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11.
We used ionic tracers to estimate the volume of old (soil and ground) water interacting with snowmelt in eleven Adirondack, NY watersheds. The contribution of old water varied from 66 to 90%, with no general relationship between old water % and soil depth to till. This approach also discriminated between watershed retention and release of particular ions to lake outlet water during snowmelt. Most watersheds released NO3 ? during snowmelt, in addition to the snowpack NO3 ?. Nitrification of snowpack NH4 + explained part of the additional NO3 ? in lake out outlet water, but some NO3 ? was likely mineralized nitrogen from soil organic matter. All watersheds retained NH4 + as well. Nitrogen release was greatest in the acidic watersheds in the southwestern Adirondacks, a region thought to be impacted by anthropogenic deposition. During snowmelt, Ca2+ and Mg2+ ions (presumably from soil exchange sites) were also released from most watersheds. In watersheds with acidic (minimum pH<4.6) lake outlet water, Al was also released during snowmelt. Thus, lake outlet water acidification during snowmelt was both buffered by cation release, and intensified by NO3 ? release. If the soil exchangeable cation pools were not replenished prior to snowmelt, or NO3 ? mobilization were increased, acidification during snowmelt would intensify. 相似文献
12.
J Schaefer K J Kramer J R Garbow G S Jacob E O Stejskal T L Hopkins R D Speirs 《Science (New York, N.Y.)》1987,235(4793):1200-1204
Cross-polarization magic-angle-spinning nuclear magnetic resonance spectroscopy has been used to determine insect cuticle composition and cross-link structure during sclerotization or tanning. Unsclerotized cuticle from newly ecdysed pupae of the tobacco hornworm, Manduca sexta L., had a high protein content with lesser amounts of lipid and chitin. Concentrations of chitin, protein, and catechol increased substantially as dehydration and sclerotization progressed. Analysis of intact cuticle specifically labeled with carbon-13 and nitrogen-15 revealed direct covalent linkages between ring nitrogens of protein histidyl residues and ring carbons derived from the catecholamine dopamine. This carbon-nitrogen adduct was present in chitin isolated from cuticle by alkaline extraction and is probably bound covalently to chitin. These data support the hypothesis that the stiffening of insect cuticle during sclerotization results primarily from the deposition of protein and chitin polymers and their crosslinking by quinonoid derivatives of catecholamines. 相似文献
13.
Subjects with type III hyperlipoproteinemia develop premature atherosclerosis and have hyperlipidemia due to an increase in cholesterol-rich very low density lipoproteins (VLDL) of abnormal electrophoretic mobility. Apolipoprotein E is a major protein constituent of VLDL and appears to be important for the hepatic uptake of triglyceride-rich lipoproteins. A new kindred of patients with type III hyperlipoproteinemia is described in which no plasma apolipoprotein E could be detected, consistent with the concept that type III hyperlipoproteinemia may be due to an absence or striking deficiency of apolipoprotein E. 相似文献
14.
Schaefer VJ 《Science (New York, N.Y.)》1946,104(2707):457-459
15.
A series of organic salts, in which the cation has been designed to have a large molecular hyperpolarizability, has been prepared. Variation of the counterion (anion) in many cases leads to materials with large powder second harmonic generation efficiencies, the highest of which is roughly 1000 times that of a urea reference. 相似文献
16.
The local effects of isolated magnetic adatoms on the electronic properties of the surface of a superconductor were studied with a low-temperature scanning tunneling microscope. Tunneling spectra obtained near magnetic adsorbates reveal the presence of excitations within the superconductor's energy gap that can be detected over a few atomic diameters around the impurity at the surface. These excitations are locally asymmetric with respect to tunneling of electrons and holes. A model calculation based on the Bogoliubov-de Gennes equations can be used to understand the details of the local tunneling spectra. 相似文献
17.
DR Link G Natale R Shao JE Maclennan NA Clark E Korblova DM Walba 《Science (New York, N.Y.)》1997,278(5345):1924-1927
A smectic liquid-crystal phase made from achiral molecules with bent cores was found to have fluid layers that exhibit two spontaneous symmetry-breaking instabilities: polar molecular orientational ordering about the layer normal and molecular tilt. These instabilities combine to form a chiral layer structure with a handedness that depends on the sign of the tilt. The bulk states are either antiferroelectric-racemic, with the layer polar direction and handedness alternating in sign from layer to layer, or antiferroelectric-chiral, which is of uniform layer handedness. Both states exhibit an electric field-induced transition from antiferroelectric to ferroelectric. 相似文献
18.
PM Lundquist R Wortmann C Geletneky RJ Twieg M Jurich VY Lee CR Moylan DM Burland 《Science (New York, N.Y.)》1996,274(5290):1182-1185
The performance of amorphous organic photorefractive (PR) materials in applications such as optical data storage is generally limited by the concentration of active molecules (chromophores) that can be incorporated into the host without forming a crystalline material with poor optical quality. In polymeric PR systems described previously, performance has been limited by the necessity of devoting a large fraction of the material to inert polymer and plasticizing components in order to ensure compositional stability. A new class of organic PR materials composed of multifunctional glass-forming organic chromophores is described that have long-term stability and greatly improved PR properties. 相似文献
19.
The use of scanning tunneling microscopy in an electrochemical environment as a tool for the nanoscale modification of gold electrodes was demonstrated. Small copper clusters, typically two to four atomic layers in height, were precisely positioned on a gold(111) electrode by a process in which copper was first deposited onto the tip of the scanning tunneling microscope, which then acted as a reservoir from which copper could be transferred to the surface during an appropriate approach of the tip to the surface. Tip approach and position were controlled externally by a microprocessor unit, allowing the fabrication of various patterns, cluster arrays, and "conducting wires" in a very flexible and convenient manner. 相似文献
20.
The extent to which synaptic activity can signal a sensory stimulus limits the information available to a neuron. We determined the contribution of individual synapses to sensory representation by recording excitatory postsynaptic currents (EPSCs) in cerebellar granule cells during a time-varying, quantifiable vestibular stimulus. Vestibular-sensitive synapses faithfully reported direction and velocity, rather than position or acceleration of whole-body motion, via bidirectional modulation of EPSC frequency. The lack of short-term synaptic dynamics ensured a highly linear relationship between velocity and charge transfer, and as few as 100 synapses provided resolution approaching psychophysical limits. This indicates that highly accurate stimulus representation can be achieved by small networks and even within single neurons. 相似文献