半刚性高分子链塌缩相变的蒙特卡洛模拟

本文主要研究半刚性高分子链在不同温度下的形态变化,特别是在低温下的塌缩相变。我们对半刚性高分子链在三维简立方点阵上进行蒙特卡洛数值模拟。计算模型中考虑了链段间的排斥势、近邻间的吸引势和局域刚性势。链的刚性程度由刚性势与吸引势的比值确定。计算证明在温度逐步下降时,柔性链的形态由高温下的无规线团逐渐收缩为低温下的无规紧缩线团;但刚性程度较高的链则首先扩展,然后再收缩为具有一定有序度的紧缩态。同时我们也证明半刚性高分子链的塌缩相变是与柔性链的形态转变相同,它们的热力学行为都符合二级相变的特征。     

端基附壁高分子模型链的塌缩转变

借MonteCarlo和模拟退火方法研究了端基附壁高分子尾形链在不同温度下的形态变化 ,链的模型采用有最近邻相互吸引作用的自避行走 .计算机实验结果表明 ,对于端基附壁的高分子尾形链 ,与自由链一样 ,当温度逐渐降低时高分子链会发生从松散的无规线团到紧密球体的塌缩转变 .计算机模拟得到了端基附壁链的均方末端距及其分量 ,均方回转半径及分布随温度的变化 .由于刚性壁的影响 ,使得有限链长的高分子尾形链与自由链相比 ,其表示链尺寸 温度关系的曲线要稍低 .模拟还发现 ,在高温时壁对链形状的影响比较大 ,壁垂直方向上尺寸明显大于平行方向的尺寸 ,后者接近于自由链的尺寸分量 .然而 ,低于θ温度时 ,尤其是完全塌缩之后 ,壁对链形状的影响已经很小 .     

高分子单链热力学量的Monte Carlo模拟

提出了一种Monte Carlo模拟方法,可以计算高分子链的热力学量.该方法采用简单立方格子上的自避行走链和紧邻相互作用模型,计算了链长200以内的链在无量纲相互作用参数△_ε/(?)T不同数值下的构象熵和Helmholtz自由能,其中△_ε为一次接触的能量,(?)为Boltzmann常数,T为绝对温度.模拟结果和直接计数法的准确结果显示出较好的一致性 并且得到排除体积和相互作用效应互相抵消的(?)条件下的参数△_ε/(?)T之值大约为-0.6且与链长无关.     

简立方格点自避行走所模拟的高分子链吸附和塌缩

采用复合Markov链法 ,针对简立方格点上的自避行走模型 ,研究了同时具有对壁的吸附作用ε1 和最近邻相互作用ε2 的高分子链的热力学性质 .相互作用能量参数ε1 和ε2 分别联系于参数α和 β .令链长N=10 0 ,由这种MonteCarlo方法可得出链的自由能FN(α ,β) ,热容2 FN(α ,β) /2 α和2 FN(α ,β) /2 β ,吸附点平均数〈m〉/N ,最近邻相互作用对平均数〈n〉/N和均方末端距对壁的垂直分量RZ2 .除已有方法由热容数据可绘出α β相图外 ,建议由结构参数〈m〉/N ,〈n〉/N和RZ2 绘制相图 ,并发现二者基本一致 .所得相图表明 ,存在 4个相区 ,分别是解吸 膨胀相 (DE) ,吸附 膨胀相 (AE) ,解吸 紧密相 (DC)和吸附 紧密相 (AC) .在伸展区和塌缩区 ,随着吸附作用的增强 ,会出现吸附相转变 .在解吸区和吸附区 ,随着自相互作用的增大 ,也将出现塌缩相转变 .相图出现了两个三相点 ,即AE AC DC三相点和AE DE DC三相点     

简立方格点自避行走所模拟的高分子链吸附和塌缩

采用复合Markov链法,针对简立方格点上的自避行走模型,研究了同时具有对壁的吸附作用ε1和最近邻相互作用ε2的高分子链的热力学性质.相互作用能量参数ε1和ε2分别联系于参数α和β.令链长N=100,由这种Monte Carlo方法可得出链的自由能FN (α ,β),热容2FN(α,β)/2α和2FN(α,β)/2β,吸附点平均数〈m〉/N,最近邻相互作用对平均数〈n〉/N和均方末端距对壁的垂直分量RZ2.除已有方法由热容数据可绘出α-β相图外,建议由结构参数〈m〉/N ,〈n〉/N和RZ2绘制相图,并发现二者基本一致.所得相图表明,存在4个相区,分别是解吸-膨胀相(DE),吸附-膨胀相(AE),解吸-紧密相(DC)和吸附 -紧密相(AC).在伸展区和塌缩区,随着吸附作用的增强,会出现吸附相转变.在解吸区和吸附区,随着自相互作用的增大,也将出现塌缩相转变.相图出现了两个三相点,即AE-AC-DC三相点和AE-DE-D C三相点.     

良溶剂中环形高分子单链的结构与动力学性质的模拟研究

本文采用多粒子碰撞动力学与分子动力学耦合的模拟方法研究了环形高分子单链在良溶剂中的静态与动态性质,并与线形分子进行了对比.研究发现,环形高分子链内粒子之间的平均距离小于线形链,即粒子排列得更加紧密;相应的均方回转半径也小于线形链,线形链与环形链的均方回转半径的比值为1.77;同时,环形链扩散的速度也比线形链快,两者比值为1.10.模拟结果揭示了扩散行为是排斥体积作用和流体力学相互作用耦合的结果,在扩散过程中,流体力学相互作用消减了排斥体积作用对扩散行为的贡献.此外,通过对有和没有流体力学相互作用的多粒子碰撞动力学得到的结果作对比,研究了流体力学相互作用对高分子静态和动态行为的影响,结果表明,流体力学相互作用使高分子链在极稀溶液中的扩散速度变快.     

金纳米球热力学稳定性的理论研究

Due to the significant importance in many applications,the thermal stability of hollow gold nano-particles has been studied theoretically by using ultra-large molecular dynamics simulation. By changing the nano-particle size while keeping the aspect ratio constant,it was found that the large gold nano-particle has better thermal stability than the small ones.     

GSH特异的人单链抗体计算机辅助结构研究

以还原型谷胱甘肽 (GSH)的两种衍物作为半抗原 ,从半人工合成的人单链抗体库中筛选得到了多株特异结合的单链抗体 .从中选择亲合力最强的单链抗体进行了基因的重组和测序 .根据由核苷酸测定结果而推导的氨基酸残基序列 ,用计算机模拟分析了两株单链抗体的空间结构 .结果显示单链抗体以二聚体形式存在 ,与单链抗体蛋白SDS_PAGE电泳结果相一致 .抗原结合部位———重链CDR3区位抗体的表面 ,且形成一个凹腔 ,因此可推测位于CDR3区的丝氨酸最可能参与硒化反应     

不同标态下标准电极电势间关系的热力学解释

利用标准吉布斯自由能变化ΔrGm与标准电极电势和标准反应平衡常数间的联系,分析不同标态下电极标准电极电势间的关系。根据这一思想,以水解离平衡为例,讨论了设计原电池求化学反应平衡常数的一般方法。     

高分子玻璃化转变过程的分子动力学模拟本科教学实验

玻璃化转变是高分子专业教学中的重要内容,转变过程中自由体积的变化和分子链段的运动是理解玻璃化转变的难点。在计算机上使用分子模拟的方法得到三维的可视化图形,可以更直观地观察到自由体积和分子链的变化。提炼近年分子模拟技术在玻璃化转变研究中的最新科研成果,得到既有理论基础又适合本科教学的课程素材。分子模拟可以实现了仪器测试无法达到的超快速升降温,并与仪器测试得到的实验结果相对照,验证了普通实验很难验证的理论观点,从而拓展了本科实验的范围,达到了很好的教学效果。     

On the Nature of the Temperature-Induced Transition From the Molten Globule to the Unfolded State of Globular Proteins

In this paper we try to perform a thermodynamic analysis of the temperature-induced transition from the molten globule to the unfolded state of globular proteins. A series of calorimetric investigations showed that this process is not associated with an excess heat capacity absorption peak, and cannot be regarded as a first-order phase transition. This result contrasts with the well-established conclusion that the thermal unfolding of the native tertiary structure of globular proteins is a first-order phase transition. First, the theoretical approach developed by Ikegami is outlined to emphasize that a second-order or gradual transition induced by temperature is expected for globular proteins when the various secondary structure elements do not interact cooperatively. Secondly, a simple thermodynamic model is presented which, taking into account the independence of the secondary structure elements among each other, is able to rationalize the shape of the experimental DSC profiles.This revised version was published online in November 2005 with corrections to the Cover Date.     

“活性中间体”与“过渡态”

活性中间体和过渡态是两个不同的概念。在许多有机化学反应中,从反应物到产物往往不是一步完成,而是经历若干步骤。在这些多步骤反应中会形成一些极不稳定的反应中间体。例如,烷烃卤代反应过程中生成的碳游离基,烯烃与卤化氢加成过程中生成的碳正离子以及芳烃亲电取代反应中生成的σ络合物—碳正离子中间体等等都是非常活泼的中间体,它们一般称为有机活性中间体。有机活性中间体除碳游离基、碳正离子外,还有碳负离子、碳烯、苯炔等等。这些活性中间体都有着共同的特征:它们都非常活泼,生成后立即与其它物质反应,存在时间很短暂。     

高聚物从高弹态到流体态的转变

将高聚物由高弹态转变为流体态的转变温度命名为流动温度Tf,该转变温度与高聚物分子量密切相关.在高聚物从高弹态转变为流体态的研究中,由于T1.1的概念忽视对高聚物分子量的依赖性,因此采用Tf的概念更为合理.本文对高聚物的流动温度Tf的讨论涉及高聚物温度-形变曲线、高弹态温区、高聚物熔体剪切粘度.从高聚物凝聚态观点来看,高聚物熔体中凝聚缠结网络中的凝聚结点是分子链的局部向列相互作用使链单元间产生平行凝聚而形成的,而高聚物从高弹态到流体态的转变正是反映了高聚物熔体中凝聚缠结网络的物理交联点,即凝聚结点状态的变化.高聚物熔体可以流动,说明熔体中凝聚网络中的凝聚结点至少是可以在瞬间内打开的,升温使凝聚结点的解凝聚状态存在的时间加长,凝聚状态存在时间减短,当升高到某一温度时,在凝聚结点解凝聚状态的时间内,分子链通过内旋转使质量中心在外加力的方向上可以发生位移,此时高聚物从高弹态转变为流体态,而此时的温度就是流动温度Tf.对高聚物流体弛豫网络的研究,是一个很有前景的研究课题.     

The Molten Globule State of a Globular Protein in a Cell Is More or Less Frequent Case Rather than an Exception

Quite a long time ago, Oleg B. Ptitsyn put forward a hypothesis about the possible functional significance of the molten globule (MG) state for the functioning of proteins. MG is an intermediate between the unfolded and the native state of a protein. Its experimental detection and investigation in a cell are extremely difficult. In the last decades, intensive studies have demonstrated that the MG-like state of some globular proteins arises from either their modifications or interactions with protein partners or other cell components. This review summarizes such reports. In many cases, MG was evidenced to be functionally important. Thus, the MG state is quite common for functional cellular proteins. This supports Ptitsyn’s hypothesis that some globular proteins may switch between two active states, rigid (N) and soft (MG), to work in solution or interact with partners.     

Theories of the Thermodynamic Properties of Pure Molten Salts

Abstract

The analytical theories may be classified into (a) the older, semi-empirical theories, in which the approximations are physically motivated and rather unsystematic; typical of these are the so-called Significant Structures and Hole theories; (b) the newer theories, which we will, rather arbitrarily, refer to as the fundamental theories; here the approximations are of a more mathematical character and more systematically introduced; the perturbation and variational theories exemplify this approach. In addition, there are the Corresponding States methods, which occupy an intermediate position, in that, while having some fundamental justification, they are generally applied in an empirical manner.     

High Glass‐Transition Temperature Polymer Networks Harnessing the Dynamic Ring Opening of Pinacol Boronates

Differential scanning calorimetry of high molar mass poly(4‐vinylphenylboronic acid, pinacol ester)s evidenced unusual reactive events above 120 °C, resulting in a high glass‐transition temperature of 220 °C. A reversible ring‐opening reactivity of pinacol boronates is proposed, involving a nucleophilic attack on the sp² boron and subsequent bridging between boron atoms by interconnected pinacol moieties to form a densely crosslinked network with high T_g. FTIR, solid‐state NMR investigations, and rheology studies on the polymer as well as double‐tagging analyses on molecular model structures and theoretical calculations further support this hypothesis and indicate a ring‐opening inducing crosslinking. When diluted in an apolar solvent such as toluene, the polymer network can be resolubilized via ring closing, thus recovering the entropically favored linear chains featuring cyclic boronate esters.     

可控相转变温度热敏高分子的制备及其在免疫分析中的应用

合成了一种新型的快速响应热敏高分子聚N－异丙基丙烯酰胺－丙烯酰胺[P(NIP-co-AA)]，通过改变丙烯酰胺的含量可以改变高分子的临界溶解温度（LCST），使之用于不同用途。其中，将相转变温度（Ttr)在37℃的热敏高分子用于免疫分析的载体，建立了夹心型荧光免疫分析兔IgG的新方法。与聚N－异丙基丙烯酰胺（PNIP）作载体相比，两灵敏度相当，但由于相转变温度的提高，使得免疫反应的温度更接近于生物体的生理环境，并使免疫反应速率得到提高。该方法线性范围为0-1000μg/L；检出限为10μg/L。用于兔血清中兔IgG的测量，结果令人满意。     

Simulation Study on the Coil‐Globule Transition and Surface Adsorption of HP Chains

The coil‐globule transition of short hydrophobic‐polar (HP) chains, composed of 24 hydrophilic monomers and 24 polar monomers, in solution and on hydrophobic surface and the adsorption of the HP chain on hydrophobic surface are simulated. The coil‐globule transition point of the HP chain is dependent on sequence of chain but is roughly independent of the surface adsorption strength. Whereas the critical adsorption point of the HP chain is roughly independent of sequence. In addition, the lowest energy states can be obtained for the HP chain in solution or on surface by Monte Carlo simulated annealing method. Results show that the statistical conformation is strongly dependent on the intrachain H‐H attraction strength and the surface adsorption strength.

     

Thermodynamic Aspects of the Vaterite-Calcite Phase Transition

Although vaterite is the least stable anhydrous calcium carbonate polymorph, it is formed as a metastable phase in some normal and pathological biomineralisation processes. In this work, thermodynamic aspects of the vaterite-calcite phase transition were comprehensively studied. Vaterite samples were prepared by different methods and characterised for the composition, crystal structure, specific surface and grain size. All products were identified to be pure vaterite by careful X-ray diffraction measurements. The enthalpy and Gibbs energy of transition were determined by precise calorimetric and potentiometric measurements. The reliability of the thermodynamic data for the vaterite-calcite phase transition derived from this work was shown by the use of different calorimetric methods to determine the enthalpy of transition and the independent measurements of heat capacity and entropy of vaterite. Our recommended values are Δ_trs G*=−2.9±0.2 kJ mol⁻¹ , Δ _trs H *=−3.4±0.2 kJ mol−1 and Δ _trs S *=−1.7±0.9 J K⁻¹ mol⁻¹ , where the uncertainties are given as twice the standard deviations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Unexpected Intermediate State for the Cylinder‐to‐Gyroid Transition in a Block Copolymer Solution

The cylinder‐to‐gyroid transition in a concentrated solution of polystyrene‐block‐polyisoprene in dibutyl phthalate has been studied using rheology and small angle X‐ray scattering. Following an appropriate temperature quench, the oriented cylinder phase transforms to the gyroid structure epitaxially. Remarkably, an intermediate state appears for a deep quench, whereas for a shallow quench the transition proceeds directly; the intermediate state exhibits scattering signatures consistent with a hexagonally perforated layer structure.