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Hydroxyurea, a selective cytotoxic agent for cells in DNA synthesis, injected intraperitoneally at 24 and 48 hours after the first painting with 1 percent croton oil, significantly reduced the tumor yield in the two-stage chemical carcinlogenesis in mouse skin. A comparable group of mice receiving hydroxyurea only once at 24 hours had a tumor induction similar to that in controls. 相似文献
105.
Temperature profiles measured regularly for 21 years reveal the interannual differences in winter-to-summer heat gain in Castle Lake, California, a small subalpine lake. Year-to-year changes in large-scale climatic surface forcing, especially the amount of snowfall from February through April (which determines the date of thaw) coupled with the early heating and wind mixing after thaw, causes this interannual variation. The seasonal thermal structure for years in which the lake gains significantly more or less heat than normal-all of the El Ni?o years and several others-shows that the depth of the mixed layer and the mixing of heat into the stratified thermocline region control the storage of heat. The temperature of the mixed layer does not reflect abnormal thermal storage. Variations in mixing during early spring, which controls the heat content at Castle Lake, may also affect the annual average of the primary productivity. 相似文献
106.
We have developed the suspended-load backpack, which converts mechanical energy from the vertical movement of carried loads (weighing 20 to 38 kilograms) to electricity during normal walking [generating up to 7.4 watts, or a 300-fold increase over previous shoe devices (20 milliwatts)]. Unexpectedly, little extra metabolic energy (as compared to that expended carrying a rigid backpack) is required during electricity generation. This is probably due to a compensatory change in gait or loading regime, which reduces the metabolic power required for walking. This electricity generation can help give field scientists, explorers, and disaster-relief workers freedom from the heavy weight of replacement batteries and thereby extend their ability to operate in remote areas. 相似文献
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Choi J Wang DY Kundu S Choliy Y Emge TJ Krogh-Jespersen K Goldman AS 《Science (New York, N.Y.)》2011,332(6037):1545-1548
Carbon-fluorine bonds are the strongest known single bonds to carbon and as a consequence can prove very hard to cleave. Alhough vinyl and aryl C-F bonds can undergo oxidative addition to transition metal complexes, this reaction has appeared inoperable with aliphatic substrates. We report the addition of C(sp(3))-F bonds (including alkyl-F) to an iridium center via the initial, reversible cleavage of a C-H bond. These results suggest a distinct strategy for the development of catalysts and promoters to make and break C-F bonds, which are of strong interest in the context of both pharmaceutical and environmental chemistry. 相似文献
110.
The carbon-fluorine bond in compounds of biological interest 总被引：3，自引：0，他引：3
P Goldman 《Science (New York, N.Y.)》1969,164(884):1123-1130