Quantum interference and decoherence in single-molecule junctions: how vibrations induce electrical current.

Quantum interference and decoherence in single-molecule junctions is analyzed employing a nonequilibrium Green's function approach. Electrons tunneling through quasidegenerate states of a molecular junction exhibit interference effects. We show that electronic-vibrational coupling, inherent to any molecular junction, strongly quenches such interference effects. This decoherence mechanism may cause significantly larger electrical currents and is particularly pronounced if the junction is vibrationally highly excited, e.g., due to inelastic processes in the resonant transport regime.

Vibrational nonequilibrium effects in the conductance of single molecules with multiple electronic states.

Vibrational nonequilibrium effects in charge transport through single-molecule junctions are investigated. Focusing on molecular bridges with multiple electronic states, it is shown that electronic-vibrational coupling triggers a variety of vibronic emission and absorption processes, which influence the conductance properties and mechanical stability of single-molecule junctions profoundly. Employing a master equation and a nonequilibrium Green's function approach, these processes are analyzed in detail for a generic model of a molecular junction and for benzenedibutanethiolate bound to gold electrodes.

Fetal compensatory renal hypertrophy with a unilateral functioning kidney.

OBJECTIVE: To evaluate the possibility of fetal compensatory renal hypertrophy with a unilateral functioning kidney. METHODS: A nomogram of renal length was established from 254 normal fetuses. Renal length was also obtained in 14 fetuses with unilateral renal agenesis and 22 fetuses with a unilateral multicystic kidney. RESULTS: Compensatory renal hypertrophy, defined as a renal length > 95th percentile for gestational age, occurred in 16 of 36 cases (44.4%). CONCLUSIONS: Compensatory renal hypertrophy is detectable in utero and may occur as early as 22 weeks' gestation.

Chemical composition measurements of the atmosphere of Jupiter with the Galileo Probe mass spectrometer.

The Galileo Probe entered the atmosphere of Jupiter on December 7, 1995. Measurements of the chemical and isotopic composition of the Jovian atmosphere were obtained by the mass spectrometer during the descent over the 0.5 to 21 bar pressure region over a time period of approximately 1 hour. The sampling was either of atmospheric gases directly introduced into the ion source of the mass spectrometer through capillary leaks or of gas, which had been chemically processed to enhance the sensitivity of the measurement to trace species or noble gases. The analysis of this data set continues to be refined based on supporting laboratory studies on an engineering unit. The mixing ratios of the major constituents of the atmosphere hydrogen and helium have been determined as well as mixing ratios or upper limits for several less abundant species including: methane, water, ammonia, ethane, ethylene, propane, hydrogen sulfide, neon, argon, krypton, and xenon. Analysis also suggests the presence of trace levels of other 3 and 4 carbon hydrocarbons, or carbon and nitrogen containing species, phosphine, hydrogen chloride, and of benzene. The data set also allows upper limits to be set for many species of interest which were not detected. Isotope ratios were measured for 3He/4He, D/H, 13C/12C, 20Ne/22Ne, 38Ar/36Ar and for isotopes of both Kr and Xe.

The composition of the Jovian atmosphere as determined by the Galileo probe mass spectrometer.

The Galileo probe mass spectrometer determined the composition of the Jovian atmosphere for species with masses between 2 and 150 amu from 0.5 to 21.1 bars. This paper presents the results of analysis of some of the constituents detected: H2, He, Ne, Ar, Kr, Xe, CH4, NH3, H2O, H2S, C2 and C3 nonmethane hydrocarbons, and possibly PH3 and Cl. 4He/H2 in the Jovian atmosphere was measured to be 0.157 +/- 0.030. 13C/C12 was found to be 0.0108 +/- 0.0005, and D/H and 3He/4He were measured. Ne was depleted, < or = 0.13 times solar, Ar < or = 1.7 solar, Kr < or = 5 solar, and Xe < or = 5 solar. CH4 has a constant mixing ratio of (2.1 +/- 0.4) x 10(-3) (12C, 2.9 solar), where the mixing ratio is relative to H2. Upper limits to the H2O mixing ratio rose from 8 x 10(-7) at pressures <3.8 bars to (5.6 +/- 2.5) x 10(-5) (16O, 0.033 +/- 0.015 solar) at 11.7 bars and, provisionally, about an order of magnitude larger at 18.7 bars. The mixing ratio of H2S was <10(-6) at pressures less than 3.8 bars but rose from about 0.7 x 10(-5) at 8.7 bars to about 7.7 x 10(-5) (32S, 2.5 solar) above 15 bars. Only very large upper limits to the NH3 mixing ratio have been set at present. If PH3 and Cl were present, their mixing ratios also increased with pressure. Species were detected at mass peaks appropriate for C2 and C3 hydrocarbons. It is not yet clear which of these were atmospheric constituents and which were instrumentally generated. These measurements imply (1) fractionation of 4He, (2) a local, altitude-dependent depletion of condensables, probably because the probe entered the descending arm of a circulation cell, (3) that icy planetesimals made significant contributions to the volatile inventory, and (4) a moderate decrease in D/H but no detectable change in (D + 3He)/H in this part of the galaxy during the past 4.6 Gyr.

The Galileo probe mass spectrometer: composition of Jupiter's atmosphere.

The composition of the jovian atmosphere from 0.5 to 21 bars along the descent trajectory was determined by a quadrupole mass spectrometer on the Galileo probe. The mixing ratio of He (helium) to H2 (hydrogen), 0.156, is close to the solar ratio. The abundances of methane, water, argon, neon, and hydrogen sulfide were measured; krypton and xenon were detected. As measured in the jovian atmosphere, the amount of carbon is 2.9 times the solar abundance relative to H2, the amount of sulfur is greater than the solar abundance, and the amount of oxygen is much less than the solar abundance. The neon abundance compared with that of hydrogen is about an order of magnitude less than the solar abundance. Isotopic ratios of carbon and the noble gases are consistent with solar values. The measured ratio of deuterium to hydrogen (D/H) of (5 +/- 2) x 10(-5) indicates that this ratio is greater in solar-system hydrogen than in local interstellar hydrogen, and the 3He/4He ratio of (1.1 +/- 0.2) x 10(-4) provides a new value for protosolar (solar nebula) helium isotopes. Together, the D/H and 3He/4He ratios are consistent with conversion in the sun of protosolar deuterium to present-day 3He.

Exposure to biogenic silica fibers and respiratory health in Hawaii sugarcane workers.

We conducted a cross-sectional environmental and medical survey of 355 male sugarcane workers in Hawaii to determine whether exposure to biogenic silica fibers (BSF) affected their respiratory health. Exposures to BSF ranged from nondetectable to more than 0.700 BSF/mL and varied by job and department. Respiratory symptoms, chest radiograph findings, and pulmonary function were not associated with BSF exposures. Cigarette smoking was associated with respiratory symptoms and pulmonary obstruction. Fifteen workers had pleural thickening or pleural plaques and 3 of these workers were exposed to BSF for more than 10 years. BSF exposure does not appear to influence the respiratory health of sugarcane workers; however, further study is warranted.

The watermelon stomach: long-term outcome in patients treated with Nd:YAG laser therapy.

Watermelon stomach is an unusual cause of gastrointestinal bleeding and iron deficiency anemia. Its etiology is unknown, but it has been reported to be associated with a variety of diseases, including autoimmune disorders and cirrhosis. We report on the long-term outcome of 15 patients (13 women, 2 men) treated with neodymium-yttrium-aluminum-garnet (Nd:YAG) laser therapy. The mean age of patients at presentation was 71.6 years (range, 59 to 85 years). Fourteen patients were transfusion-dependent, requiring an average of 9.6 units of blood in the 12 months preceding diagnosis and treatment. Associated diseases included scleroderma (3 patients), mixed connective tissue disease (1 patient), history of cancer (3 patients), cryptogenic cirrhosis (3 patients), and chronic renal failure (3 patients). In 7 of 9 patients who had an antinuclear antibody test, an elevated titer greater than 1:160 in a speckled pattern was noted. Nd:YAG laser coagulation therapy was administered to all patients without complications and was successful in reducing bleeding in every case. Five patients died during the course of follow-up without signs of recurrent gastrointestinal bleeding. The remaining 10 patients have had both endoscopic and hematologic improvement during a mean follow-up period of 4.4 years from the time of initial diagnosis (range, 2 to 8 years). The 10 survivors are no longer transfusion-dependent and have stable hematocrits.

Exposure to methyl tert-butyl ether and benzene among service station attendants and operators.

Concerns for atmospheric pollution from auto exhaust have led to the blending of "oxygenates" with motor fuels. The most common oxygenate, methyl tert-butyl ether (MTBE) is currently required within several metropolitan areas (Denver and Phoenix) in the range of 12% of the motor fuel. Amendments to the Clean Air Act may expand this requirement to as many as 44 other areas of the United States in the near future. In consideration of the magnitude of potential uncontrolled exposures from its extensive use and a related concern involving the potential influence of MTBE blending on exposures to other constituents of gasoline (particularly benzene), an evaluation of exposures among service station attendants and operators was undertaken at the request, and in cooperation with, the American Petroleum Institute during the latter part of 1990. For application of the survey results to a broad audience, three categories or types of service stations were identified with regard to MTBE use and exposure potential: a) service stations that do not use MTBE or use it only as an octane enhancer, b) service stations with seasonal requirements to use 12-15% MTBE (the Denver, Colorado, and Phoenix, Arizona, metropolitan areas), and c) service stations equipped with stage II (active) vapor recovery systems (several coastal areas, most notably Southern California). At the two sampled service stations that use only minimal amounts of MTBE (less than 1%), only 1 of 32 personal breathing zone (PBZ) samples from attendants was above the analytical limit of detection, reported at 0.16 ppm. The geometric mean concentration of benzene among this same population (n = 32) was 0.04 ppm.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of selenium on growth, S-adenosylmethionine and polyamine biosynthesis in human colon cancer cells.

Selenium metabolism and polyamine biosynthesis are linked in their common requirement for S-adenosylmethionine. The effects of selenium supplementation (0.1 to 6.0 ppm) on growth, polyamine biosynthesis and S-adenosylmethionine were examined in two human colon cancer cell lines, WiDr and HT29. WiDr cells were very sensitive to selenium with a significant decrease in 3H-thymidine incorporation and cell number to doses above 0.25 ppm. HT29 cells were less sensitive. In HT29 cells, ornithine decarboxylase activity and its product putrescine decreased in parallel with the growth inhibitory effects of selenium. Similar changes were not noted in WiDr cells. Spermidine and spermine content were conserved in both cell lines exposed to cytotoxic doses of selenium. S-adenosylmethionine was increased in HT29 cells at cytotoxic doses of selenium (1.0 to 6.0 ppm).

An epidemic of dermatitis at a large construction site.

During 6 months in 1986, an epidemic of dermatitis occurred among more than 5600 workers at a single large construction site in the United States. To identify its cause, we used the monthly nurses' log of visits to the medical facility to characterize the outbreak by person, place, and time. Follow-up studies included a survey of carpenters and skin testing of laboratory animals. Workers were more than twice as likely to visit the medical facility for a skin-related complaint in 1986 compared with 1985 (relative rate [RR] = 2.6, 95% confidence interval [CI] 2.2-3.2). Carpenters experienced the greatest increased risk (RR = 9.7, 95% CI 5.5-17.3). We found a strong association between dermatitis and handling of fire-retardant lumber and plywood. Carpenters working only with fire-retardant lumber had a rate of dermatitis 4 times that of carpenters who worked exclusively with untreated wood (RR = 3.6, 95% CI 1.5-8.6). Carpenters who occasionally worked with fire-retardant lumber and plywood were at moderate risk (RR = 2.18, 95% CI 0.7-6.7). Although laboratory tests showed that phosphate compounds could be leached with water from the fire-retardant wood, an extract of these phosphates did not irritate the skin of laboratory animals. We concluded the epidemic was a result of handling fire-retardant lumber but could not identify the specific chemical agent. In view of the observed association, construction workers should be advised to handle this material with caution, especially in high temperature and humidity.

Plasma observations near uranus: initial results from voyager 2.

Extensive measurements of low-energy positive ions and electrons in the vicinity of Uranus have revealed a fully developed magnetosphere. The magnetospheric plasma has a warm component with a temperature of 4 to 50 electron volts and a peak density of roughly 2 protons per cubic centimeter, and a hot component, with a temperature of a few kiloelectron volts and a peak density of roughly 0.1 proton per cubic centimeter. The warm component is observed both inside and outside of L = 5, whereas the hot component is excluded from the region inside of that L shell. Possible sources of the plasma in the magnetosphere are the extended hydrogen corona, the solar wind, and the ionosphere. The Uranian moons do not appear to be a significant plasma source. The boundary of the hot plasma component at L = 5 may be associated either with Miranda or with the inner limit of a deeply penetrating, solar wind-driven magnetospheric convection system. The Voyager 2 spacecraft repeatedly encountered the plasma sheet in the magnetotail at locations that are consistent with a geometric model for the plasma sheet similar to that at Earth.

The biosynthesis, processing, and secretion of laminin by human choriocarcinoma cells.

Laminin, a glycoprotein component of basal laminae, is synthesized and secreted in culture by a human malignant cell line (JAR) derived from gestational choriocarcinoma. Biosynthetically labeled human laminin subunits A (Mr approximately 400,000) and B (Mr = 200,000 doublet) are glycoslyated with asparagine-linked high mannose oligosaccharides that are processed to complex oligosaccharides before the laminin molecule is externalized by the cell. The rate-limiting step in the processing of the asparagine-linked glycans of laminin is at the point of action of alpha-mannosidase I since the principal laminin forms that accumulate in JAR cells contain Man9GlcNAc2 and Man8GlcNAc2 oligosaccharide units. The combination of subunits to form the disulfide-linked laminin molecule (Mr approximately 950,000) occurs rapidly within the cell at a time when the subunits contain these high mannose oligosaccharides. The production of laminin is limited by the availability of the A subunit such that excess B subunit forms accumulate intracellularly as uncombined B and a disulfide-linked B dimer. Pulse-chase kinetic studies establish these B forms as intermediates in the assembly of the laminin molecule. The fully assembled laminin undergoes further oligosaccharide processing and translocation to the cell surface, but uncombined B and B dimer are neither processed nor secreted to any significant extent. Therefore, laminin subunit combination appears to be a prerequisite for intracellular translocation, processing, and secretion. The mature laminin that contains complex oligosaccharides does not accumulate intracellularly but is rapidly externalized upon completion, either secreted into the culture medium (25%) or associated with the cell surface (75%) as determined by susceptibility to degradation by trypsin. About one-third of the laminin molecules secreted or shed by JAR cells into the chase medium contain a smaller A subunit form that appears to have been modified by limited proteolytic cleavage. The putative proteolytic event is closely timed to the release of the laminin into the culture medium.

Urine phthalate determinations as an index of occupational exposure to phthalic anhydride and di(2-ethylhexyl)phthalate.

Although it has been estimated that over 600 000 workers in the United States are exposed to di(2-ethylhexyl)phthalate (DEHP), an animal carcinogen, and that over 100 000 are exposed to phthalic anhydride (PA), few data are available on levels of phthalates in biological fluids of these workers. For a determination of occupational exposure to PA and DEHP at a plant manufacturing DEHP from PA and 2-ethylhexanol, air samples were taken for PA and DEHP, and pre- and postshift urine samples were collected for the determination of total phthalates. Urine samples were obtained from 48 workers in jobs with high exposure to phthalates and from 47 workers in jobs with low exposure. The airborne concentrations of DEHP ranged from 20 to 4 110 micrograms/m3, and the concentrations of PA ranged from 4 to 203 micrograms/m3. The most heavily exposed workers had the highest mean postshift urine phthalate concentration (geometric mean 7.6 nmol/ml) (p = 0.015), and also the greatest mean increase (4.4 nmol/ml) in preshift to postshift urine phthalate levels. Twofold increases over the shift in urine phthalate concentration and postshift phthalate levels of greater than 10 nmol/ml were observed in 8 (25%) of 32 chemical operators, but in none of 52 other workers. These data suggest that measurement of urine phthalate levels may have utility for monitoring the exposure of workers manufacturing or using PA.

An analysis of exposure to styrene in the reinforced plastic boat-making industry.

To evaluate patterns of occupational exposure to styrene monomer in the boat-building industry, we conducted industrial hygiene surveys in seven fiberglass-reinforced plastic boat-fabrication plants. A total of 397 personal breathing zone air samples were collected on workers in four key job categories: gel coating and hull, deck, and small-parts lamination. We found that exposure to styrene in 234 (59%) of the personal samples exceeded the NIOSH-recommended time-weighted average (TWA) standard of 50 parts per million (ppm), and 96 (24%) of these samples exceeded the OSHA eight-hour TWA permissible exposure limit of 100 ppm. From highest to lowest average exposure potential, the job categories ranked in the following order: hull lamination (range 2-183 ppm, mean 78 ppm), deck lamination (range 12-160 ppm, mean 73 ppm), gel coating (range 5-94 ppm, mean 48 ppm), and small-parts lamination (range 9-130 ppm, mean 45 ppm). The former two categories composed a higher-exposure group, and the latter two tended to form a lower-exposure group. Exposures in these jobs appear to be proportional to resin consumption. Statistical analyses indicated that the parameter that most affects exposure potential is job category. Within a job category, part size, configuration, and surface area are important determinants of exposure. The principal reason for high exposures was the absence of control technologies and, in particular, the absence of adequate ventilation.

A kinetic comparison of the processing and secretion of the alpha beta dimer and the uncombined alpha and beta subunits of chorionic gonadotropin synthesized by human choriocarcinoma cells.

Human choriocarcinoma cells (JAR) synthesize the alpha and beta subunits of the glycoprotein hormone chorionic gonadotropin (hCG) (R.W. Ruddon, C.A. Hanson, A. H. Bryan, G.J. Putterman, E.L. White, F. Perini, K. S. Meade, and P.H. Aldenderfer (1980) J. Biol. Chem. 255, 1000-1007). In addition to the hCG dimer (alpha beta), JAR cells secrete uncombined alpha and beta subunits into the culture medium (L.A. Cole, R.J. Hartle, J.A. Laferla, and R.W. Ruddon (1983) Endocrinology 113, 1176-1178). Pulse-chase studies with [35S]methionine or [3H]mannose were carried out in order to compare free alpha, free beta, and the alpha beta dimer with regard to the kinetics of synthesis, N-linked oligosaccharide processing, and secretion and to determine the kinetics of alpha-beta subunit combination. A panel of three antisera was used to immunoprecipitate directly the free subunits and the alpha beta dimer sequentially from the same cell lysates and culture media. The alpha subunit of hCG was synthesized in a slight molar excess (1.2-1.5-fold) over the beta subunit, and alpha beta dimer was rapidly formed by combination of the intracellular alpha and beta precursors. Dimer formation was already apparent in JAR cells following a 10-min biosynthetic labeling incubation with [35S]methionine. The combination of subunits ceased by 30 min of chase even though 51% of alpha and 44% of beta remained free within the cells. Combination of the alpha and beta precursors had occurred before their N-linked oligosaccharides were processed beyond the Man8GlcNAc2 structure. The initial trimming of glucosyl and mannosyl units from the high-mannose oligosaccharides of the hCG precursors occurred more rapidly for free alpha and CG-alpha than for free beta and CG-beta. JAR cells accumulated alpha precursors bearing mostly Man8GlcNAc2 units and beta precursors bearing Man8GlcNAc2 units that represent the substrates of the rate-limiting step in the secretory pathway. In spite of the fact that their N-linked oligosaccharides were trimmed at different rates, free alpha, free beta, and alpha beta dimer were all secreted into the medium at the same rate, with a half-time of 35 min. The secreted hCG forms were stable in the chase medium between 4 and 8h, indicating that extracellular degradation, combination of free subunits to form dimer, or dissociation of dimer to form free subunits did not occur.(ABSTRACT TRUNCATED AT 400 WORDS)

An epidemic of psychogenic illness in an electronics plant.

We investigated an outbreak of illness in electronics plant workers. Questionnaire data revealed that 98 employees experienced symptoms, including light- headedness , headache, sleepiness, and numbness/tingling of the face or extremities. Attack rates by work station followed no apparent pattern. Extensive medical and environmental evaluation, including air sampling for numerous solvents and gases, provided no physical or chemical explanation for the epidemic. Compared with well employees, ill employees were more commonly female, complained more frequently of bothersome odors, and believed that greater danger existed of the illness recurring. Blood gas analyses of seven of 11 ill workers showed respiratory alkalosis, consistent with hyperventilation. The poorly defined nature of this illness, the absence of exposures to environmental contaminants in concentrations exceeding recommended limits, and the evidence of hyperventilation suggest that this outbreak was an incident of industrial psychogenic illness.

The use of drugs to dissect the pathway for secretion of the glycoprotein hormone chorionic gonadotropin by cultured human trophoblastic cells.

Agents that affect intracellular cation and pH gradients and inhibit energy production have been tested for their ability to modulate the processing and secretion of the free alpha subunit and the alpha beta dimer of human chorionic gonadotropin (hCG) by cultured human trophoblastic cells (JAR). Incubation of JAR cells with monensin or nigericin, monovalent cation ionophores that produce equilibration of Na+ and K+ across cellular membranes, dicyclohexylcarbodiimide, an agent that inhibits intracellular membrane ATPases, and methylamine, which neutralizes intracellular pH gradients, produced similar effects on hCG processing and secretion. All these agents inhibited the processing of the asparagine-linked oligosaccharide chains of free alpha subunit and the alpha and beta subunits contained in the hCG dimer. Moreover, after treatment of JAR cells with these agents, there was an intracellular accumulation of precursor forms and an inhibition of secretion of "mature" forms of hCG. Monensin affected the processing and secretion of hCG subunits differently at different concentrations. At 5 X 10(-7) M, monensin inhibited the processing of the asparagine-linked oligosaccharides of hCG without altering the rate-limiting step in the secretory pathway or blocking hCG secretion. The intracellular hCG subunit precursors in both control and monensin-treated cells contained a similar array of high mannose oligosaccharides, predominantly of the Man8GlcNAc2 and Man9GlcNAc2 types. However, monensin-treated cells secreted hCG subunits that contained endo H-sensitive oligosaccharides of the high mannose (mostly Man5GlcNAc2) and hybrid types rather than the endo H-resistant complex chains synthesized by control cells. Nevertheless, a full complement of serine-linked oligosaccharides was added to the hCG-beta subunit in monensin-treated cells. These results indicate that the intracellular movement of hCG from the rough endoplasmic reticulum to the cell surface was not inhibited by monensin at a concentration that impaired Golgi-localized steps in the processing of asparagine-linked oligosaccharides. At 5 X 10(-6) M, monensin significantly inhibited secretion of hCG and created a new rate-limiting step in the processing pathway. hCG subunits bearing Man5GlcNAc2 units accumulated intracellularly, suggesting that the equilibration of intracellular Na+/K+ pools blocked oligosaccharide processing at an intra-Golgi point, perhaps by inhibiting movement of the glycoprotein hormone from the "cis" to the "trans" Golgi compartment. Since the other drugs mentioned above produced similar effects on hCG processing and secretion, it appears that maintenance of intracellular cation and pH gradients is necessary for the intra-Golgi transport of glycoprotein hormones.(ABSTRACT TRUNCATED AT 400 WORDS)