关于空间群判断失误的札记I.最小素单胞的选取

导出反映晶体结构周期的最小素单胞是正确判断空间群的第一步,其中以还原单胞的选择最为理想。它不仅具有选择的唯一性,且便于确定布拉维格子的类型。本文还讨论单胞选取过小和过大的实例,并给出正确的单胞和空间群以及原子坐标。     

BaCe_(0.8)Y_(0.2)O_(3-α)固体电解质的离子导电性及其燃料电池性能

用高温固相反应合成了BaCe0.8Y0.2O3-α固体电解质用氢浓差电池和氧浓差电池方法研究了它的离子导电特性。以该氧化物为固体电解质多孔性Pt为电极材料组成氢空气燃料电池测定了该燃料电池的电流电压特性。研究发现BaCe0.8Y0.2O3-α在氢气中几乎是一个纯的质子导体在氧气中是一个氧离子和电子空穴的混合导体其燃料电池的开路电压OCV接近于理论值最大输出电流密度约为820mA·cm-21000℃最大输出功率密度约为200mW·cm-21000℃放电性能稳定具有良好的电池性能。     

Mechanical response analysis and power generation by single-cell stretching.

To harvest useful information about cell response due to mechanical perturbations under physiological conditions, a cantilever-based technique was designed, which allowed precise application of arbitrary forces or deformation histories on a single cell in vitro. Essential requirements for these investigations are a mechanism for applying an automated cell force and an induced-deformation detection system based on fiber-optical force sensing and closed loop control. The required mechanical stability of the setup can persist for several hours since mechanical drifts due to thermal gradients can be eliminated sufficiently (these gradients are caused by local heating of the cell observation chamber to 37 degrees C). During mechanical characterization, the cell is visualized with an optical microscope, which enables the simultaneous observation of cell shape and intracellular morphological changes. Either the cell elongation is observed as a reaction against a constant load or the cell force is measured as a response to constant deformation. Passive viscoelastic deformation and active cell response can be discriminated. The active power generated during contraction is in the range of Pmax= 10(-16) Watts, which corresponds to 2500 ATP molecules s(-1) at 10 k(B)T/molecule. The ratio of contractive to dissipative power is estimated to be in the range of 10(-2). The highest forces supported by the cell suggest that about 10(4) molecular motors must be involved in contraction. This indicates an energy-conversion efficiency of approximately 0.5. Our findings propose that, in addition to the recruitment of cell-contractile elements upon mechanical stimulation, the cell cytoskeleton becomes increasingly crosslinked in response to a mechanical pull. Quantitative stress-strain data, such as those presented here, may be employed to test physical models that describe cellular responses to mechanical stimuli.     

Molecular aspects of sickle cell disease.

Sickle cell hemoglobin (hemoglobin S) contains valine instead of glutamic acid in position 6 of the β-chain. Few detectable conformational differences exist between hemoglobin S and normal adult hemoglobin (hemoglobin A). Following loss of oxygen, sickle cell hemoglobin (hemoglobin S) self associates to form a liquid crystal phase which distorts the erythrocyte into the sickle shape thereby resulting in the clinical symptomology associated with sickle cell anemia. This gel contains deoxyhemoglobin S monomers and polymers in equilibrium. The polymerization process is known to have a negative temperature coefficient, to be pH dependent, and to be extremely dependent on hemoglobin S concentration. The polymerization of deoxyhemoglobin S appears to be entropically driven and occurs in two kinetic phases, a delay period and a rapid polymerization process. The polymer consists of tubules containing six or eight strands of deoxyhemoglobin S tetramers which align with one another. Each strand is would around the tubule with a pitch of about 3000 Å, but the precise nature of the intermolecular hemoglobin S contacts is not known. Subsequent alignment of the tubules occurs and results in the tactoids observed in (S/S) erythrocytes. While many of the details of polymer formation and structure remain to be elucidated, several attempts to chemically alter the sickling phenomenon have been carried out. As yet, however, no satisfactory chemical treatment has been discovered.     

Bioactivity-guided isolation of new antiproliferative compounds from Juniperus foetidissima Willd.

Based on a literature survey on cytotoxic medicinal plants, Juniper species were identified as interesting source of antitumor compounds. Using bioassay-guided fractionation against Caov-4 cancer cells on acetone extract of leaves and branchlets of Juniperus foetidissima led to the isolation of a new 3H-benzofuaran-2-one: 4-methyl-3-methoxy-3H-benzofuaran-2-one (1), a new sesquiterpene: 4,9(α)-dihydroxy-nardosin-6-en (2) and an already known labdane-type diterpene: 15-hydroxy-8(17),13(E)-labdadiene-19-carboxilic acid (3). Compounds 1–3 exhibited cytotoxic effects, with moderate cytotoxicity against the EJ-138 bladder and CAOV-4 ovary cancer cell lines.     

Five new ent-kaurane diterpenes from Annona squamosa L. pericarps

Abstract

In the present study, five new ent-kaurane diterpenes including 4α-hydroxy-17,19-dinor-ent-kaurane-16-one (1), 4β-hydroxy-16β-H-18-nor-ent-kaurane-17-oic acid (2), 4β,17-dihydroxy-16α-acetoxy-18-nor-ent-kaurane (3), Annosquamosin Z (4) and 16α-H-ent-kaurane-17,18-dioic acid, 17-methy ester (5) were isolated from Annona squamosa L. pericarp. The compounds were also evaluated for their cytotoxic activities against SMMC-7721 and HepG2 cell lines, among which compound 3 exhibited potent cytotoxicity with IC₅₀ value of less than 20?μM.