一种改进的快速全局k均值聚类分割算法

针对k均值聚类算法的聚类结果易受初始中心随机选择和噪声的影响,本文提出了一种改进的快速全局k均值聚类算法,该方法将聚类中心选择转化为一系列子问题,解决了k均值原始聚类中心的随机选择而导致聚类结果不正确的问题,通过中值方法选择新一轮聚类中心,缓解了k均值聚类结果易受噪声影响的问题.实验结果表明,本文提出方法提高了图像聚类分割的准确性.     

鄂尔多斯地区不同生境类型对苔藓植物多样性和丰富度的影响

对鄂尔多斯地区皇甫川人工治理区和阿贵庙自然保护区不同生境类型下的苔藓植物的多样性和丰富度进行研究.结果表明,维管植物的种类和分布影响苔藓植物的生长和分布模式,其创造的微生境为苔藓植物提供了避难所,但维管植物产生的地被物(枯枝、落叶)又抑制了苔藓植物在其下的分布.苔藓植物α多样性的变化不仅与生境的变化有关,还与构成生境的群落结构要素有关,这些结构要素包括群落的层片结构,维管植物的郁闭度、盖度、种类组成,地被物的盖度和分布格局,藻类的盖度和分布格局.苔藓植物β多样性的变化与群落结构的关系更为密切,相似群落结构间的β多样性相似系数明显高于结构存在差异的群落.     

A novel nuclear export activity in HIV-1 matrix protein required for viral replication

An important aspect of the pathophysiology of human immunodeficiency virus type-1 (HIV-1) infection is the ability of the virus to replicate in non-dividing cells. HIV-1 matrix (MA), the amino-terminal domain of the Pr55 gag polyprotein (Pr55), bears a nuclear localization signal that promotes localization of the viral preintegration complex to the nucleus of non-dividing cells following virus entry. However, late during infection, MA, as part of Pr55, directs unspliced viral RNA to the plasma membrane, the site of virus assembly. How MA can mediate these two opposing targeting functions is not understood. Here we demonstrate that MA has a previously undescribed nuclear export activity. Although MA lacks the canonical leucine-rich nuclear export signal, nuclear export is mediated through the conserved Crm1p pathway and functions in both mammalian cells and yeast. A mutation that disrupts the MA nuclear export signal (MA-M4) mislocalizes Pr55 and genomic viral RNA to the nucleus, thereby severely impairing viral replication. Furthermore, we show that MA-M4 can act in a dominant-negative fashion to mislocalize genomic viral RNA even in the presence of wild-type MA. We conclude that the MA nuclear export signal is required to counteract the MA nuclear localization signal, thus ensuring the cytoplasmic availability of the components required for virion assembly.     

卧龙自然保护区土壤中有机氯农药的浓度水平及来源分析

为了研究有机氯农药(OCPs)的分布趋势、组成特征和来源, 采用气相色谱-电子捕获检测器(GC-ECD)分析了四川卧龙自然保护区2005年春秋两季山地土壤中OCPs的浓度. 结果显示, 卧龙地区土壤中OCPs浓度在10^-2~10^-1 ng/g数量级, 属于清洁背景水平. 同时, 土壤中OCPs的含量具有春季高、秋季低的特点, 这和冬季蓄积在降雪中的OCPs在春季随融雪和温度上升而释放有关. 而夏季的高温辐射使土壤中的部分OCPs又重新挥发进入大气, 夏季土壤中微生物作用活跃加速了OCPs的降解, 这也是造成了OCPs季节变化的原因. 高山草甸和林线下土壤样品季节变化幅度的不同与OCPs的物化性质及植被覆盖率密切相关. 高山草甸土壤中OCPs的浓度随海拔上升而增高, 大气湿沉降清除效率沿海拔梯度的增加是形成这种“冷凝结现象”的重要因素. 春秋两季土壤中的OCPs组成特征与大气相关样品较为一致, 而与典型污染土壤不同, 这说明卧龙地区土壤中OCPs主要来自大气传输.     

共轨柴油机燃用生物柴油的排气颗粒粒径分布及核态纳米颗粒生成机理研究

在新一代轿车用共轨柴油机上对生物柴油及柴油燃烧排放的颗粒粒径分布和纳米颗粒的生成机理进行了研究．试验结果表明,柴油机的颗粒粒径分布为双态分布：核态和聚集态,基本以50nm为分界线．在2000r／min,50N．m工况下,颗粒粒径分布为单一的聚集态分布,当转矩超过100N．m后,颗粒粒径分布从单态转化为双态分布．对于本试验中的生物柴油混合燃料,当燃料混合比小于60％,发动机高负荷下其粒径分布也为双态,相同条件纯生物柴油的颗粒粒径分布仅为单一聚集态．低负荷时,对比所有燃料的颗粒粒径分布都为单一聚集态．生物柴油混合燃料可以明显降低柴油机的聚集态颗粒数量浓度．而颗粒中核态纳米颗粒的形成可解释为随着负荷的增大,燃料的耗油量和排气温度的上升,燃烧生成的SO2被氧化催化器转化为SO3,其水化物硫酸的形成促成了核态纳米颗粒的形成．所以生物柴油混合燃料的硫含量决定带有氧化催化器的轻型柴油机的纳米颗粒的形成和数量．     

湿地状态对石油污染和植物长势影响的模拟研究

模拟实验中建立了一系列不同状态的“湿地桶”,桶中装有从黄河三角洲采集的被石油污染的土壤和当地的湿地植物。通过监测湿地植物（芦苇、香蒲等）的长势和化验土壤中总石油烃（TPH）及油和脂（O/G）随时间的变化,以探讨湿地状态对石油污染和植物长势的影响。结果显示：湿地环境对土壤中的石油污染有明显的降解作用,芦苇等挺水植物的生长量与积水深度呈正相关,土壤中少量含油并不构成对湿地植物生长的威胁。此实验可为建立油田的湿地缓冲带提供科学依据。     

Rapid magnetic reconnection in the Earth's magnetosphere mediated by whistler waves

Magnetic reconnection has a crucial role in a variety of plasma environments in providing a mechanism for the fast release of stored magnetic energy. During reconnection the plasma forms a 'magnetic nozzle', like the nozzle of a hose, and the rate is controlled by how fast plasma can flow out of the nozzle. But the traditional picture of reconnection has been unable to explain satisfactorily the short timescales associated with the energy release, because the flow is mediated by heavy ions with a slow resultant velocity. Recent theoretical work has suggested that the energy release is instead mediated by electrons in waves called 'whistlers', which move much faster for a given perturbation of the magnetic field because of their smaller mass. Moreover, the whistler velocity and associated plasma velocity both increase as the 'nozzle' becomes narrower. A narrower nozzle therefore no longer reduces the total plasma flow-the outflow is independent of the size of the nozzle. Here we report observations demonstrating that reconnection in the magnetosphere is driven by whistlers, in good agreement with the theoretical predictions.     

用纳米压痕法测量电沉积镍镀层的力学性能

随着科学技术的发展和薄膜材料的广泛应用,对于薄膜材料的性能的研究已经越来越引起人们的重视,尤其是其力学性能.因此,着重对电沉积镍涂层中的力学性能如硬度和杨氏模量等进行了研究.首先介绍了用纳米压痕的方法测量薄膜的硬度与杨氏模量的原理和方法,然后对电沉积后未经处理、经真空热处理和激光处理的样品进行了载荷-深度曲线的测试.结果表明,经过真空热处理的样品硬度明显减小,而经激光处理后的样品的硬度比未经处理的样品要高. 同时,对于同一样品,加不同的载荷导致压痕深度不同,随着压痕深度的增加其测得的硬度值和杨氏模量值都会有明显的下降.     

Homodimer formation of retinoid X receptor induced by 9-cis retinoic acid.

Retinoid response pathways are mediated by two classes of receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). A central question is whether distinct response pathways are regulated by these two classes of receptors. The observation that the stereoisomer 9-cis-retinoic acid binds with high affinity to RXRs suggested that this retinoid has a distinct role in controlling RXR activity, but it was almost simultaneously discovered that RXRs function as auxiliary receptors for RARs and related receptors, and are essential for DNA binding and function of those receptors. Hence, although RARs seem to operate effectively only as heterodimeric RAR/RXR complexes, RXRs themselves apparently function predominantly, if not exclusively, as auxiliary receptors. Here we report that 9-cis-retinoic acid induces RXR homodimer formation. Our results demonstrate a new mechanism for retinoid action by which a ligand-induced homodimer mediates a distinct retinoid response pathway.     

Formation of ordered ice nanotubes inside carbon nanotubes

Following their discovery, carbon nanotubes have attracted interest not only for their unusual electrical and mechanical properties, but also because their hollow interior can serve as a nanometre-sized capillary, mould or template in material fabrication. The ability to encapsulate a material in a nanotube also offers new possibilities for investigating dimensionally confined phase transitions. Particularly intriguing is the conjecture that matter within the narrow confines of a carbon nanotube might exhibit a solid-liquid critical point beyond which the distinction between solid and liquid phases disappears. This unusual feature, which cannot occur in bulk material, would allow for the direct and continuous transformation of liquid matter into a solid. Here we report simulations of the behaviour of water encapsulated in carbon nanotubes that suggest the existence of a variety of new ice phases not seen in bulk ice, and of a solid-liquid critical point. Using carbon nanotubes with diameters ranging from 1.1 nm to 1.4 nm and applied axial pressures of 50 MPa to 500 MPa, we find that water can exhibit a first-order freezing transition to hexagonal and heptagonal ice nanotubes, and a continuous phase transformation into solid-like square or pentagonal ice nanotubes.