High-resolution scanning x-ray diffraction microscopy

Coherent diffractive imaging (CDI) and scanning transmission x-ray microscopy (STXM) are two popular microscopy techniques that have evolved quite independently. CDI promises to reach resolutions below 10 nanometers, but the reconstruction procedures put stringent requirements on data quality and sample preparation. In contrast, STXM features straightforward data analysis, but its resolution is limited by the spot size on the specimen. We demonstrate a ptychographic imaging method that bridges the gap between CDI and STXM by measuring complete diffraction patterns at each point of a STXM scan. The high penetration power of x-rays in combination with the high spatial resolution will allow investigation of a wide range of complex mesoscopic life and material science specimens, such as embedded semiconductor devices or cellular networks.     

Ultrafast x-ray diffraction of transient molecular structures in solution

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.     

Electron diffraction of wet biological membranes

Electron diffraction patterns were obtained for the first time from single wet phospholipid bilayers and from wet human erythrocyte membranes by using a temperature-controlled electron microscope hydration stage. Selective area diffraction showed the existence of semicrystalline domains. A structural transition was observed at the transition temperature of the wet dipalmitoyl lecithin bilayer.     

Single crystals of transfer RNA: an x-ray diffraction study

Large single crystals of formylmethionyl transfer RNA have been prepared. An x-ray diffraction study shows that the material crystallizes in a hexagonal lattice with a equal to 170 angstroms, c equal to 234 angstroms. The diffraction pattern extends to spacings just under 20 angstroms at present. The crystals are heavily hydrated, containing 88 percent water.     

Single-crystal-like diffraction data from polycrystalline materials

A method for solving structures from powder diffraction data was developed, and its validity was demonstrated on three complex structures. The method uses a textured sample and exploits the high intensity and parallel nature of synchrotron radiation. In principle, crystal structures as complex as those routinely solved by single-crystal methods can be determined with this approach. For example, the as-synthesized form of the zeolite UTD-1, with 69 nonhydrogen atoms in the asymmetric unit, could be solved directly. With this method, a larger range of structural complexity becomes accessible to scientists interested in the structures of polycrystalline materials that cannot be grown as single crystals.     

Structures from anomalous diffraction of native biological macromolecules

Crystal structure analyses for biological macromolecules without known structural relatives entail solving the crystallographic phase problem. Typical de novo phase evaluations depend on incorporating heavier atoms than those found natively; most commonly, multi- or single-wavelength anomalous diffraction (MAD or SAD) experiments exploit selenomethionyl proteins. Here, we realize routine structure determination using intrinsic anomalous scattering from native macromolecules. We devised robust procedures for enhancing the signal-to-noise ratio in the slight anomalous scattering from generic native structures by combining data measured from multiple crystals at lower-than-usual x-ray energy. Using this multicrystal SAD method (5 to 13 equivalent crystals), we determined structures at modest resolution (2.8 to 2.3 angstroms) for native proteins varying in size (127 to 1148 unique residues) and number of sulfur sites (3 to 28). With no requirement for heavy-atom incorporation, such experiments provide an attractive alternative to selenomethionyl SAD experiments.     

Thiamine pyrophosphate hydrochloride: stereochemical aspects from an x-ray diffraction study

The crystal structure of thiamine pyrophosphate has been determined by a three-dimensional x-ray analysis. The conformation of the molecule in the crystalline state is determined by the formal charge distribution within the molecule which exists as a zwitterion with the negative pyrophosphate chain folded back over the positive, ring portion of the molecule. The oxygen atoms in the pyrophosphate group are in the staggered conformation when viewed along the phosphorus-phosphorus axis. Even though the pyrophosphate is present in this compound as the monoionized monoester, the configuration is the same as that present in the inorganic pyrophosphate ion. From a comparison of three different crystal structures containing the thiamine moiety and from studies with atomic models, it seems plausible that the basic molecular conformation observed in this crystal is maintained in the catalytically active molecule. Knowledge of the detailed crystal structure provides new insight into the biochemical mechanism of reactions catalyzed by thiamine pyrophosphate.     

Photoacoustic spectroscopy of biological materials

A new technique for performing optical spectroscopy on solids has been developed. Photoacoustic spectra of cytochrome c and hemoglobin show how this technique can be used to obtain information about optical absorptions and subsequent de-excitations in solid biological materials, particularly those which cannot readily be studied by conventional means.     

Laser-stimulated luminescence used to measure x-ray diffraction of a contracting striated muscle

An integrating x-ray area detector that operates on the basis of laser-stimulated luminescence was used in a diffraction study of muscle contraction. The area detector has a dynamic range of 1 to 10(5), a sensitivity about 60 times greater with approximately 1/300 as much fog background as x-ray film. It is erasable and reusable but, like film, can integrate at a practically unlimited counting rate. The high sensitivity and wide dynamic range of the detector resulted in a sufficient reduction in the exposure time to make possible the recording of a clear x-ray diffraction pattern, with up to 2.0-nanometer axial spacing, from a contracting frog skeletal muscle in as little as 10 seconds with synchrotron radiation. During the isometric contraction of the muscle, most of the actin diffraction lines increased in intensity without noticeable changes in their peak positions. Changes also occurred in diffraction intensities from the myosin heads. The results indicate that during contraction the structure of the actin filaments differs from that in the rigor state, suggesting a possible structural change in the actin subunits themselves; the myosin heads during contraction retain the axial periodicity of the myosin filament and become aligned in a more perpendicular manner to the actin filaments.     

Time-resolved X-ray diffraction studies of the B in equilibrium D structural transition in the DNA double helix

Because of the relation between topology and function, there has been much interest in the structural transitions of the various conformations of DNA polymers. The x-ray fiber diffraction analysis system at the Daresbury Synchrotron Radiation Source was used to study the reversible transition between the B and D forms of the synthetic DNA poly[d(A-T)].poly[d(A-T)]. The gradual progression of conformations between these two forms indicates that the DNA double helix does not undergo a change of handedness during this transition.     

不同材料地膜覆盖对玉米生物学性状的影响

【目的】比较同一气候条件下不同材料地膜覆盖的集雨效果,为选择高效集雨保墒、环保无污染的覆盖材料提供理论依据。【方法】在2008-2009年于渭北旱塬选用普通地膜、生物降解膜和液态膜3种材料,在玉米不同时期进行覆盖(生育期覆盖和周年覆盖),以不覆盖的传统平作为对照,比较不同覆盖材料对土壤含水量、温度及玉米生长发育、生物量和产量的影响。【结果】与对照相比,普通地膜和生物降解膜覆盖处理,5～10cm土层土壤平均温度分别增加了1.6和1.3℃,0～200cm土壤平均含水量分别增加了7.8%和7.3%,玉米生育期普遍提前,株高和生物量较对照显著增加(P0.05);液态膜覆盖表现不稳定。2年试验中,玉米生育期覆盖普通地膜、生物降解膜和液态膜处理的籽粒产量较对照分别提高19.96%,19.67%和4.77%,水分利用效率平均提高30.67%,29.69%和7.36%;周年覆盖普通地膜、生物降解膜和液态膜籽粒产量比对照分别提高23.75%,23.90%和5.31%,水分利用效率提高32.06%,31.59%和8.44%;生育期和周年覆盖普通地膜、生物降解膜的增产作用均达显著水平(P0.05)。【结论】覆盖生物降解膜和普通地膜均能显著增加作物产量和水分利用效率,且两者之间无明显差异,因此生物降解膜可以替代普通地膜应用于农业生产。     

Time-resolved holography with photoelectrons

Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse.     

应用生物素-链霉亲和素的时间分辨荧光免疫分析检测玉米赤霉烯酮

采用基于生物素(Biotin)-链霉亲和素(Strepavidin)的时间分辨荧光免疫分析(TRFIA)技术建立了快速灵敏的玉米赤霉烯酮(ZEN)检测方法.以strepavidin包被板条,加入生物素化的ZEN-BSA,结合在板条上的ZEN-BSA与标准/样本中的游离ZEN共同竞争限量的抗ZEN单克隆抗体,用稀土离子Eu3+标记的羊抗鼠IgG进行示踪,建立了间接竞争的ZEN-TRFIA.该方法的检测灵敏度为0.2 ng/ml,测量范围是0.2～200.0 ng/ml,批内、批间变异系数分别为5.7%和8.3%,平均回收率为91.1%.与玉米赤霉醇的平均交叉反应是12.3%,与赭曲霉素A、脱氧雪腐镰刀菌烯醇、黄曲霉素B1无交叉反应,说明该方法的特异性较好.相关试剂在4 ℃放6个月后各检测参数基本无变化,说明该方法稳定.样品同时用ZEN-TRFIA和商品ELISA试剂盒测定样品ZEN含量,两者的相关系数为 0.966 4,说明该方法测量准确.ZEN-TRFIA具有测量准确、检测灵敏度高、检测范围宽、操作简便、标记物稳定等优点,适合于ZEN大量筛查的应用.     

氯霉素的直接竞争时间分辨荧光免疫检测法

[目的]采用时间分辨荧光技术建立氯霉素（CAP）直接竞争免疫检测法（CAPTRFIA）。[方法]以羊抗鼠IgG二抗为固定相,游离CAP与生物素化的CAP人工抗原（CAP—BSA）共同竞争限量的CAP抗体;用铕标记的链霉亲和素示踪。[结果]该方法的灵敏度为0．016ng／ml。用于牛奶样品检测的添加回收率为89．6％,批内和批间变异系数分别为5．5％和10．1％,与甲砜霉素的交叉反应率〈0．01％。[结论]CAPTRFIA直接竞争法灵敏度高、特异性好、性能稳定,适于高通量筛查,有良好的应用前景。     

Time-resolved three-dimensional concentration measurements in a gas jet

Turbulence can greatly influence reaction and heat transfer rates in fluids. The topology of the three-dimensional interface between mixing fluids directly determines the location and degree of reaction. The time-resolved measurement of the three-dimensional concentration field in a transitional gas jet is reported. A thin sheet of laser light was swept through the flow volume by a rotating mirror in a time brief enough that motion of the gas was minimal. The light sheet illuminated different parallel planes within the flow, and light scattered from particles seeding the jet was imaged onto a detector. The series of two-dimensional measurements made during one scan of the flow volume constituted a full three-dimensional mapping of structures within the flow. Computer graphics software was used to reconstruct and visualize three-dimensional surfaces of constant concentration and the magnitude of the concentration gradient vector over such surfaces.