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排序方式: 共有293条查询结果,搜索用时 55 毫秒
21.
Static and dynamic mechanical deflections were electrically induced in cantilevered, multiwalled carbon nanotubes in a transmission electron microscope. The nanotubes were resonantly excited at the fundamental frequency and higher harmonics as revealed by their deflected contours, which correspond closely to those determined for cantilevered elastic beams. The elastic bending modulus as a function of diameter was found to decrease sharply (from about 1 to 0.1 terapascals) with increasing diameter (from 8 to 40 nanometers), which indicates a crossover from a uniform elastic mode to an elastic mode that involves wavelike distortions in the nanotube. The quality factors of the resonances are on the order of 500. The methods developed here have been applied to a nanobalance for nanoscopic particles and also to a Kelvin probe based on nanotubes. 相似文献
22.
Microorganisms living in anoxic rice soils contribute 10 to 25% of global methane emissions. The most important carbon source for CH4 production is plant-derived carbon that enters soil as root exudates and debris. Pulse labeling of rice plants with 13CO2 resulted in incorporation of 13C into the ribosomal RNA of Rice Cluster I Archaea in the soil, indicating that this archaeal group plays a key role in CH4 production from plant-derived carbon. This group of microorganisms has not yet been isolated but appears to be of global environmental importance. 相似文献
23.
Benson DE Conrad DW de Lorimier RM Trammell SA Hellinga HW 《Science (New York, N.Y.)》2001,293(5535):1641-1644
We report a flexible strategy for transducing ligand-binding events into electrochemical responses for a wide variety of proteins. The method exploits ligand-mediated hinge-bending motions, intrinsic to the bacterial periplasmic binding protein superfamily, to establish allosterically controlled interactions between electrode surfaces and redox-active, Ru(II)-labeled proteins. This approach allows the development of protein-based bioelectronic interfaces that respond to a diverse set of analytes. Families of these interfaces can be generated either by exploiting natural binding diversity within the superfamily or by reengineering the specificity of individual proteins. These proteins may have numerous medical, environmental, and defense applications. 相似文献
24.
Skowyra D Koepp DM Kamura T Conrad MN Conaway RC Conaway JW Elledge SJ Harper JW 《Science (New York, N.Y.)》1999,284(5414):662-665
Control of cyclin levels is critical for proper cell cycle regulation. In yeast, the stability of the G1 cyclin Cln1 is controlled by phosphorylation-dependent ubiquitination. Here it is shown that this reaction can be reconstituted in vitro with an SCF E3 ubiquitin ligase complex. Phosphorylated Cln1 was ubiquitinated by SCF (Skp1-Cdc53-F-box protein) complexes containing the F-box protein Grr1, Rbx1, and the E2 Cdc34. Rbx1 promotes association of Cdc34 with Cdc53 and stimulates Cdc34 auto-ubiquitination in the context of Cdc53 or SCF complexes. Rbx1, which is also a component of the von Hippel-Lindau tumor suppressor complex, may define a previously unrecognized class of E3-associated proteins. 相似文献
25.
Open carbon nanotubes were filled with molten silver nitrate by capillary forces. Only those tubes with inner diameters of 4 nanometers or more were filled, suggesting a capillarity size dependence as a result of the lowering of the nanotube-salt interface energy with increasing curvature of the nanotube walls. Nanotube cavities should also be less chemically reactive than graphite and may serve as nanosize test tubes. This property has been illustrated by monitoring the decomposition of silver nitrate within nanotubes in situ in an electron microscope, which produced chains of silver nanobeads separated by high-pressure gas pockets. 相似文献
26.
Zusammenfassung In Nährlösung nach Bonner und Addicott (1937) wurde der Einfluß hoher Konzentrationen von Penicillin G (1,5 · 10–3 bis 6 · 10–3 g/ml) auf Aufnahme und Einbau von35S-Methionin in das Eiweiß verschiedener Entwicklungsabschnitte isolierter Wurzeln von Erbsenkeimlingen untersucht. Die Methionin-Aufnahme wurde bei den drei Wurzelsegmenten übereinstimmend durch 6 · 10–3 g Penicillin je ml Nährlösung leicht gehemmt und durch 1,5 und 3 · 10–3 g/ml gefördert, und zwar durch die schwächere Konzentration am stärksten. Die absolute Größe des Methionin-Einbaus in das Eiweiß der Wurzelzellen und die darin erreichte spezifische Methionin-Markierung werden durch das Penicillin noch weniger beeinflußt als die Stoffaufnahme. Bei Einwirkung von Penicillinmengen, die das Wurzelwachstum stark herabsetzen, weicht die Intensität des Stoffeinbaus von den unbehandelten Kontrollen (ohne Penicillin) nur unerheblich ab.
Summary In nutrient solutions (Bonner and Addicott 1937) the influence of high concentrations of penicillin G (1,5 · 10–3 up to 6 · 10–3 g/ml) was investigated upon uptake and incorporation of35S-methionine into the proteins in segments of different developmental stage of isolated roots of pea seedlings. In all three root segments uptake of methionine was slightly depressed by 6 · 10–3 g penicillin per ml nutrient solution, and stimulated by 1,5 · 10–3 and 3 · 10–3 g/ml, most strongly by the weaker concentration. The absolute quantity of methionine incorporation into proteins of root cells and the specific methionine activity reached within these cells are less influenced by penicillin than the uptake. In solutions with penicillin quantities strongly inhibiting root growth the intensity of methionine incorporation differs but insignificantly from that of tests without penicillin.
(1937) G ( 1,5 · 10–3 6 · 10–3 / 35S- . : 6 · 10–3 , 1,5 3 · 10–3 / -; . «» , , , . , , ( ).相似文献
27.
Summary Hydrogen oxidation in soil was measured at low (1 ppmv) and high (300 ppmv) H2 concentrations to distinguish between the activities of abiontic soil hydrogenases and Knallgas bacteria, respectively. The two activities also showed distinctly different pH optima, temperature optima, and apparent activation energies. The pH optima for the soil hydrogenase activities were similar to the soil pH in situ, i.e., pH 8 in an slightly alkaline garden soil (pH 7.3) and pH 5 in an acidic cambisol (pH 4.6–5.4). Most probable number determinations in the alkaline acidic soils showed that Knallgas bacterial populations grew preferentially in neutral or acidic media, respectively. However, H2 oxidation activity by Knallgas bacteria in the acidic soil showed two distinct pH optima, one at pH 4 and a second at pH 6.4–7.0. The soil hydrogenase activities exhibited temperature optima at 35–40°C, whereas the Knallgas bacteria had optima at 50–60°C. The apparent activation energies of the soil hydrogenases were lower (11–23kJ mol-1) than those of the Knallgas bacteria (51–145 kJ mol-1). Most of the soil hydrogenase activity was located in the upper 10 cm of the acidic cambisol and changed with season. The seasonal activity changes were correlated with changes in soil moisture and soil pH. 相似文献
28.
Incubation of soil under low partial pressures of acetylene (10 Pa) is a widely used method to specifically inhibit nitrification
due to the suicide inhibition of ammonium monooxygenase (AMO), the first enzyme in NH4
+ oxidation by nitrifying bacteria. Although the inhibition of AMO is irreversible, recovery of activity is possible if new
enzyme is synthesized. In experiments with three different soils, NH4
+ concentrations decreased and NO3
– concentrations increased soon after acetylene was removed from the atmosphere. Recovery of NO production started immediately
after the removal of acetylene. The release rates of NO and N2O were higher in soil samples which were only preincubated with 10 Pa acetylene than in those which were kept in the presence
of 10 Pa acetylene. In the permanent presence of 10 Pa acetylene, NH4
+ and NO3
– concentrations stayed constant, and the release rates of NO and N2O were low. These low release rates were apparently due to processes other than nitrification. Our experiments showed that
the blockage of nitrification by low (10 Pa) acetylene partial pressures is only reliable when the soil is kept in permanent
contact with acetylene.
Received: 17 July 1996 相似文献
29.
Oxygen profiles and methane turnover in a flooded rice microcosm 总被引:19,自引:0,他引:19
Summary Dissolved O2 was depleted within the top 3.5-mm surface layer of flooded rice soil microcosms without plants. In planted microcosms, however, O2 was detectable down to at least 40 mm in depth. O2 concentrations in the uppermost soil layers of microcosms with rice plants were higher in the light than in the dark, indicating O2 production by photosynthesis. The CH4 emission rates were nearly identical for illuminated and for darkened microcosms, demonstrating that the photosynthetically produced O2 did not increase CH4 oxidation in the rhizosphere. In contrast, CH4 emission rates increased when the microcosms were incubated under an N2 atmosphere, indicating that transport of O2 from the atmosphere into the rhizosphere was important for CH4 oxidation. CH4 emission under air accounted for only 10%–20% of the cumulative CH4 production determined in cores taken from the microcosms. Apparently, 80%–90% of the CH4 produced was oxidized in the rhizosphere and thus was not emitted. 相似文献
30.
Nitrification and denitrification are, like all biological processes, influenced by temperature. We investigated temperature
effects on N trace gas turnover by nitrification and denitrification in two soils under two experimental conditions. In the
first approach ("temperature shift experiment") soil samples were preincubated at 25 °C and then exposed to gradually increasing
temperatures (starting at 4 °C and finishing at 40–45 °C). Under these conditions the immediate effect of temperature change
was assessed. In the second approach ("discrete temperature experiment") the soil samples were preincubated at different temperatures
(4–35 °C) for 5 days and then tested at the same temperatures. The different experimental conditions affected the results
of the study. In the temperature shift experiment the NO release increased steadily with increasing temperature in both soils.
In the discrete temperature experiment, however, the production rates of NO and N2O showed a minimum at intermediate temperatures (13–25 °C). In one of the soils (soil B9), the percent contribution of nitrification
to NO production in the discrete temperature experiment reached a maximum (>95% contribution) at 25 °C. In the temperature
shift experiment nitrification was always the dominant process for NO release and showed no systematic temperature dependency.
In the second soil (soil B14), the percent contribution of nitrification to NO release decreased from 50 to 10% as the temperature
was increased from 4 °C to 45 °C, but no differences were evident in the discrete temperature experiment. The N2O production rates were measured in the discrete temperature experiment only. The contribution of nitrification to N2O production in soil B9 was considerably higher at 25–35 °C (60–80% contribution) than at 4–13 °C (15–20% contribution).
In soil B14 the contribution of nitrification to N2O production was lowest at 4 °C. The effects of temperature on N trace gas turnover differed between the two soils and incubation
conditions. The experimental set-up allowed us to distinguish between immediate effects of short-term changes in temperature
on the process rates, and longer-term effects by which preincubation at a particular temperature presumably resulted in the
adaptation of the soil microorganisms to this temperature. Both types of effects were important in regulating the release
of NO and N2O from soil.
Received: 20 October 1998 相似文献