共查询到20条相似文献，搜索用时 468 毫秒
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甲氧苄啶是广谱、高效、低毒的抗菌药和杀菌剂,优良的甲氧苄啶可以治疗畜禽细菌感染等疾病,但较强的疏水性使甲氧苄啶溶出速率低、生物利用率小,限制临床应用范围。而环糊精是一种新型药用辅料,可以做载体。大量研究证实,采取饱和水溶液法、搅拌法等制备方法进行包合时,在水溶液中,环糊精亲脂腔体中的水分子形成氢键的倾向得不到充分满足。因此,整个分子或药物脂质体部分可以取代环糊精体中水分子,从而降低系统能量形成包合物,能显著增加药物的溶解度或溶出速率、提高药物生物利用度和疗效,具有广阔的发展前景。 相似文献
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孙桂清 《农村实用科技信息》2007,(4):13-13
各地推广的测土配方施肥方法归纳起来有三大类六种方法:第一类是地力分区法:第二类是目标产量法,包括养分平衡法和地力差减法;第三类是田间试验法,包括肥料效应函数法、养分丰缺指标法、氮磷钾比例法。 相似文献
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聚乙烯醇与氢氧化铜形成包合物，可使氢氧化铜在8≤pH≤12的水溶液中增溶和吸收光谱红移。以pH=10的Cu2 -PVA-H20体系在740nm波长下，用分光光度计测定铜(Ⅱ)盐，其含量在10～50mg／100mL范围内可以得到理想的结果。 相似文献
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11．测土配方施肥有几种方法？
测土配方施肥的方法归纳起来有三大类六种方法：第一类是地力分区法；第二类是目标产量法，包括养分平衡法和地力差减法；第三类是田间试验法，包括肥料效应函数法、养分丰缺指标法、氮磷钾比例法。 相似文献
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锌盐共沉淀石墨炉原子吸收光谱法测定水质中Cr+6 总被引：2，自引：0，他引：2
进行了用石墨炉原子吸收光谱分析法测定液体食品(饮用品)中Cr^ 6的技术研究。结果表明：在样品溶液中加入氢氧化锌沉淀剂，调节pH为8～9，可使氢氧化锌与Cr^ 3离子形成共沉淀，从而有效地分离样品溶液中的Cr^ 3；再用石墨炉原子吸收光谱仪测定水质中的Cr^ 6。这样，不仅消除了Cr^ 3离子的干扰，而且可利用石墨炉2700℃高温原子化的特性，使各种基体干扰物质在铬元素原子化之前挥发殆尽，避免了仪器测定的背景干扰，从而大大提高了分析结果的准确性。 相似文献
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MASON HS 《Science (New York, N.Y.)》1957,125(3259):1185-1188
The enzymes which catalyze reactions of molecular oxygen occur in three principle classes: (i) oxygen transferases, (ii) mixed function oxidases, and (iii) electron transferases. The first class catalyzes the transfer of a molecule of molecular oxygen to substrate. The second class catalyzes the transfer of one atom of the oxygen to substrate; the other atom undergoes two-equivalent reduction. The third class catalyses the reduction of molecular oxygen to hydrogen peroxide or to water. 相似文献
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Bis(3-fluorosalicylaldehyde) ethylenediimine cobalt(II), a reversible oxygen-carrying compound, has been crystallized as a tetramer with two oxygen molecules bound to the four cobalt atomns, each oxygen molecule bridging between two metal atoms. These dimers are further linked by two bonds between the oxygen atom of the salicylaldehyde and the cobalt atom. The oxygen-to-oxygen distance, 1.21 angstroms in molecular oxygen, has increased to 1.308(28) angstroms (where the number in parentheses is the estimated standard deviation) in this compound, the shortest distance yet observed in sutch a bridging arrangement. 相似文献
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Hayes KF Roe AL Brown GE Hodgson KO Leckie JO Parks GA 《Science (New York, N.Y.)》1987,238(4828):783-786
A novel application of x-ray absorption spectroscopy has provided structural information for ions sorbed at oxide-water interfaces. As an example, in situ extended x-ray absorption fine structure (EXAFS) measurements of adsorbed selenate and selenite ions at ah alpha-FeOOH(goethite)-water interface have been performed; these measurements show that selenate forms a weakly bonded, outer-sphere complex and that selenite forms a strongly bonded, inner-sphere complex. The selenite ion is bonded directly to the goethite surface in a bidentate fashion with two iron atoms 3.38 angstroms from the selenium atom. Adsorbed selenate has no iron atom in the second coordination shell of selenium, which indicates retention of its hydration sphere upon sorption. This method provides direct structural information for adsorbed species at solid-liquid interfaces. 相似文献
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Schreier WJ Schrader TE Koller FO Gilch P Crespo-Hernández CE Swaminathan VN Carell T Zinth W Kohler B 《Science (New York, N.Y.)》2007,315(5812):625-629
Femtosecond time-resolved infrared spectroscopy was used to study the formation of cyclobutane dimers in the all-thymine oligodeoxynucleotide (dT)18 by ultraviolet light at 272 nanometers. The appearance of marker bands in the time-resolved spectra indicates that the dimers are fully formed approximately 1 picosecond after ultraviolet excitation. The ultrafast appearance of this mutagenic photolesion points to an excited-state reaction that is approximately barrierless for bases that are properly oriented at the instant of light absorption. The low quantum yield of this photoreaction is proposed to result from infrequent conformational states in the unexcited polymer, revealing a strong link between conformation before light absorption and photodamage. 相似文献
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有机酸对土壤磷吸附的影响 总被引：6，自引：0，他引：6
以棕红壤为对象,采用等温平衡吸附法,研究了不同有机酸种类和浓度对土壤磷吸附的影响。结果表明：不同种类的有机酸、其功能基（－ＣＯＯＨ,－ＯＨ）数目多的对降低土壤吸磷量的影响大,供试五种有机酸的影响大小顺序为：柠檬酸＞草酸＞酒石酸＞苯甲酸＞乙酸;在同一ｐＨ下,随加入有机酸浓度的升高,土壤吸磷量减少;有机酸与磷的不同加入方式对磷吸附量的影响也不同,先吸附磷后再加入有机酸的吸磷量最大,吸附有机酸后离心弃去有机酸液再加入磷的吸磷量次之,吸附有机酸后再加入磷的吸磷量最小。 相似文献
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Cyclobutane-type pyrimidine dimers in polynucleotides 总被引：17，自引：0，他引：17
R B Setlow 《Science (New York, N.Y.)》1966,153(734):379-386
The formation of cyclobutane-type dimers between adjacent pyrimidine residues in model polynucleotides or DNA may be represented by the general scheme See pdf 379.pdf Whereas the formation of all other known photoproducts follows the irreversible path See pdf 379.pdf Thus dimers are distinguished from other photoproducts by the fact that they can be monomerized, as well as formed, by ultraviolet irradiation. At large incident fluxes of photons the steady-state value of dimers depends on wavelength and pH, as well as on other characteristics of the surrounding medium. The number of dimers in an irradiated polynucleotide may be decreased by purely photochemical means, whereas this is not true for most other photoproducts, for which continued irradiation, irrespective of wavelength, always results in the formation of more photoproduct (37). The wavelength dependence of the steady-state for dimers is also reflected in the biological activity of irradiated transforming DNA. This experiment and the fact that photoreactivating enzyme plus visible light monomerizes dimers (and has not been demonstrated to have any effect on other photoproducts) are the strongest lines of experimental evidence that pyrimidine dimers of the cyclobutane type are biologically important lesions and can account for a large fraction of the effects of ultraviolet light on DNA in solution. Insofar as DNA is one of the more important biological structures, such dimers, when formed, account for a large part of the effects of ultraviolet radiation on biological systems. 相似文献
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通过大量培养不同来源的92个黑木耳菌株,筛选得到1株高产红褐色素的菌株,命名为Aua09,提取其产生的色素,并研究pH、温度、光照、氧化剂、还原剂对色素稳定性的影响。结果表明:该色素的最大吸收峰为304nm,易溶于碱性溶液,溶于甲醇、丙酮、乙醇等有机溶剂,不溶于乙醚,酸性条件下,色素溶解度随pH降低而减小,pH值为2以下时色素基本上全部沉淀;光对色素稳定性影响不大,有较好的耐高温性能,抗氧化还原性强。 相似文献
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染料与木材结合机理的研究 总被引：8，自引：1，他引：7
为探索染料在染色木材中的存在形式以及染料分子与木材分子的结合形态,该研究选用毛白杨做试材,用两种酸性染料通过真空浸注方式对其进行染色,运用傅立叶变换红外光谱仪对木材染色前后进行红外光谱分析.结果表明:在不加入其他助剂的情况下,两种染料对毛白杨染色没有出现新的吸收峰,没有新的官能团产生;染色过程中木材发生了物理和化学变化,其部分官能团的吸收强度发生了改变,染料分子的部分官能团占据了木材中的部分官能团的位置,也存在覆盖现象. 相似文献
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Ihee H Lorenc M Kim TK Kong QY Cammarata M Lee JH Bratos S Wulff M 《Science (New York, N.Y.)》2005,309(5738):1223-1227
We report direct structural evidence of the bridged radical (CH2ICH2.) in a polar solution, obtained using time-resolved liquid-phase x-ray diffraction. This transient intermediate has long been hypothesized to explain stereo-chemical control in many association and/or dissociation reactions involving haloalkanes. Ultrashort optical pulses were used to dissociate an iodine atom from the haloethane molecule (C2H4I2) dissolved in methanol, and the diffraction of picosecond x-ray pulses from a synchrotron supports the following structural dynamics, with approximately 0.01 angstrom spatial resolution and approximately 100 picosecond time resolution: The loss of one iodine atom from C2H4I2 leads to the C-I-C triangular geometry of CH2ICH2.. This transient C2H4I then binds to an iodine atom to form a new species, the C2H4I-I isomer, which eventually decays into C2H4 + I2. Solvent dynamics were also extracted from the data, revealing a change in the solvent cage geometry, heating, and thermal expansion. 相似文献
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Alkalinity and formation of zeolites in saline alkaline lakes 总被引：1，自引：0，他引：1
The solubility of rhyolitic glass increases with increasing alkalinity, whereas the ratio of silicon to aluminum decreases with increasing alkalinity. The strong correlation observed between alkalinity and zeolite mineralogy in saline, alkaline lakes is thought to be a function of this relationship between pH and the Si/Al ratio. It is suggested that this function is a result of the reaction between silicic glass and alkaline solution whereby (i) a gel forms, whose Si/Al ratio is controlled by the Si/Al ratio of the solution, and (ii) a zeolite forms from the gel, whose Si/Al ratio is, in turn, controlled by the composition of the gel. 相似文献