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.

Damage Localization, Sensitivity of Energy Release and the Catastrophe Transition

Large earthquakes can be viewed as catastrophic ruptures in the earth’s crust. There are two common features prior to the catastrophe transition in heterogeneous media. One is damage localization and the other is critical sensitivity; both of which are related to a cascade of damage coalescence. In this paper, in an attempt to reveal the physics underlying the catastrophe transition, analytic analysis based on mean-field approximation of a heterogeneous medium as well as numerical simulations using a network model are presented. Both the emergence of damage localization and the sensitivity of energy release are examined to explore the inherent statistical precursors prior to the eventual catastrophic rupture. Emergence of damage localization, as predicted by the mean-field analysis, is consistent with observations of the evolution of damage patterns. It is confirmed that precursors can be extracted from the time-series of energy release according to its sensitivity to increasing crustal stress. As a major result, present research indicates that the catastrophe transition and the critical point hypothesis (CPH) of earthquakes are interrelated. The results suggest there may be two cross-checking precursors of large earthquakes: damage localization and critical sensitivity.

Two-Dimensional Kinetics Of Beta(2)-Integrin And Icam-1 Bindings Between Neutrophils And Melanoma Cells In A Shear Flow

Cell adhesion, mediated by specific receptor-ligand interactions, plays an important role in biological processes such as tumor metastasis and inflammatory cascade. For example, interactions between beta(2)-integrin ( lymphocyte function-associated antigen-1 and/or Mac-1) on polymorphonuclear neutrophils (PMNs) and ICAM-1 on melanoma cells initiate the bindings of melanoma cells to PMNs within the tumor microenvironment in blood flow, which in turn activate PMN-melanoma cell aggregation in a near-wall region of the vascular endothelium, therefore enhancing subsequent extravasation of melanoma cells in the microcirculations. Kinetics of integrin-ligand bindings in a shear flow is the determinant of such a process, which has not been well understood. In the present study, interactions of PMNs with WM9 melanoma cells were investigated to quantify the kinetics of beta(2)-integrin and ICAM-1 bindings using a cone-plate viscometer that generates a linear shear flow combined with a two-color flow cytometry technique. Aggregation fractions exhibited a transition phase where it first increased before 60 s and then decreased with shear durations. Melanoma-PMN aggregation was also found to be inversely correlated with the shear rate. A previously developed probabilistic model was modified to predict the time dependence of aggregation fractions at different shear rates and medium viscosities. Kinetic parameters of beta(2)-integrin and ICAM-1 bindings were obtained by individual or global fittings, which were comparable to respectively published values. These findings provide new quantitative understanding of the biophysical basis of leukocyte-tumor cell interactions mediated by specific receptor-ligand interactions under shear flow conditions.

Attractor crisis and bursting in a fluid flow with two no-slip directions

Characteristic burtsing behavior is observed in a driven, two-dimensional viscous flow, confined to a square domain and subject to no-slip boundaries. Passing a critical parameter value, an existing chaotic attractor undergoes a crisis, after which the flow initially enters a transient bursting regime. Bursting is caused by ejections from and return to a limited subdomain of the phase space, whereas the precrisis chaotic set forms the asymptotic attractor of the flow. For increasing values of the control parameter the length of the bursting regime increases progressively. Passing another critical parameter value, a second crisis leads to the appearance of a secondary type of bursting, of very large dynamical range. Within the bursting regime the flow then switches in irregular intervals from the primary to the secondary type of bursting. Peak enstrophy levels for both types of bursting are associated to the collapse of a primary vortex into a quadrupolar state.

Numerical simulation of evolution-induced catastrophe

A numerical simulation of damage evolution in a two-dimensional system of micocracks is presented. It reveals that the failure is induced by a cascade of coalescences of microcracks, and the fracture surface appears fractal. A model of evolution-induced catastrophe is introduced. The fractal dimension is found to be a function of evolution rule only. This result could qualitatively explain the correlation of fractal dimension and fracture toughness discovered in experiments.

三维直叶栅非定常流动的并行计算研究

流动分离直接关系到压气机运行的安全性与效率，对分离流动的研究是叶轮机械真实流动研究中的一个重大课题。本文针对三维压气机单转子叶片中截面所构成的三维直叶栅跨音速分离流开发了通用数值计算程序。该程序基于B-L湍流模型及高精度差分方法。多种工况的数值计算显示本程序结果与实验值吻合比较理想，验证了程序的正确性。10°攻角下分离区脉动压力的频谱与实验结果的数量级吻合，说明本程序能够较好地模拟大攻角分离流这种非定常复杂流动，为了提高计算规模及计算速度，作者对程序进行了并行化并针对微机机群系统进行了并行优化，实际计算表明本程序具有较高的并行效率。

Catastrophic Rupture Induced Damage Coalescence in Heterogeneous Brittle Media

In heterogeneous brittle media, the evolution of damage is strongly influenced by the multiscale coupling effect. To better understand this effect, we perform a detailed investigation of the damage evolution, with particular attention focused on the catastrophe transition. We use an adaptive multiscale finite-element model (MFEM) to simulate the damage evolution and the catastrophic failure of heterogeneous brittle media. Both plane stress and plane strain cases are investigated for a heterogeneous medium whose initial shear strength follows the Weibull distribution. Damage is induced through the application of the Coulomb failure criterion to each element, and the element mesh is refined where the failure criterion is met. We found that as damage accumulates, there is a stronger and stronger nonlinear increase in stress and the stress redistribution distance. The coupling of the dynamic stress redistribution and the heterogeneity at different scales result in an inverse cascade of damage cluster size, which represents rapid coalescence of damage at the catastrophe transition.

Laser-Induced Particle Jet And Its Ignition Application In Premixed Combustible Gases

A hot particle jet is induced as a laser pulse from a free oscillated Nd:YAG laser focused on a coal target. The particle jet successfully initiates combustion in a premixed combustible gas consisting of hydrogen, oxygen, and air. The experiment reveals that the ionization of the particle jet is enhanced during the laser pulse. This characteristic is attributed to the electron cascade process and the ionization of the particles or molecules of the target. The initial free electrons, which are ablated from the coal target, are accelerated by the laser pulse through the inverse Bremsstrahlung process and then collide with the neutrals in the jet, causing the latter to be ionized.

IL-8-induced L-selectin shedding regulates its binding kinetics to PSGL-1

L-selectin plays a crucial role in inflammation cascade by initiating the tethering and rolling of leukocytes on endothelium wall. While many L-selectin molecules are rapidly shed from the cell surface upon activation, the remaining membrane-anchored L-selectin may still play an important role in regulating leukocyte rolling and adhesion with different binding kinetics. Here we developed an in vitro model to activate Jurkat cells via interlukin-8 (IL-8) and quantified the two-dimensional (2D) binding kinetics, using a micropipette aspiration assay, of membrane-anchored L-selectin to P-selectin glycoprotein ligand 1 (PSGL-1) ligand coupled onto human red blood cells (RBCs). The data indicated that L-selectin shedding reduced the amount of membrane-anchored L-selectin and lowered both its reverse and forward rates. These results suggested that the rolling dynamics of activated leukocytes was determined by two opposite impacts: reducing the surface presentation would enhance the rolling but lowering the kinetic rates would decrease the rolling. This finding provides a new insight into understanding how L-selectin shedding regulates leukocyte rolling and adhesion.

Two mechanisms for population inversion via two-colour coherent excitation in three-level systems

We analyse the physical origin of population inversion via continuous wave two-colour coherent excitation in three-level systems by dressing the inverted transition. Two different mechanisms are identified as being responsible for the population inversion. For V-configured systems and cascade (E) configured systems with inversion on the lower transition, the responsible mechanism is the selective trapping of dressed states, and the population inversion approaches the ideal value of 1. For Lambda-configured systems and Xi-configured systems with inversion on the upper transition, population inversion is based on the selective excitation of dressed states, with the population inversion tending towards 0.5. As the essential difference between these two mechanisms, the selective trapping of dressed states occurs in systems with strong decay into dressed states while the selective excitation appears in systems with strong decay out of dressed states.

Three-photon-excited upconversion luminescence of YVO4 single crystal by infrared femtosecond laser irradiation

We report on the upconversion luminescence of a pure YVO4 single crystal excited by an infrared femtosecond laser. The luminescent spectra show that the upconversion luminescence comes from the transitions from the lowest excited states T-3(1), T-3(2) to the ground state (1)A(1) of the VO43-. The dependence of the fluorescence intensity on the pump power density of laser indicates that the conversion of infrared irradiation to visible emission is dominated by three-photon excitation process. We suggest that the simultaneous absorption of three infrared photons promotes the VO43- to excited states, which quickly cascade down to lowest excited states, and radiatively relax to ground states, resulting in the broad characteristic fluorescence of VO43-. (c) 2005 Optical Society of America.

自由空间激光远距离传输的地面模拟研究

根据透镜的傅里叶变换性质，提出了采用光学傅里叶变换加级联光学成像放大并结合有限口径接收的方法来实现自由空间激光光束远距离传输的实验室模拟。由此原理设计了自由空间激光远距离传输模拟装置，该装置主要由大口径、长焦距的傅里叶变换平行光管和三级成像放大镜所组成，最大等效传输距离达2.4×10^5km，可用于星间激光通信终端综合通信性能的评估，在设定的误码率下测量终端可能的通信距离，或者在设定的作用通信距离下检测通信的误码率。

基于RS编码及网格编码调制的光PPM通信纠错技术

分析了RS（Reed-Solomon）纠错码和网格编码调制（Trellis Coded Modulation，TCM）技术在光脉冲位置调制通信中的应用．在此基础上，提出了以RS码作为外码，以网格编码调制技术作为内码，用于光脉冲位置调制通信的新编码方案，能以几乎不减少通信速率的优势，提高传统RS码系统在时变带限光信道中的通信性能．模拟研究了在不同空间光信道条件下，传统RS码的符号正确传输率和误码率，并对网格编码调制的编码增益、RS码与网格编码调制级联时的编码增益进行了仿真研究，证实了本方案的有效性．

三峡大坝的修建对库区动物的影响

世界上最大的大坝—三峡大坝―于2003年6月正式建成并蓄水，三峡水库的水位由蓄水前的84m上升至蓄水后的139m。三峡大坝的修建与蓄水，已经显著地改变了三峡库区陆地生态系统和水域生态系统的生态环境。三峡大坝的修建将如何影响生活在这一区域（包括陆地和水域）的野生动植物资源，是个亟待解决的问题。 对于陆地生态系统，三峡大坝的修建，使三峡库区的生态环境进一步破碎化（Fragmentation）和岛屿化（Island）。为了了解三峡大坝的修建对三峡库区野生动物所产生的影响，我们选择适应性强且对环境变化敏感的啮齿动物为研究对象，在三峡库区选取两个岛屿（洛碛岛和皇华岛）及其对岸为研究点，在野外调查了啮齿动物的群落组成、种群密度与分布，并用胃容物分析和稳定性同位素分析方法研究了两个研究点优势种啮齿动物的食物来源与组成。结果表明，两个研究点间啮齿动物群落组成有很大差异，这主要是由于啮齿动物分布的地域性差异引起的。在同一个研究点，岛屿上的啮齿动物群落组成与对岸的没有差异，但在岛屿上的啮齿动物种群密度却明显高于其对岸的，这说明随着库区水位的大幅度提升，岛屿的面积急剧减小，使原本栖息在水边的啮齿动物不得不向岛屿的中上部迁移，致使岛屿上的啮齿动物的种群密度迅速增加。啮齿动物的食物来源有四种：C3非豆科植物，C3豆科植物，C4植物及水生生物。不同种啮齿动物的食物种类组成也不同，四川短尾鼩（Anourosorex squamipes）的食物中有水生生物的组成成分，但其它啮齿动物食物中不含水生生物。不同食物来源在啮齿动物的整体食物中所占比例也不同，并呈明显的季节性变化，这种变化与田间的农作物种类密切相关。动物组织的稳定性同位素组成不仅可以示踪动物的食物来源，也反映了动物的生存状况。我们的结果显示：对岸啮齿动物的稳定性同位素组成相对集中，并且不同种啮齿动物间没有重叠。这一方面说明对岸啮齿动物的食物来源相对丰富，啮齿动物可以选择各自喜食食物，在此环境下的啮齿动物种间竞争相对较弱，至少是种间食物竞争较弱。岛屿上啮齿动物的稳定性同位素值分布范围远大于在对岸的，并且种间稳定性同位素组成有明显的重叠部分。稳定性同位素组成的重叠意味着有相似的食物来源，也即栖息生境相似。这说明岛上的啮齿动物种间存在较剧烈的食物竞争关系。在资源限制情况下，多种啮齿动物不得不利用共同的食物来源。换句话说，岛屿化过程将加剧岛屿上啮齿动物的种间竞争。 洛碛岛上的四川短尾鼩因为可以利用水生食物来源，这使得它在岛屿化过程中处于有利地位。随着岛屿化进程的加剧，四川短尾鼩在岛屿上的优势将更加明显。而皇华岛屿上的褐家鼠的食物来源单一，会因岛屿面积的进一步减小，食物来源更加缺乏，它们将不得不改其食物组成或面临消失。而白腹巨鼠的栖息环境靠近江边，将因水位的上升进一步上移，它们的栖息环境与普通田鼠的发生重叠，使得白腹巨鼠与普通田鼠发生栖息地的竞争与食物竞争，并面临更多的人为因素干扰。 对于水域生态系统，长江是我国最大的河流，也是我国淡水鱼类最丰富的区域。三峡大坝的修建已经显著改变了三峡库区的水文特征。为了了解三峡大坝的修建是否会改变三峡库区的有机物组成，并进而影响到三峡库区水生生物的食物来源和组成，我们选择了三个水文特征不同的研究点（洛碛江段、皇华城江段和茅坪江段）调查了三峡库区的常见鱼种类组成，并用稳定性同位素方法研究了三峡库区洪水前后的有机物组成变化与鱼类的食物网模型，用稳定性同位素划分了鱼类食物网结构及鱼类体长与其肌肉δ15N值间的关系。结果表明，三峡大坝的修建，已经显著改变了皇华城和茅坪江段的水文特征，同时也“干扰”了生活在这一区域的鱼类。适应于流水环境的鱼类在库区回水处和大坝附近几乎消失，而喜欢静态环境的其它鱼类却得到极大的发展，例如鲢鱼（H. molitrix）和草鱼（ C. idellus）。 洪水前后，三峡库区的有机物组成成分有明显改变。洪水前，水中有机物主要以河流自身生产力产生的有机物为主（浮游植物、藻类等）;洪水后，水中有机物主要以外来有机物为主（陆地植物、土壤有机物和从上游带来的有机物）。对于三个食物网模型：河流连续体模型（RCC）、脉冲模型（FPC）和河流生产力模型（RPM），河流生产力模型能更好的解释三峡库区的水生生物的食物来源，即三峡库区的水生生物的食物主要来源于河流自身生产力产生的有机物。但外来有机物作为水生生物的一种辅助食物来源，在洪水期间起到不可忽视的作用。 在本研究中，鱼类体长与其δ15N值间的关系与选取鱼的种类有关，比如南方大口鲇（Silurus asotus）的体长与其肌肉的δ15N值呈明显负相关关系（R2≈0.5），而鱖鱼（Siniperca）、铜鱼（Coreius guichenoti）和草鱼（Ctenopharyngodon idellus）的体长与其肌肉的δ15N值却呈正相关关系。 三峡库区常见鱼类主要以杂食性和广普性鱼类为主，食物网结构十分复杂。通过胃容物分析和稳定性同位素分析，三峡库区水生生物营养级间的判别值约为3.1‰。依据鱼类肌肉的δ15N值，三峡库区的常见鱼类可以划分为四个营养级：草食性鱼类（herbivorous fish）、初级杂食性鱼类（1ºomnivorous fish）、次级杂食性鱼类（2ºomnivorous fish）和食鱼性鱼类（piscivorous fish）。营养级间没有明确的分界限，鱼类的营养级从2到4.8，是连续分布的营养级结构，从而更真实的反映了自然界中动物的捕食关系和在食物网中的位置。 自然丰度变化的稳定性同位素(Stable isotope)作为一种天然的示踪物，在动物生态学上已经得到广泛应用。动物的稳定性同位素可以清楚的示踪动物的食物来源、食物组成、栖息地情况和生存状况等多种信息，结合传统的胃容物分析，或其它的粪便分析、储藏物分析等，稳定性同位素技术在动物生态学研究方面必将得到更广泛的应用。

西藏湍流蛙类一新种

在西藏墨脱采集到1种与Odorrana graminea、O.chloronota和O.livida体背绿色和指端膨大具沟的湍流蛙类相近的物种,经鉴定为新种,命名为墨脱臭蛙Odorrana zhaoi.新种具有以下主要特征:上唇缘具有金黄色条纹;鼓膜圆形,显著,TD:ED为0.56;背侧褶弱,背部皮肤除后部有小疣粒外光滑,体侧有疣粒和小刺;4肢背面有横斑纹;无外蹠突;雄性具1对外声囊和咽胸部椭圆形小刺团,无肱腺.