barriers and two sites (3B2S) that includes double occupancy and ion-ion repulsion. Best-fit energy barrier profiles for Na+, Li+, and K+ were obtained by nonlinear curve fitting of I-V data using the 3B2S model. The Na(+)-Li+ and Na(+)-K+ mole-fraction experiments do not exhibit an anomalous mole-fraction effect. However, the 3B2S model is able to account for the biphasic dependence of unitary conductance on symmetrical [Na+] that is suggestive of multiple occupancy and the monotonic dependence of unitary current on the mole fraction of Na+ that is compatible with single or multiple occupancy. The best-fit 3B2S barrier profiles also successfully predict bi-ionic reversal potentials for Na(+)-Li+ and Na(+)-K+ in both orientations across the channel. Our experimental and modeling results reconcile the dual personality of ion permeation through Na+ channels, which can display features of single or multiple occupancy under various conditions. To a first approximation, the 3B2S model developed for this channel does not require corrections for vestibule surface charge. However, if negative surface charges of the protein do influence conduction, the conductance behavior in the limit of low [Na+] does not correspond to a Gouy-Chapman model of planar surface charge.

Fires and human behaviour : Canter, D. (2nd Edn). David Fulton Publishers, London, 1990, 258 pp, 228 refs.

Psychological aspects of the experience of fires, Canter the concept of panic, Sime a survey of behaviour in fires, Wood fat fires - a domestic hazard, Whittington and Wilson domestic, multiple occupancy and hospital fires, Canter, Breaux and Sime human behaviour in fires in total institutions, Haber fires in nursing facilities, Lerup, Cronrath and Liu a model of behaviour in fires applied to a nursing home fire, Edelman, Herz and Bickman a comparison of two disasters, Bradford and King's Cross, Canter.

Helium bubbles in molybdenum investigated by positron annihilation spectroscopy

Abstract Positron lifetime and Doppler broadening measurements have been performed in homogeneously helium implanted (715 appm) molybdenum. A new stage beyond the temperature range of stability of voids has been observed. The shorter lifetime componet τ1 = 158 ± 2 ps of 60% intensity is assigned to the positron state in helium-decorated dislocation loops while the longer lifetime component τ2 = 408 ± 5 ps of 40% intensity is explained in terms of positron trapping at voids with a small percentage of helium associated with them. Dissociation of helium from loops is observed in the range 300–800 K followed by loop annealing above 900 K. A sharp reduction of τ2 around 1300 K is assigned to multiple helium occupancy of voids, resulting in their transformation into stable bubbles. Helium retention in bubbles is found stable up to the highest annealing temperature of 1700 K. The bubble parameters deduced from the positron lifetime results reveal that the pressure in the bubbles is maintained at the near-equilibrium value and that the vacancy mode of bubble growth is operative, in contrast to the reported growth of athermal bubbles in molybdenum following very low energy helium implantation where loop punching mechanism seems to control the excess pressure in bubbles. A brief comparison is also made between the observed behaviour of helium with that of hydrogen in molybdenum and the differences are discussed.

Inequalities in health and health service use: Evidence from the general household survey

The General Household Survey data file for 1982 was examined to identify variations in self-reported morbidity and health service use between socio-economic groups and geographical areas in Great Britain. Both acute and chronic morbidity varied with socio-economic status. Morbidity was more strongly related to housing tenure and car availability than to occupational class. A north-west to south-east gradient in sickness was observed, although morbidity was comparatively high in Wales and comparatively low in Scotland, taking mortality differences into account. The highest age-adjusted morbidity ratios were for females in multiple occupancy inner city areas. Service use rates in relation to reported sickness showed little systematic variation. There was an indication that lack of car transport was an inhibiting factor for the sick in rural areas.

Multiple role juggling and daily mood states in working mothers: An experience sampling study.

The effects of multiple role juggling (i.e., simultaneously attending to demands of different roles) on daily mood states of employed mothers were examined. Ss completed activity and mood questionnaires 8 times a day for 8 days. Multiple role juggling had immediate negative effects on task enjoyment and mood. However, contrast effects and habituation to role juggling occurred when mood and satisfaction were examined over time. Furthermore, mood states tended to spill over from one episode to the next within a day, but contrast effects were found across days. These results reflect the complex nature of psychological adjustment to multiple role occupancy.

Detecting precatalytic conformational changes in F1-ATPase with 4-benzoyl(benzoyl)-1-amidofluorescein, a novel fluorescent nucleotide site-specific photoaffinity label.

A novel photoaffinity label for studies with the F1-ATPase has been synthesized and found to be an effective reporter of subunit conformational changes that occur in this enzyme upon multiple nucleotide-binding site occupancy. The new probe, 4-benzoyl(benzoyl)-1-amidofluorescein (BzAF), which possesses structural similarity to purine nucleotides, exhibits bifunctional characteristics that enable it to bind covalently to the exchangeable nucleotide sites on beef heart F1 (via photoactivation of the benzophenone moiety) and, once covalently linked, emit environmentally sensitive fluorescence (via selective excitation of the fluorescein moiety). BzAF binds competitively with ATP in the absence of illumination, with a KI of 50 microM. Under actinic irradiation necessary for generating the covalently reacting diradical triplet state of benzophenone, BzAF behaves as a nucleotide site-directed photoaffinity label of exchangeable (catalytic) sites, and the resulting photoinhibition of ATPase activity displays pseudo first-order rate-saturation kinetics that support formation of a dissociable BzAF.F1 complex (k-1/k1 = 58 microM) prior to covalent binding. The BzAF-induced photoinactivation is protectable with native nucleotide ligand (e.g. MgADP, Kprotect = 0.4 mM). Added corroboration of a catalytic cooperativity mechanism for F1 was obtained by finding a molar stoichiometric ratio [( 3H]BzAF:F1) of 1 required for complete inhibition of ATPase activity. Steady-state fluorescence studies with a unisite-labeled BzAF.F1 complex (a catalytically inactive species on which at least one exchangeable nucleotide-binding site remains unoccupied) display a saturable fluorescence quenching of the bound fluorescein upon titration with MgADP, but no change with MgAMP. These data imply that the filling of more than one of the catalytic binding sites/mol of F1 with nucleotide signals a precatalytic conformational adjustment that is transmitted between catalytic sites and across the beta-alpha-beta domain of the enzyme's subunit structure.

A new approach to simulating disorder in crystals

The technique of calculating lattice dissociation energies using static, minimum lattice energy, ionic models has been extended to allow for multiple occupancy of the ionic sites. A particular lattice site can have a fraction x of an ionic species A and a fraction y of an ionic species B, where the position of each can be relaxed separately along with the unit cell dimensions until an equilibrium is reached. Various degrees of long and short range order can be modelled. This technique has been applied to the mineral sillimanite, Al2SiO5, to calculate the effect on the lattice energy of (Al, Si) ordering over the tetrahedral sites. It is found using this method that (Al, Si) ordering with space group Pbmn stabilizes the material by 29.25 kcal/mol (Aliv-O-Aliv), with respect to the completely disordered material.

Light scattering determination of subband structure and population of modulation-doped multiple quantum wells

We show that electronic Raman scattering measurements of the plasmon dispersion in combination with calculations of the random phase approximation dielectric response and self-consistent electronic subband calculations can determine the subband structure and populations of modulation-doped GaAs/AlxGa1−xAs multiple quantum wells with multiple subband occupancy. Thus we present a contactless optical alternative to measurements of Shubnikov–de Haas oscillations for the determination of carrier concentrations. The same technique can also be applied for the characterization of parallel conduction from doped AlGaAs layers in modulation-doped heterostructures.

Temperature dependence of the Fermi level in the two-zone hybrid model of doped semiconductors

On the basis of the two-zone hybrid model of doped semiconductors suggested earlier (see Phys. Status Solidi (b), vol.156, p.487 (1989)) have analysed the behaviour of the Fermi level and the degree of ionisation of the impurity band over a wide temperature range. The calculation of the density of states is performed using a modified Matsubara-Toyozawa method (1960) in which the so-called multiple occupancy corrections are fully taken into account. The results obtained are compared with the corresponding ones of a simpler delta model which is a good approximation of the well known Kane and Morgan density of states relations.

A maxi calcium-activated potassium channel from chick lens epithelium

The apical membrane of embryonic chick lens epithelium contains at high density, a large conductance K+ channel whose open probability is increased by Ca++ at the inner surface of the membrane and by depolarization. The conductance of the channel when it is fully open in symmetrical 150 mM K+ solutions is 214 +/- 3 pS (mean +/- std. error). The current through the channel is a function of the K+ concentration. Gating (open probability) at positive transmembrane voltages increases as the internal [Ca++] is raised above 10(-7) M. The open probability decreases monotonically as the transmembrane voltage is made more negative. The channel is at least 87 times more permeable to K+ than to Na+ or Li+ and shows appreciable permeability to Rb+ and NH4+. It has at least three subconductance levels amounting to approximately 3/4, 1/2, and 1/4 the fully open unitary conductance. The occurrence of these subconductance levels is highly variable from one patch to another. The channel is blocked by physiological levels of internal Na+ but not over a physiological voltage range. This block is partially overcome by elevated external K+. This K+ channel from chick lens epithelium is blocked by a number of compounds known to block BK channels in other tissues. Here we show that decamethonium and Ba++ are effective blockers when added to the inner bathing solution at concentrations greater than .1 mM. Tetraethylammonium, Cs+, quinine, quinidine and Ba++ are all effective blockers when applied to the outer side of the channel in the .1 mM - 5 mM range. With the exception of internal Ba++, all of these compounds produce a fast flicker-type blockade. We use a one-site model to quantify the blockade caused by these flicker producing agents. The voltage dependence of the blockade by Cs+ suggests that this channel probably allows multiple occupancy.

Nuclear spin relaxation due to the translational diffusion of hydrogen in BCC metals

The authors calculate the spectral density functions required for the theoretical determination of nuclear spin relaxation rates due to magnetic dipolar coupling between diffusing hydrogen spins in BCC metals. Both the like-spin (hydrogen-hydrogen) and unlike-spin (metal-hydrogen) contributions are determined using the Monte Carlo method of Faux, Ross and Sholl (1986). Results are obtained for hydrogen-to-metal ratios of 0.12, 0.3 and 0.6 for the simple hopping model (no multiple occupancy of sites) and for a multiple site-blocking model in which the hydrogen spins block all sites as far as the second or third neighbour. For the simple hopping model, the results are in very good agreement with the multiple-scattering theory of Sankey and Fedders (1980), in the high- and low-temperature limits. For the multiple site-blocking models, it is found that the BPP model (due to Bloembergen, Purcell and Pound, 1948) and Torrey model (1953), can differ significantly from the Monte Carlo result, particularly at higher concentrations. The results obtained for D/T1, where D is the tracer diffusion coefficient and T1 is the spin-lattice relaxation time, are compared to the experimental result on NbH0.6. The Monte Carlo method gives a value which is 2/3 of the experimental value if blocking to either the second or the third neighbour is assumed. Agreement with experiment may be obtained if it is assumed that the hydrogen diffuses by a combination of jumps to nearest-neighbour and second-nearest-neighbour sites.

Properties of single potassium-selective ionic channels from the apical membrane of rabbit corneal endothelium

The corneal endothelium from several species contains a highly conductive, flickery potassium selective channel in the apical membrane. The channel flicker is not due to blockade. The single channel has an inwardly rectifying current-voltage relationship under symmetrical ionic conditions. Based on reversal potential measurements, it is more than 40:1 selective for potassium over sodium. It also shows subconductance levels. In bathing solutions lacking chloride and bicarbonate, its open probability is less than 0.1 and is quite independent of voltage in the physiological voltage range. Bicarbonate in the bath or DIDs in the pipette increase channel activity. The channel is blocked by external cesium in the 0.5 to 5 mM range. The steepness of the voltage dependence of this blockade is consistent with multiple occupancy.

Working and Non-Working Students: Roles, Support and Well-Being

Abstract A questionnaire containing measures of physical and psychological symptoms and social support was completed by 142 MSW students. Students were grouped into three categories: Full-time students with no job, full-time students with part-time jobs, and part-time students with full-time jobs. Full-time students with part-time jobs reported more symptoms than full-time students with no jobs, a finding interpreted as consistent with a demands/resources analysis of stress. The latter group did not differ from part-time students with full-time jobs, despite the difference in number of roles held. Analysis controlling for associated variables indicated that it was the specific combination of full-time student and part-time work, not the total number of roles occupied, that was associated with higher distress. Evidence for a “buffering” effect of social support was found only for this high distress group. Results are discussed in terms of recent formulations concerning stress and multiple role occupancy, a...

Multiple deuterium occupancy of vacancies in Pd and related metals.

The binding energies of up to six deuterium atoms into monovacancies of Pd, Ni, Fe, Cu, Nb, and Mo have been calculated using the effective-medium theory. For all metals it is found that at least six D atoms can be accommodated in the vacancy. Of the systems studied the deuterium-deuterium interaction is found to be least repulsive for Pd. This multiple occupancy of the vacancy defect can have potentially significant implications for the recently proposed fusion of D atoms in Pd.

How electrolyte shielding influences the electrical potential in transmembrane ion channels

The electrical potential due to fixed charge distributions is strongly altered in the vicinity of a membrane and notably dependent on aqueous electrolyte concentration. We present an efficient way to solve the nonlinear Poisson-Boltzmann equation applicable to general cylindrically symmetric dielectric geometries. It generalizes Gouy-Chapman theory to systems containing transmembrane channels. The method is applied to three channel systems: gramicidin, gap junction, and porin. We find that for a long, narrow channel such as gramicidin concentration variation has little influence on the electrical image barrier to ion permeation. However, electrolyte shielding reduces the image induced contribution to the energy required for multiple occupancy. In addition, the presence of electrolyte significantly affects the voltage profile due to an applied potential, substantially compressing the electric field to the immediate vicinity of the pore itself. In the large diameter channels, where bulk electrolyte may be assumed to enter the pore, the electrolyte greatly reduces the image barrier to ion permeation. At physiological ionic strengths this barrier is negligible and the channel may be readily multiply occupied. At all ionic strengths considered (l greater than 0.005 M) the image barrier saturates rapidly and is essentially constant more than one channel radius from the entrance to the pore. At lower ionic strengths (l less than 0.016 M) there are noticeable (greater than 20 mV) energy penalties associated with multiple occupancy.

N.M.R. studies of water proton relaxation in Sephadex bead suspensions

The effects of morphology and diffusive exchange on N.M.R. water proton transverse relaxation times in a model heterogeneous system consisting of packed Sephadex beads are interpreted within a unified theoretical framework based on the Bloch-Torrey equations. The relaxation depends on the radii of the Sephadex beads, the cross-linking density of the dextran and varies with the C.P.M.G. interpulse spacing. The larger beads show multiexponential relaxation with up to three components. Without recourse to concepts such as ‘bound-water’ or ‘multiple site occupancy’ we show how the data can be simply understood as arising from diffusive exchange between regions having different relaxation times and resonance frequencies.

Intermolecular vibrational coupling associated with the 2ν2 features of the Raman spectrum of strontium nitrate

A comparative study of the Raman spectra of chemical and physically mixed crystals of Sr(14NO3)2 and Sr(15NO3)2 shows that the doubled features observed in the 1630 cm−1 (2ν2) region result from intermolecular vibrational coupling and not from multiple site occupancy.

Activity and Plural Occupancy of Burrows in Botta's Pocket Gopher Thomomys Bottae

Fourteen Thomomys bottae pocket gophers were radiotracked during 2 reproductive seasons in central New Mexico. Activity (proportion of time spent away from the nest) was low in both sexes (18.8%), but occurred throughout the 24-hr day. There were no significant peaks in activity although gophers were generally more active in the late afternoon. Previous studies have observed high levels of plural or multiple occupancy of single burrows during the breeding season; however, I found only three instances of short-term plural occupancy, suggesting checks on the reproductive status of nearest neighbors rather than cohabitation. Results of previous studies may have been biased by the trapping techniques employed.