共查询到20条相似文献，搜索用时 15 毫秒
1.
Cruzan JD Braly LB Liu K Brown MG Loeser JG Saykally RJ 《Science (New York, N.Y.)》1996,271(5245):59-62
Measurement of the far-infrared vibration-rotation tunneling spectrum of the perdeuterated water tetramer is described. Precisely determined rotational constants and relative intensity measurements indicate a cyclic quasi-planar minimum energy structure, which is in agreement with recent ab initio calculations. The O-O separation deduced from the data indicates a rapid exponential convergence to the ordered bulk value with increasing cluster size. Observed quantum tunneling splittings are interpreted in terms of hydrogen bond rearrangements connecting two degenerate structures. 相似文献
2.
Resonant ion-dip infrared spectroscopy has been used to record size-specific infrared spectra of C(6)H(6)-(H(2)O)n clusters with n = 1 through 7 in the O-H stretch region. The O-H stretch spectra show a dramatic dependence on cluster size. For the n = 3 to 5 clusters, the transitions can be divided into three types-attributable to free, pi hydrogen-bonded, and single donor water-water O-H stretches-consistent with a C(6)H(6)-(H(2)O)n structure in which benzene is on the surface of a cyclic (H(2)O)n cluster. In n = 6 and 7 clusters, the spectra show distinct new transitions in the 3500 to 3600 wave number region. After comparison of these results with the predictions of ab initio calculations on (H(2)O)n clusters, these new transitions have been assigned to double donor O-H stretches associated with the formation of a more compact, noncyclic structure beginning with (H(2)O)(6). This is the same size cluster for which ab initio calculations predict that a changeover to noncyclic (H(2)O)n structures will occur. 相似文献
3.
郑新宇 《湖南农业大学学报(自然科学版)》2012,38(2):225-228
采用紫外光谱法和电化学法研究了水杨酸(SA)与鲱鱼精DNA的相互作用。紫外吸收光谱法结果表明,SA与DNA的相互作用为嵌插方式。在pH 5.8的NaH2PO4-Na2HPO4缓冲液中对SA进行循环伏安扫描,SA在玻碳电极上于1.070 V(Ag/AgCl参比电极)有1个阳极氧化峰,随着DNA的加入,氧化峰电流降低,峰电位正移,进一步表明SA嵌插到DNA分子的双螺旋结构中,二者相互作用生成了一种非电活性的超分子复合物。通过电化学方法可计算出SA与DNA复合物的结合比为3∶1,结合常数β为2.53×1012。 相似文献
4.
以丙三醇和2％质量分数硼酸的复配物为防腐剂,以脲醛树脂胶为胶黏剂,采用响应面分析法对室内用马尾松防腐胶合板的制作工艺进行优化,并用傅立叶变换红外光谱（FTIR）进行表征分析．结果表明,马尾松防腐胶合板的最佳制作工艺为：防腐液中丙三醇的质量分数1．91％,干燥温度75℃,干燥时间4h．在此条件下制作的马尾松防腐胶合板胶合强度为1．39MPa,与理论预测值基本一致．FTIR分析表明硼酸与丙三醇混合后反应生成硼酸酯,硼酸酯对木材结构和胶粘剂的固化不会造成不良影响,因此制作的马尾松防腐胶合板胶合强度可以达到国家标准GB／T9846-2004中Ⅱ类胶合板的要求． 相似文献
5.
Atomic clusters containing from two to several hundred atoms offer the possibility of studying the transition from molecules to crystalline solids. The covalent group IV elements carbon, silicon, and germanium are now being examined with this long-range objective. These elements are particularly interesting because of the very different character of their crystalline solids and because they are intermediate between metals and insulators in the nature of their bonding. Small mass-selected atom cluster ions are formed by pulsed laser techniques and identified by time-of-flight methods. Laser photoexcitation is used to study the relative stability of these clusters and their modes of fragmentation. These modes for C(n)(+) clusters, which tend to fragment with a characteristic loss of a neutral C(3), are found to be different from the modes for Si(n)(+) and Ge(n)(+) clusters, which tend to fragment to "magic" clusters such as Si(4)(+), Si(6)(+) and Si(10)(+). These experimental results can be accounted for by recent theoretical calculations of the ground-state structure and stability of small silicon and carbon clusters. Several theoretical approaches give consistent results, showing that small silicon clusters are compact and different from small fragments of the bulk crystal. Calculations show that carbon clusters change from linear structures toward cyclic structures as the cluster size increases, but with significant odd-even differences. 相似文献
6.
7.
大豆生物解离纤维素是生物解离技术提取大豆油脂过程中产生的不溶性纤维素,合理利用该纤维素有利于生物解离技术推广。试验以大豆生物解离纤维素为基材,添加丙三醇后利用延流法制备可食性膜,测定不同丙三醇添加量下可食性膜拉伸强度、断裂伸长率、水蒸气透过率、玻璃化转变温度、颜色及亮度、水溶性。结果表明,丙三醇对可食性膜多种特性产生影响。丙三醇添加量增加,可食性膜断裂伸长率、水蒸气通过率、亮度及水溶性升高,拉伸强度及玻璃化转变温度降低。利用扫描电子显微镜观察可食性膜表面微观结构发现,添加丙三醇可提高可食性膜表面致密性与均一性;红外吸收光谱测定结果表明,丙三醇与大豆生物解离纤维素间产生氢键作用,在成膜过程中破坏纤维素分子间酯键,削弱纤维素分子间作用力。研究为大豆生物解离纤维素可食性膜的生产提供理论依据。 相似文献
8.
We observed coherent proton tunneling in the cyclic network of four hydrogen bonds in calix[4]arene. The tunneling frequency of 35 megahertz was revealed by a peak in the magnetic field dependence of the proton spin-lattice relaxation rate measured with field-cycling nuclear magnetic resonance in the solid state at temperatures below 80 kelvin. The amplitude of the coherent tunneling peak grows with temperature according to a Boltzmann law with energy D/kB = (125 +/- 10) kelvin (where kB is Boltzmann's constant). The tunneling peak can be interpreted in the context of level crossings in the region where the tunneling frequency matches the proton Larmor frequency. The tunneling spectrum reveals fine structure that we attribute to coupling between the hydrogen bonds in the network. The characteristics of the tunneling peak are interpreted in the context of the potential energy surface experienced by the hydrogen atoms in the network. 相似文献
9.
Cyclic urinary leukopoietic activity in gray collie dogs 总被引：6，自引：0，他引：6
The urinary activities for bone marrow colony formation were measured on consecutive 24-hour urine samples from two gray collie dogs with cyclic neutropenia and from two normal collies. The activity varied cyclically in the gray collies with a peak activity developing during the neutropenic phase, which antecedes the return of blood neutrophils. The activity fell to undetectable levels after the blood neutrophil counts returned to the normal range. The urine of normal dogs showed no activity. Since the dogs with cyclic neutropenia have been shown to have periodic hematopoiesis, these data suggest a regulatory hormonal role for the substance measured by this assay. 相似文献
10.
The cyclic water hexamer, a higher energy isomer than the cage structure previously characterized in the gas phase, was formed in liquid helium droplets and studied with infrared spectroscopy. This isomer is formed selectively as a result of unique cluster growth processes in liquid helium. The experimental results indicate that the cyclic hexamer is formed by insertion of water molecules into smaller preformed cyclic complexes and that the rapid quenching provided by the liquid helium inhibits its rearrangement to the more stable cage structure. 相似文献
11.
The "spliceosome": yeast pre-messenger RNA associates with a 40S complex in a splicing-dependent reaction 总被引：110，自引：0，他引：110
The in vitro splicing reactions of pre-messenger RNA (pre-mRNA) in a yeast extract were analyzed by glycerol gradient centrifugation. Labeled pre-mRNA appears in a 40S peak only if the pre-mRNA undergoes the first of the two partial splicing reactions. RNA analysis after extraction of glycerol gradient fractions shows that lariat-form intermediates, molecules that occur only in mRNA splicing, are found almost exclusively in this 40S complex. Another reaction intermediate, cut 5' exon RNA, can also be found concentrated in this complex. The complex is stable even in 400 mM KCl, although at this salt concentration, it sediments at 35S and is clearly distinguishable from 40S ribosomal subunits. This complex, termed a "spliceosome," is thought to contain components necessary for mRNA splicing; its existence can explain how separated exons on pre-mRNA are brought into contact. 相似文献
12.
豺和果子狸毛发的傅立叶变换红外光谱特征对比研究 总被引：1，自引：0，他引：1
使用傅立叶变换红外光谱(Fr-IR)对果子狸(Paguma larvata)和豺(Cuon alpinus)的毛发进行了检测和分析.结果表明,两种动物毛发的FT-IR图谱具有明显的种间差异,主要表现在出峰范围和最强峰位置的不同;同种动物颈部、腹部和背部针毛的FT-IR图谱具有较好的一致性,表明不同部位针毛的角蛋白分子结构、构型和构像相近;针毛和绒毛的红外图谱都具有共同的特征,均可反映出该物种毛发组织的特征性成分,但针毛的特征性峰高、峰面积更典型,FT-IR光谱特征更稳定. 相似文献
13.
Lowering of pH from 7.40 to 6.60 significantly decreased the rate of glycerol formation in fat cells incubated with either 1-norepinephrine or theophylline. When these cells were incubated with both 1-norepinephrine and theophylline, the glycerol formation proceeded at maximal rate and was quite similar at pH 7.40 and 6.60. These results indicate that the inhibitory effect of acidosis on the lipolytic action of 1-norepinephrine is exerted on the process which activates lipase. 相似文献
14.
All-Carbon Molecules: Evidence for the Generation of Cyclo[18]carbon from a Stable Organic Precursor
Diederich F Rubin Y Knobler CB Whetten RL Schriver KE Houk KN Li Y 《Science (New York, N.Y.)》1989,245(4922):1088-1090
The unambiguous structural characterization of a single-sized all-carbon molecule requires its chemical synthesis. For cyclo[18]carbon, ab initio calculations predict a relatively stable, cyclic D9h ground state geometry with alternating C-C (1.36 angstroms) and C identical withC (1.20 angstroms) bonds. The synthesis and x-ray crystal structure of a direct precursor to C(18) are described. The analysis of laser flash heating experiments on this precursor by time-of-flight mass spectroscopy shows a sequence of retro-Diels-Alder reactions leading to C(18) as the predominant fragmentation pattern. Structural evidence is provided for the generation of an all-carbon molecule from a well-characterized organic precursor. 相似文献
15.
Melanin,DNA replication,and autophagy affect appressorium development in Setosphaeria turcica by regulating glycerol accumulation and metabolism 
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下载免费PDF全文 GUO Xiao-yue LIU Ning LIU Bing-hui ZHOU Li-hong CAO Zhi-yan HAN Jian-min DONG Jin-gao 《农业科学学报》2022,21(3):762-773
Setosphaeria turcica (syn. Exserohilum turcicum) is the pathogenic fungus of maize (Zea mays) that causes northern leaf blight, which is a major maize disease worldwide. Melanized appressoria are highly specialized infection structures formed by germinated conidia of S. turcica that infect maize leaves. The appressorium penetrates the plant cuticle by generating turgor, and glycerol is known to be the main source of the turgor. Here, the infection position penetrated by the appressorium on maize leaves was investigated, most of the germinated conidia entered the leaf interior by directly penetrating the epidermal cells, and the appressorium structure was necessary for the infection, whether it occurred through epidermal cells or stomata. Then, to investigate the effects of key factors in the development of the appressorium, we studied the effects of three inhibitors, including a melanin inhibitor (tricyclazole, TCZ), a DNA replication inhibitor (hydroxyurea, HU), and an autophagy inhibitor (3-methyladenine, 3-MA), on appressorium turgor and glycerol content. As results, appressorium turgor pressure and glycerol concentration in the appressorium reached their highest levels at the mature stage of the appressorium under the control and inhibitor treatments. The three inhibitors had the greatest effects on appressorium turgor pressure at this stage. Glycogen and liposomes are the main substances producing glycerol. It was also found inhibitors affected the distribution of glycogen and liposomes, which were detected in the conidia, the germ tube, and the appressorium during appressorium development. This study provides profound insight into the relationship between appressorium turgor pressure and glycerol content, which was affected by the synthesis of melanin, DNA replication, and autophagy in the developing appressorium during a S. turcica infection. 相似文献
16.
为了探究在不同条件下蜂蜜中的甘油含量,通过甘油酶制剂法测定其含量,实验结果表明,利用甘油酶制剂法能较稳定的测定蜂蜜中的甘油含量,其加标回收率实验的回收率在95.7%~115.0%之间,精密度实验甘油含量的RSD在2%~7%,重复性实验甘油含量的RSD在1%~5%;同时pH为3.00~7.00时,蜂蜜中的甘油含量变化较稳定,在7 d内,通过此法测定蜂蜜中甘油的含量无明显变化,在4~25℃条件下,蜂蜜中甘油含量变化较稳定。同时采用此法对不同蜂蜜中的甘油含量进行测定,能够达到生产上基本测定的要求。 相似文献
17.
A fluorometric-analysis procedure, used to quantitate indoles, confirmed the presence of 5-hydroxytryptamine (5-HT) in an arachnid; there was unimodal cyclic production of 5-HT in brain and intestinal tissues over a 24-hour period. The same tissues produced 5-HT after 80-day culture; bimodal cyclic output was indicated during continuous 24-hour study. One peak occurred at 0200 hours, at the same time as the peak in vivo, suggesting an endogenously controlled mechanism of secretion. The second peak occurred at midmorning, a time when production in vivo was lowest, suggesting that there is a possible feedback-control mechanism in the organism that inhibits the endogenous output of 5-HT. 相似文献
18.
Cox DJ 《Science (New York, N.Y.)》1966,152(3720):359-361
An asymmetric initial boundary is often formed when a synthetic boundary cell is used in the analytical ultracentrifuge. Model calculations show that, when such a boundary diffuses, the peak of the corresponding gradient pattern moves. The movement of the peak resulting from diffusion, when superimposed on the sedimentation of the boundary, may produce errors in the measurement of the sedimentation coefficient. 相似文献
19.
Pérez C Muckle MT Zaleski DP Seifert NA Temelso B Shields GC Kisiel Z Pate BH 《Science (New York, N.Y.)》2012,336(6083):897-901
Theory predicts the water hexamer to be the smallest water cluster with a three-dimensional hydrogen-bonding network as its minimum energy structure. There are several possible low-energy isomers, and calculations with different methods and basis sets assign them different relative stabilities. Previous experimental work has provided evidence for the cage, book, and cyclic isomers, but no experiment has identified multiple coexisting structures. Here, we report that broadband rotational spectroscopy in a pulsed supersonic expansion unambiguously identifies all three isomers; we determined their oxygen framework structures by means of oxygen-18-substituted water (H(2)(18)O). Relative isomer populations at different expansion conditions establish that the cage isomer is the minimum energy structure. Rotational spectra consistent with predicted heptamer and nonamer structures have also been identified. 相似文献
20.
Oscillation of cyclic adenosine monophosphate concentration during the myocardial contraction cycle 总被引：3，自引：0，他引：3
G Brooker 《Science (New York, N.Y.)》1973,182(115):933-934
The concentration of adenosine 3',5'-monophosphate (cyclic AMP) rises and falls during each myocardial contraction cycle. Peak concentrations of cyclic AMP precede peak development of systolic tension. Epinephrine alters the normal oscillation in myocardial cyclic AMP and increases both diastolic and systolic concentrations of the cyclic nucleotide. These transient changes in myocardial cyclic AMP indicate a potential role for cyclic AMP as a beat-to-beat regulator of myocardial contractility. 相似文献