Changes in culture growth dynamics and cell size in a Scenedesmus quadricauda upon action of potassium dichromate

The effect of potassium dichromate in concentrations of 0.5 to 10 mg/l on a laboratory culture of Sc. quadricauda algae was studied in standard conditions. The total cell numbers decreased at potassium dichromate concentrations over 1 mg/l, and the proportion of living cells decreased at all studied concentrations. A positive correlation was found between changes in cell size and their numbers at toxin concentrations of 1 and 3 mg/l, and a negative correlation was found between the relative size and the cell numbers at 3 and 10 mg/l. This may be due to different intensity of growth inhibition and cell division under the influence of the toxin. The culture sensitivity to the toxin increased in autumn and decreased in the spring.

中国暖温带辽东栎林主要树种共存机制的研究

物种共存是植物群落生态学的一个重要问题。对物种共存的研究可以促进人们对生态系统结构与功能的认识，并为生物多样性保护及恢复提供理论指导。在本论文中，通过对辽东栎－棘皮桦林主要树种空间格局及空间关系、竞争作用和繁殖策略的研究，初步确定本地区辽东栎林主要树种共存的机制。 空间点格局分析表明辽东栎、棘皮桦和五角枫的幼苗、幼树和大树均呈聚集分布。辽东栎幼苗和幼树的分布与同种大树个体正相关，与棘皮桦的大树个体负相关，而与死树个体无显著的相关关系。这些结果说明种子散播和萌生繁殖过程影响辽东栎树木个体的空间格局。棘皮桦幼苗和幼树的分布在小的空间尺度上与同种活树和死树个体正相关，而与其它树种的活树及死树个体的分布呈负相关，说明萌生是该物种繁殖的一个重要途径。五角枫的幼苗和幼树呈聚集分布，并与同种大树个体的分布有关，说明其种子有限的散播范围影响其分布。辽东栎大树个体的分布与棘皮桦大树个体在小尺度上呈负相关，表明这两个树种之间可能存在竞争关系。 离散模型（the diffusion model）和生长动态模型（the growth dynamics model）分析表明在一些树种之间存在单向的种间竞争，例如，黄花柳对白桦和棘皮桦的竞争作用，棘皮桦对辽东栎，辽东栎对五角枫，和五角枫对山杨的竞争作用。这些单向竞争主要是林冠层的大树个体对林下层小个体的竞争。在这些树种中没有发现种内竞争。但是，各树种的生长率（G (t, x)）与生长率方差（ ）呈直线相关，而且样地中树木的死亡率较低，说明树种间的竞争强度很小。因此我们推测非对称的种间竞争在塑造该森林群落结构和物种共存过程中的作用不大。 五角枫、辽东栎、糠椴和山杨的幼苗和幼树在森林的林下层占优势，密度和出现频率均较高，可以形成稳定的幼苗库。五角枫、辽东栎和糠椴的幼苗大小级结构呈倒J形，山杨幼苗的大小结构呈单峰形。棘皮桦幼苗很少，但幼树相对较多，说明该树种可以形成幼树库。辽东栎幼苗在林窗中密度较大，而幼树的密度与林窗的出现与否无明显的相关关系，说明林窗的形成对新更新苗有显著影响，而林窗内的幼树可能是由林窗形成前已经建立的幼苗发展而来。林冠下的辽东栎幼苗多数为萌生起源，寿命较长，一旦上层大树死亡，可以迅速生长，进入林冠层。因此，林冠层下的幼苗库和林窗更新对辽东栎种群动态至关重要。五角枫幼苗和幼树的耐荫性或避荫性较强，可以形成巨大的长期存在的幼苗库和幼树库，以维持其种群的更新。糠椴和山杨的幼苗，主要起源于根的萌蘖。由于母树提供营养，幼苗生长速度较其它三个树种快。但它们对土壤湿度和营养的要求较高，这限制了它们很难占据其它生境。 虽然，在所研究的处于不同演替阶段的群落中，树种的组成有所不同，但辽东栎、大叶白腊和五角枫的幼苗均非常丰富，可以形成幼苗库。辽东栎的幼苗库大部分为萌生苗，而大叶白腊的幼苗库主要由实生苗构成。大叶白腊的幼苗种群中小个体丰富，它们生长速度很慢，在林下采取一种“坐－等”（sit-and-wait）的策略。在三个树种中，五角枫幼苗主枝的伸长生长率最高，而且生长率与幼苗的基径和生境的光照强度正相关。 通过以上研究，我们初步得出结论：竞争作用在研究的辽东栎－棘皮桦林主要树种共存中起的作用很小;形成稳定长久的幼苗库是这些树种的主要繁殖策略。因此，理解这些树种共存机制的关键在于这些树种幼苗库或幼树库的不同起源，及幼苗和幼树对生境因子的不同生态响应所创造的丰富的更新生态位。

濒危植物疏花水柏枝种群生物学和回归引种的研究

　本文对三峡库区濒危植物疏花水柏枝的种子形成与传播、种子萌发与幼苗定居、植株生长动态等种群生物学特性及其与环境的关系，特别是对水淹胁迫的生理学与生态学响应进行了研究，并以此为基础结合疏花水柏枝抢救工程的实施，对疏花水柏枝回归引种的相关生态学问题进行了探讨。 　　疏花水柏枝植株的生殖分配极高，每株所形成的花枝、花序和种子数量都比较多，其种子传播的方式有风传播和水传播，种子直接传播是疏花水柏枝种群进行扩增的主要途径，自然分布疏花水柏枝种群因此而常常成群分布。种子通过江水的二次传播是沿长江呈星状分布的疏花水柏枝各种群间进行基因交流的主要途径。限制疏花水柏枝种子萌发的环境因素主要是土壤含水量，这也是导致该物种在自然条件下仅分布于消落带的关键因素。疏花水柏枝一年生幼苗的成活率以及衡量植株生长状况的生物量、枝条数和根系长度都随着密度的增加而显著下降，表明密度对疏花水柏枝一年生幼苗的定居与生长有明显的制约作用。疏花水柏枝每年夏天都要经历长时间的水淹，在该时期无论是否被水淹，其幼苗地上部的生长都处于停滞（休眠），水淹后植株生物量都有一定程度的减少，但在各种水淹程度处理与对照之间除根深有显著差异外，幼苗的存活率、生物量、一级枝数、一级枝长度、根冠比的差异都不显著，表明水淹对幼苗的定居和生长没有显著影响，疏花水柏枝是通过快速的生物量丧失和快速恢复来适应水淹阶段的环境胁迫的。夏季适当的水淹不仅不会影响植株的生长，反而有利于该物种躲避这个阶段河滩上的高温天气，提高幼苗的成活率。 　　疏花水柏枝对水淹的生理生态适应机制较复杂。夏季水淹过后，植株进入生长恢复阶段，恢复初期植株的日均蒸腾强度与日均光合速率随着其在夏季被淹没时间的增加而增加，说明夏季水淹有利于植株尽快恢复生长。生长季节的水淹胁迫会使疏花水柏枝植株最初的光合作用和蒸腾作用的能力有所下降，但是长期水淹后，植株会形成适应，其光合与水分生理又会恢复到正常状态。疏花水柏枝植株也会在生理生化上对水淹胁迫作出响应。在夏天洪水季节无论是否被水淹，植株的可溶性糖总糖与蔗糖的都含量上升，根系活力、丙二醛（MDA）含量、过氧化物酶(POD)活性、多酚氧化酶(PPO)活性在各种水淹处理与对照之间以及淹水后与淹水前之间没有显著差异，从生理上表明疏花水柏枝的生长在夏季处于生长停滞（休眠）状态，其生理生化变化是植株对休眠的响应。夏天洪水季节结束后，各处理植株的多酚氧化酶活性、过氧化物酶活性、蔗糖与总糖含量在生长恢复阶段显著下降，丙二醛含量、根系活力则分别有所增加，表明植株在生理生化上也得到恢复，且植株在夏季是否受水淹胁迫对生长恢复阶段植株的生理生化变化没有显著影响。生长阶段的水淹胁迫会导致丙二醛含量、多酚氧化酶、过氧化物酶活性上升，其中前两者的变化显著，而根系活力、可溶性总糖含量则在水淹后表现为下降。但是经长期水淹后，各种生理生化指标会逐渐恢复到正常水平，表明该物种对水淹的适应能力较强，水淹并不会对植株形成长期的生理压力。 　　疏花水柏枝被回归引种于长江三峡的几个主要支流上，形成几个相对隔离的重建种群。种群遗传多样性、年龄结构、分布格局、繁殖与扩展等生物学特性是种群持续发展的基础，疏花水柏枝种群的重建中必须予以考虑。各移栽点应分别从不同的地点采集移栽材料，且以生长旺盛、繁殖力强、遗传性状稳定的3-10年植株为主。在疏花水柏枝种群恢复与重建中目前所面临的主要问题是如何增强被隔离的种群间的基因交流、促进种群的种子扩散与萌发、协调新建种群与当地物种的关系、营造有利于新建种群定居与生长的生态环境。

Effects of seed dot layer and thin GaAs spacer layer on the structure and optical properties of upper In(Ga)As quantum dots

The structure and optical properties of In(Ga)As with the introduction of InGaAlAs or InAlAs seed dot layers are investigated. The area density and size homogeneity of the upper InGaAs dots are efficiently improved by the introduction of a buried layer of high-density dots. Our explanation for the realization of high density and size homogeneity dots is presented. When the GaAs spacer layer is too thin to cover the seed dots, the upper dots exhibit some optical properties like those of a quantum well. By analyzing the growth dynamics, we refer to this kind of dot as an empty-core dot. (C) 2003 American Institute of Physics.

Structure and optical properties of upper In(Ga)As quantum dots on a different seed layer with a thin GaAs spacer layer

The structure and optical properties of In(Ga)As grown with the introduction of InGaAlAs or InAlAs seed dots layers are investigated. The area density and size homogeneity of the upper InGaAs dots are efficiently improved with the introduction of a layer of high-density buried dots. When the GaAs spacer layer is too thin to cover the seed dots, the upper dots exhibit the characterization of a quantum well. By analyzing the growth dynamics, we refer to it as an empty-core structure dot. (C) 2002 Elsevier Science B.V. All rights reserved.

Violet/blue emission from hydrogenated amorphous carbon films deposited from energetic CH3+ ions and ion bombardment

Considering the complexity of the general plasma techniques, pure single CH3+ ion beams were selected for the deposition of hydrogenated amorphous (a) carbon films with various ion energies and temperatures. Photoluminescence (PL) measurements have been performed on the films and violet/blue emission has been observed. The violet/blue emission is attributed to the small size distribution of sp(2) clusters and is related to the intrinsic properties of CH3 terminals, which lead to a very high barrier for the photoexcited electrons. Ion bombardment plays an important role in the PL behavior. This would provide further insight into the growth dynamics of a-C:H films. (C) 2002 American Institute of Physics.

九寨沟湖泊湿地植物群落与植物地理研究

九寨沟湖泊湿地在维持九寨沟的生态平衡中起着重要的作用，在旅游产业的发展下，湿地生态系统及生物多样性面临着较大的威胁。尽管九寨沟湿地具有重要的生态价值，但目前对其研究尚比较薄弱。湿地植物群落和植物地理研究可以为湿地资源的可持续利用和监测保护提供科学依据。作者从2004年8月到2007年11月对九寨沟湿地的植物物种组成、地理分布、优势植物群落的结构、生长动态、湿地土壤种子库进行了调查研究。主要结果如下： 1. 九寨沟湿地物种组成、地理分布特点及湿地植物群落特点 九寨沟湿地共有苔藓植物8科13属16种，维管植物为48科107属199种。九寨沟湿地植物的地理成份较为丰富，维管植物在科级水平上有7种地理分布型（变型），在属级水平上有13种地理分布型（变型）， 在种级水平上共有29种地理分布型（变型）。九寨沟湿地植物以温带成份和我国特有成份为主，同时兼有热带、亚热带成份和环极—高山成份。九寨沟湿地植物的分布表现出明显的垂直地带性和水平地带性。湿地植物群落可划分21个群落类型，不同植物群落类型的物种多样性及物种组成存在较大的差异。九寨沟湿地植物的物种多样性和群落多样性以及较高的生产力特征，是维持其湿地生态景观多样性和稳定性的基础。 2. 土壤、水环境、海拔等对湿地植物的分布及生物多样性的影响 九寨沟湿地土壤、水等环境因子存在较大的差异。帕米尔苔草和宽叶香蒲等群落的凋落物较多，土壤有机碳、土壤总磷较高，可能是九寨沟湿地的重要土壤碳库。 九寨沟湿地植物沿水环境梯度的分布规律表现为：沉水植物（轮藻—篦齿眼子菜，水苦荬，杉叶藻）——挺水植物（水木贼，芦苇，宽叶香蒲）——湿生草本（苔草、节节草、披散木贼）——湿生灌木（柳灌丛，小檗灌丛）等。海拔也影响湿地植物的物种组成。 水深对物种多样性有影响，水深与物种丰富度负相关。随着水深的增加，水木贼、芦苇、杉叶藻、宽叶香蒲等群落的物种多样性下降；在长期淹水和季节性淹水的地方，水木贼群落物种多样性存在显著差异。土壤总氮与水木贼群落物种丰富度正相关。 3. 土壤营养元素、水环境对植物生长的影响 水深影响湿地植物生物量的分配。芦苇无性系分株在47 cm水深的环境中单株平均生物量最大；在干滩地中（地面水深0 cm），叶生物量百分比最大，而茎生物量百分比最小，茎的生物量百分比和生长速率随水深的增加而增加；在较干的滩地生境中，开花率、花序的生物量百分比明显大于水较深的生境。 水深与水木贼地上生物量负相关，但水木贼地上生物量在长期淹水和季节性淹水的地方没有显著的差异。在水浅的地方，杉叶藻、水木贼、芦苇等植物群落中，其他伴生物种的生物量占样方总生物量的百分比较大。 土壤有机碳、土壤总氮、土壤总磷等对湿地植物生物量的影响比较大：宽叶香蒲地上生物量与土壤总磷正相关；水木贼地上生物量与土壤总氮正相关；杉叶藻地上生物量与土壤有机碳正相关。 水深、土壤营养成分对湿地植物高度、密度等有影响。水木贼的平均高度在季节性淹水的地方比长期淹水的地方低，平均密度在长期淹水的地方比季节性淹水的地方低；除了5月份，其他观察月份水木贼的密度都与水深负相关，同时与土壤有机碳正相关。另外，芦苇密度与土壤有机碳含量正相关，宽叶香蒲密度与水深负相关，帕米尔苔草高度与土壤有机碳负相关。 4. 优势植物群落的动态变化 在优势植物群落中，优势种的高度、密度、盖度、生物量等在群落中占绝对优势。除五花海，水木贼群落的物种组成、高度、生物量在两年间没有显著的变化。芦苇群落的物种丰富度在近两年有所增加。 湿地植物生长表现为明显的季节动态，生长的峰值大多在7月－8月。优势植物群落的物候与水文周期有关。湿地植物群落的物种组成和密度，可以作为对湿地监测和保护的生物指示。 5. 九寨沟湿地土壤种子库特征及其在湿地生物多样性恢复中的作用 水深和现存植被物种丰富度可以解释湿地土壤种子库的变化。水深可以解释表层物种丰富度45%的变化。现存植被物种丰富度可以分别解释10 cm土层、2－5 cm土层及5－10 cm土层土壤种子库45%、48%和25%的变化。 湿地土壤种子库的密度为0－15945粒m-2, 种子库中共发现23个物种。现存植被优势物种和种子库优势物种不同。各层土壤种子库密度和物种丰富度并不存在显著的差异，但第二层土壤种子库密度最大。海拔、现存植被优势种盖度、土壤总磷、土壤总氮、土壤有机碳对湿地土壤种子库的密度和垂直结构没有影响。土壤种子库物种丰富度小于地上植被物种丰富度。湿地土壤种子库与地上植被的相关性不大。在浅水区域，湿地土壤种子库在湿地植被恢复中有一定作用。但在深水区域，保护现存植被更重要。 The lakeshore wetlands are valuable ecological units of the Jiuzhaigou lakes. Pressure for travel industry development pose a continuing and severe threat to the biodiversity-support function of the wetland system. Despite the ecological importance of wetlands in Jiuzhaigou, they are so far poorly studied. Both general plant communties and biogeographical studies are needed in order to attain basis for sustainable use the wetland resources and adequate protection of these areas. The present study was undertaken to examine aquatic plants distribution and the species compositon, structure and growth dynamics of their communities with variations of environmental factors along altitudes, water depth and soil properities gradients in Jiuzhaigou. Analysis of field survey data collected during August 2004 and November 2007 in lakeshore wetlands in Jiuzhaigou National Nature Reserve, Sichuan, China. The results were as following: (i) Species composition and biogeography in wetland vegetation 8 families, 13 genus, 16 species of moss and 48 families, 107 genus and 199 species of vascular plants in Jiuzhaigou wetlands were found. The floristic compositions were abundunt. Ten geographical distribution types at family level, 13 geographical distributions types at generic level and 29 geographical distribution types at specific level in vascular plants were found. Most species in Jiuzhaigou wetlands are temperate elements and Chinese endemic elements, with a few of tropical and subtropical and some circumarctic elements. And the plant distributions show clear vertical and horizontal patterns. There were 21 major wetland plant community types. Species composition and species richness in different plant communities are different. The species diversity and plant community diversity and their high biomass are the basis for the diversity and stability of wetland landscapes in Jiuzhaigou. (ii) Water depth, soil nutrients and altitudes influence on the species diversity and plant distribution. Total phosphorous and organic cabon in soil were higher in C. pamiernensis and T. latifolia communities, where are important cabon reservoirs in Jiuzhaigou wetlands. Along gradients of water depth, among populations of the dominant plant species present: submerged macrophytes (Chara vulgaris, Potagemonton pectinatus, Veronica anagalis-aquatica，Hippuris vulgaris), emergent macrophytes (Equisetum fluviatile, Phragamites australis, Typha latifolia), helophytes (Carex pamirensis )and shrubs (Salix sp., Berberis sp. ). Altitudes influence on the assemblage of plant communities. Water depth negatively correlated with species richness. Specie richness showed differences between permanently flooded sites and seasonally flooded sites in E. fluvatile communities. And total nitrogen in soil was negatively correlated with species richness in E. fluviatile communities. Altitudes show no significant influence on species richness, but in fact, through our analyses, they do have influence on the assemblage of wetland plants. (iii) Water depth, soil nutrients influence on the plant growth Water depth influences the biomass allocation in Phragmities australis. The average aboveground biomass of a single ramet (4.2 g) was the largest in the habitat with water level 47 cm above the soil surface. At the habitat with water level under soil surface 15 cm (－15 cm), the leaf biomass percentage (of the total ramet biomass) was the largest (46.1%), and the height and percentage of ramose ramets ( with branches on stem )(of the total ramets in a plot) were found obviously different. The deeper in water, the larger the biomass percentage and growth rate of stems were. The flowering rate and biomass of panicles were greater in shallow water than those in deep water. Water depth negatively correlated with aboveground biomass of E. fluviatile. However, above-ground biomass of E. fluviatile showed no significant difference between permanently flooded sites and seasonally flooded sites. But in shallow water, more biomasses of accompanying species were found in dominant plant communities such as H. vulgaris communities, E. fluviatile communities and P. australis communities. Water depth, soil nutrients influence on shoot density and shoot length of wetland plants. The shoot density of E. fluviatile was correlated to water depth in all growth months. Annual average density was significantly lower at permanently flooded sites than at seasonally flooded sites. But the annual average shoot length was significantly lower at seasonally flooded sites than at permanently flooded sites. (iv) Growth dynamics of dominant communities in Jiuzhaigou wetland The shoot length and shoot density, coverage and biomass of domiant species were dominated in plant communities. The species composition increased in P. australis communities in recent two years. The species richness in E. fluviatile communities showed no difference between 2005 and 2007. The above-ground biomass and shoot density in Five－flower Lake from July 2005 to July 2007 were significantly different, while in other sites, the differences were not significant. Shoot height, shoot density and above-ground biomass showed significant seasonal changes in all sites. Growth dynamics correlated with the cycle of water levels in lakes. Most plants growth parameters peaked at July or August. The biomass of T. latifolia peaked in August. But the shoot length of T. latifolia in deeper water peaked in July. The shoot length of E. fluviatile increased significantly from May to August except in seasonally flooded sites in Arrow-bamboo Lake. The species composition of communities and shoot density can be used as bioindicators in Jiuzhaigou wetland. (v) Soil seed bank in Jiuzhaigou wetland and its role in vegetation restoration Seed density in all soil layer samples was negatively correlated to water depth. Water depth can explain 45% variance of species richness in surface layer in sediment. Species richness in extant vegetation can explain 45%, 48%, 25% variance of species richness in total 10 cm and in 2－5 cm and 5－10 cm layer sediment respectively. Mean seed densities in wetlands ranged from 0 to 15945 m–2. A total of 23 species germinated in seed bank. The dominant species in seed bank and extant vegetation showed great difference. The total number of species and seedlings that germinated in different layers was not significantly different. But the second layer had the greatest seed density. In shallow water, seed bank can contribute to vegetation restoration, while in deeper water, protection of extant vegetation may be a better strategy.

Dynamics of nonlinear error growth and season-dependent predictability of El Nino events in the Zebiak-Cane model

With the intermediate-complexity Zebiak-Cane model, we investigate the 'spring predictability barrier' (SPB) problem for El Nino events by tracing the evolution of conditional nonlinear optimal perturbation (CNOP), where CNOP is superimposed on the El Nino events and acts as the initial error with the biggest negative effect on the El Nino prediction. We show that the evolution of CNOP-type errors has obvious seasonal dependence and yields a significant SPB, with the most severe occurring in predictions made before the boreal spring in the growth phase of El Nino. The CNOP-type errors can be classified into two types: one possessing a sea-surface-temperature anomaly pattern with negative anomalies in the equatorial central-western Pacific, positive anomalies in the equatorial eastern Pacific, and a thermocline depth anomaly pattern with positive anomalies along the Equator, and another with patterns almost opposite to those of the former type. In predictions through the spring in the growth phase of El Nino, the initial error with the worst effect on the prediction tends to be the latter type of CNOP error, whereas in predictions through the spring in the decaying phase, the initial error with the biggest negative effect on the prediction is inclined to be the former type of CNOP error. Although the linear singular vector (LSV)-type errors also have patterns similar to the CNOP-type errors, they cover a more localized area than the CNOP-type errors and cause a much smaller prediction error, yielding a less significant SPB. Random errors in the initial conditions are also superimposed on El Nino events to investigate the SPB. We find that, whenever the predictions start, the random errors neither exhibit an obvious season-dependent evolution nor yield a large prediction error, and thus may not be responsible for the SPB phenomenon for El Nino events. These results suggest that the occurrence of the SPB is closely related to particular initial error patterns. The two kinds of CNOP-type error are most likely to cause a significant SPB. They have opposite signs and, consequently, opposite growth behaviours, a result which may demonstrate two dynamical mechanisms of error growth related to SPB: in one case, the errors grow in a manner similar to El Nino; in the other, the errors develop with a tendency opposite to El Nino. The two types of CNOP error may be most likely to provide the information regarding the 'sensitive area' of El Nino-Southern Oscillation (ENSO) predictions. If these types of initial error exist in realistic ENSO predictions and if a target method or a data assimilation approach can filter them, the ENSO forecast skill may be improved. Copyright (C) 2009 Royal Meteorological Society

Modeling of Ammonothermal Growth of Nitrides

Bulk single crystals of GaN and AlN can be grown from supercritical fluids using the ammonothermal method, which utilizes ammonia as fluid rather than water as in the hydrothermal process. In this process, a mineralizer such as amide, imide or nitride is used to attack a bulk nitride feedstock at temperatures from 200°C to 500°C and pressures from 1 to 4 kbar. Ammonothermal systems have been modeled here using fluid dynamics, thermodynamics and heat transfer models. The nutrient is considered as a porous media bed and the fluid flow is simulated using the Darcy-Brinkman-Forchheimer model. The resulting governing equations are solved using the finite volume method. The effects of particle size on flow pattern and temperature distribution in an autoclave are analyzed.

Forced Dissociation of Selectin-ligand Complexes Using Steered Molecular Dynamics Simulation

Selectin-ligand interactions are crucial to such biological processes as inflammatory cascade or tumor metastasis. How transient formation and dissociation of selectin-ligand bonds in blood flow are coupled to molecular conformation at atomic level, however, has not been well understood. In this study, steered molecular dynamics (SMD) simulations were used to elucidate the intramolecular and intermolecular conformational evolutions involved in forced dissociation of three selectin-ligand systems: the construct consisting of P-selectin lectin (Lec) and epidermal growth factor (EGF)-like domains (P-LE) interacting with synthesized sulfoglycopeptide or SGP-3, P-LE with sialyl Lewis X (sLeX), and E-LE with sLeX. SMD simulations were based on newly built-up force field parameters including carbohydrate units and sulfated tyrosine(s) using an analogy approach. The simulations demonstrated that the complex dissociation was coupled to the molecular extension. While the intramolecular unraveling in P-LESGP-3 system mainly resulted from the destroy of the two anti-parallel sheets of EGF domain and the breakage of hydrogen-bond cluster at the Lec-EGF interface, the intermolecular dissociation was mainly determined by separation of fucose (FUC) from Ca2+ ion in all three systems. Conformational changes during forced dissociations depended on pulling velocities and forces, as well as on how the force was applied. This work provides an insight into better understanding of conformational changes and adhesive functionality of selectin-ligand interactions under external forces.

Forced extension of P-selectin construct using steered molecular dynamics

P-selectin, a 70-nm-long cellular adhesive molecule, possesses elastic and extensible properties when neutrophils roll over the activated endotheliam of blood vessel in inflammatory reaction. Transient formation and dissociation of P-selectin/ligand bond on applied force of blood flow induces the extension of P-selectin and relevant ligands. Steered molecular dynamics simulations were performed to stretch a single P-selectin construct consisting of a lectin (Lec) domain and an epithelial growth factor (EGF)-like domain, where P-selectin construct was forced to extend in water with pulling velocities of 0.005-0.05 nm/ps and with constant forces of 1000-2500 pN respectively. Resulting force-extension profiles exhibited a dual-peak pattern on various velocities, while both plateaus and shoulders appeared in the extension-time profiles on various forces. The force or extension profiles along stretching pathways were correlated to the conformational changes, suggesting that the structural collapses of P-selectin Lec/EGF domains were mainly attributed to the burst of hydrogen bonds within the major beta sheet of EGF domain and the disruptions of two hydrophobic cores of Lee domain. This work furthers the understanding of forced dissociation of P-selectin/ligand bond.

A Novel Counter Sheet-Flow Sandwich Cell Culture Device For Mammalian Cell Growth In Space

Cell culture and growth in space is crucial to understand the cellular responses under microgravity. The effects of microgravity were coupled with such environment restrictions as medium perfusion, in which the underlying mechanism has been poorly understood. In the present work, a customer-made counter sheet-flow sandwich cell culture device was developed upon a biomechanical concept from fish gill breathing. The sandwich culture unit consists of two side chambers where the medium flow is counter-directional, a central chamber where the cells are cultured, and two porous polycarbonate membranes between side and central chambers. Flow dynamics analysis revealed the symmetrical velocity profile and uniform low shear rate distribution of flowing medium inside the central culture chamber, which promotes sufficient mass transport and nutrient supply for mammalian cell growth. An on-orbit experiment performed on a recovery satellite was used to validate the availability of the device.

Strengthening Mechanisms And Dislocation Dynamics In Twinned Metal Nanowires

Microtwins are frequently observed in face-centered-cubic (fcc) metal nanowires with low stacking fault energy. The authors have previously reported that the tensile Yield strength of copper nanowires can be increased by, the presence of twin boundaries. lit this work, simulations are carried out under both uniaxial tension and compression loading, to demonstrate that the strengthening effects are inherent to these nanowires, independent of the loading condition (tensile/compressive). It appears that the strengthening mechanism of the twinned nanowires can be attributed to stress redistribution due to the change of crystallographic orientations across twin boundaries, which requires larger external stress to make them Yield as compared to the twin-free wire.