用异核多维NMR技术研究蛋白质动力学

蛋白质在溶液中的三维空间结构、动力学与蛋白质生物功能的关系是在分子水平上理解生命现象的重要基础. NMR技术在研究蛋白质动力学方面具有独特的优势,所能表征的运动过程相关时间尺度很广. 文章综述了异核多维NMR技术研究蛋白质动力学的实验技术和理论方法,介绍了描述蛋白质动力学的内运动参量的意义和Model-Free 方法,并举例说明¹⁵N弛豫测量实验被用于研究蛋白质及其与配体复合物的动力学.      

高温高压下电解质溶液谱学研究的进展

高温高压下电解质溶液研究在理论和工业应用上都具有重要的意义，拉曼光谱、红外光谱、紫外可见光谱、中子和X射线衍射、以及X射线吸收精细结构方法都已经用于它的研究。随着温度升高，溶液的结构发生了变化，离子的缔合度增加，内层配位水的数目减少，出现了离子的多核簇组成。除了静态结构的研究外，也用拉曼光谱进行溶液的动力学探讨。水热金刚石压腔装置是高温高压电解质溶液研究的一个重要的进步，在水热金刚石压腔装置中，拉曼光谱和X射线吸收精细结构两种方法具有重要的应用潜力。     

先进固体NMR技术研究高分子结构与动力学

随着固体NMR理论和谱仪硬件技术的不断发展,近年来固体NMR技术在高分子多尺度结构与动力学研究领域中正发挥着越来越重要的作用. 多脉冲及高速魔角旋转（MAS）等质子高分辨技术的发展使得高灵敏度的¹H谱可有效地用于高分子化学结构与链间相互作用的检测;基于化学键（J-耦合）相关和通过空间（偶极耦合）相互作用的各种二维异核相关谱NMR新技术,使得复杂高分子的链结构得以严格解析. 基于MAS下同核和异核偶极-偶极相互作用、化学位移各向异性等各向异性相互作用重聚的系列新技术,使得研究者可在采用高分辨¹H或¹³C 检测信号的同时检测准静态下的各向异性相互作用,进而获得与之密切相关的结构和动力学信息. 通过质子偶极滤波技术可有效检测多相聚合物中的界面相与相区尺寸、高分子共混物中的相容性等问题. 在动力学的研究中,通过质子间自旋扩散的有效压制技术和化学位移各向异性的重聚,目前已经可以有效地获取链段上单个化学键的快速局域运动以及链段的超慢分子运动. 上述丰富的多尺度NMR技术可以使研究者在不同空间和时间尺度上对高分子聚合物的微观结构、相分离和动力学行为等进行详细的研究,进而阐明高分子微观结构与宏观性能的关联. 该文以固体NMR中最主要的2类核（¹H和¹³C）的检测技术为主线,简单介绍近年来固体NMR领域的一些最新研究进展及其在高分子结构和动力学研究中的应用.     

超临界水流化床反应器加料方式优化的数值模拟

超临界水流化床反应器是应用于生物质气化制氢工程中的一种新型反应器,它避免了以往管式反应器容易出现结渣、堵管的不足,而且还具有床内升温迅速、生物质气化充分以及产氢率高等优点。由于采用超临界水作为流化工质,传统流化床的设计方法已经不能满足设计要求。本文基于欧拉双流体模型对超临界水流化床加料方式进行了数值模拟研究,建立了最优加料口结构评价方法,分析了加料口结构、加料速度比以及初始床层高度对超临界水流化床颗粒分布的影响。本文研究进一步完善了超临界水流化床的设计理论。     

脱氧核糖核酸的核磁共振研究简介

从NMR谱峰归属,NMR约束及结构计算,NMR谱的主要特征,化学位移的环流效应,以及溶液动力学等几个方面简要地介绍了核磁共振在DNA结构和动力学研究中的应用. 指出了DNA的NMR研究中存在着的一些问题. 这些问题主要包括：分子力场中的二面角参数不符合非规整DNA结构中的许多二面角;用分子模拟软件重建DNA结构时没有很好的重现性;DNA的化学位移环流效应理论尚未成熟;“无模型(model-free)”模型在定量解释异核稳态NOE时不理想. 然而,NMR所提供的结构和动力学信息仍然是十分重要的并且极大地深化了人们对DNA的了解.     

超临界水流化床内煤气化过程建模与仿真(2):气化反应动力学模型及气化规律

本文成功建立了煤超临界水气化动力学模型,其中包括煤在超临界水中热解、液化、固相残碳及液化产物的蒸汽重整等均相和非相反应。该动力学模型能准确反应煤在超临界水中气化特征。在前述第一部分工作的基础上,耦合该气化动力学模型,对煤在超临界水流化床中气化过程进行了建模。通过该模型研究了宽温度参数范围下反应器内典型反应速率、反应组分分布演变规律,揭示了反应器内部化学反应特征与气化规律。研究加深了对超临界水流化床内煤气化过程的认识,对超临界水流化床反应器的优化、放大以及实际运行具有指导意义。     

高聚物微观结构的固体NMR研究

核磁共振波谱是研究高聚物结构和动力学的有效手段,特别是固体高分辨NMR实验方法的不断发展及谱仪技术的进步,使这方面的研究不断深入. 文中简述了若干固体高分辨NMR技术在固态高聚物结构研究中的应用和重要进展. 部分实验工作在Varian UNITYplus 400MHz NMR谱仪的固体单元上完成.     

物质动态结构的非弹性散射研究

在静态结构的基础上，考虑原子的振动，分子键的振动、转动和振－振运动的结构称为动态结构，它是用非弹性散射方法来测定的。本文评述研究动态结构的实验方法：中子非弹性散射，非弹性X射线散射、核非弹性散射和Raman散射以及红外吸收谱。简单介绍了声子散射理论基础，继后，分三节描述了用这些方法来研究动态结构的若干结果。1）结晶物质，包括晶内，表面和界面，高纯近完整晶体中杂质引起的，多晶中、薄膜和纳米晶体中的点阵动力学研究；2）非晶物质，包括非晶固体，高聚合物，生物大分子，准晶和液体的动态结构研究；3）高温超导体的点阵动力学研究，文末给出了小结和最后评论。     

反式聚乙炔的极化子静态性质研究

在Su-Schrieffer-Heeger（SSH）模型基础上,考虑长程电子关联的影响,研究反式聚乙炔分子链中空穴极化子与电子极化子的稳态性质。比较不同关联强度下两类极化子的晶格位形、能级、电荷密度波、以及吸收谱线的变化。加深对分子链中极化子的认识,该理论方法也可推广到高分子链中其它元激发的静态性质研究中,为不同类型元激发动力学性质研究奠定基础。     

基于周期性边界条件的分块量子化学方法对液态水的从头算分子动力学模拟（英文）

本文利用分块量子化学方法,实现了在周期性边界条件下应用二阶微扰(MP2)理论对液态水的从头算分子动力学模拟.通过采用aug-cc-pVDZ基组,MP2理论可以精确地描述水分子之间的相互作用势能面,因而在描述水的各项理化性质方面,MP2有望提供比密度泛函理论更加精确的结果.本研究计算了多种水的结构及动力学性质,包括径向分布函数,扩散系数,偶极矩,三个临近氧原子的角度分布,氢键结构,都得到了与实验观测一致的结果.因此,周期性边界量子分块方法可以作为一种研究水的物质结构的可靠理论方法,并且有望促进水科学领域争议性问题的解决.同时,该方法具有普适性和可扩展性,为有效应用高精度量子化学从头算方法计算其他凝聚态体系提供了理论框架.     

Structure of supercritical water: The concept of critical isotherm as a percolation threshold

Computer simulations are used to study the rearrangements of hydrogen-bonded structures of water upon transition to the supercritical state. It is shown that the destruction of the infinite hydrogen-bonded cluster, i.e., crossing the percolation threshold occurs in the subcritical region and that, at the critical temperature, structural variations reach the point where the fluid acquires the properties of a system with two types of ordering. The existence of tetrahedral clusters in supercritical water is confirmed only at high pressure.     

Structure and dynamics of water: from ambient to supercritical

Our recent works on supercritical water are reviewed. In order to elucidate the hydrogen bonding state of supercritical water, the proton chemical shift of the water proton is measured at temperatures up to 400 °C and densities of 0.19, 0.29, 0.41, 0.49, and 0.60 g/cm³. The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. The chemical shift is then related to the average number of hydrogen bonds in which a water molecule is involved. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one for a water molecule at the densities larger than the critical. The density dependence of the chemical shift at supercritical temperatures is analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities. The dipole moment of water at supercritical states is also estimated from the number of hydrogen bonds. The dynamical counterpart of our structural study of supercritical water has been performed by NMR relaxation measurements. Using D₂O, we measured the spin-lattice relaxation time and determined the reorientational relaxation time as a function of the water density and temperature. It is then found that while the reorientational relaxation time decreases rapidly with the temperature in the subcritical condition, it is a weak function of the density in the supercritical conditions.     

Conversion of veratrole and sodium lignosulfonate in the sub- and supercritical water

The conversion of veratrole and sodium lignosulfonate (SLS) in sub-and supercritical water at temperatures of 200–430°C, pressure of 220 atm, and residence time in the reactor of 2.5–10 min is studied. The soluble products of SLS depolymerization are analyzed by HPLC and GC-MS. Thermogravimetric, elemental, IR, and ¹H NMR analyses of the insoluble products of the conversion of SLS in supercritical water (SCW) are performed. It is demonstrated that the methoxyaryl ether bonds and aromatic structures do not undergo destruction in SCW. The formation of the insoluble precipitate is explained by decarboxylation and desulfonation of lignin polymer fragments.     

Hydrogen bonding and dipole moment of water at supercritical conditions: a first-principles molecular dynamics study

We present a first-principles molecular dynamics study of water near and above the critical point ( T = 647 K, rho = 0.32 g/cm(3)). We find that the systems undergo fast dynamics with continuous formation and breaking of H bonds. At low density, the system fragments mostly into trimers, dimers, and single molecules. At a higher density, more complex structures appear and an extended, albeit very dynamical, H-bond network can be identified. These structures have important consequences for the screening properties of the system. This offers a clue to understanding the peculiar chemical behavior of a supercritical system and allows thermodynamical tuning of its solvent properties.     

Structure of supercritical water

This review paper gives the more up to date information on the structure of supercritical water, at different thermodynamic states, for densities ranging from 0.2 to 0.7 g.cm⁻³, studied by neutron scattering. The experimental partial pair correlation functions g_OH(r), G_HH(r) and g_OO(r) are compared with the results of molecular dynamics simulations using the (SPCE) model potential for water. The results confirm that hydrogen bonding is still present in dense supercritical water and are in good agreement with data obtained by NMR and Raman spectroscopies.     

Structured water chains in external electric fields

ABSTRACT

We study the structural, energetic and electronic properties of the structured water chain clusters within the density functional theory. We refer the structured water chains to those water clusters that have specific geometric patterns stretched along one direction. External electric field required to keep the structures open chain, thereby preventing them to form closed structures, is applied along the length of the chain. The structures are essentially periodic with basic repeating unit consisting of the corner- or edge-sharing 4-, 5- or 6-membered ring water clusters. Our calculations underscore the possible existence of such structured water clusters in the electrostatic environments, which we simulate in its simplicity employing a dipolar, uniform and static electric field. Analysis reveals that the 5-membered ring water chain clusters, i.e. the pentamer chain clusters have the lowest average dipole moment per water molecule while the threshold field, that marks the onset of the field-induced closure of the HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) energy gap, is highest, followed by that for the tetramer and hexamer chains. The results suggest that the pentamer chains are the most stable clusters over a wide range of electric fields.     

Correlation of porous and functional properties of food materials by NMR relaxometry and multivariate analysis

The porous properties of food materials are known to determine important macroscopic parameters such as water-holding capacity and texture. In conventional approaches, understanding is built from a long process of establishing macrostructure-property relations in a rational manner. Only recently, multivariate approaches were introduced for the same purpose. The model systems used here are oil-in-water emulsions, stabilised by protein, and form complex structures, consisting of fat droplets dispersed in a porous protein phase. NMR time-domain decay curves were recorded for emulsions with varied levels of fat, protein and water. Hardness, dry matter content and water drainage were determined by classical means and analysed for correlation with the NMR data with multivariate techniques. Partial least squares can calibrate and predict these properties directly from the continuous NMR exponential decays and yields regression coefficients higher than 82%. However, the calibration coefficients themselves belong to the continuous exponential domain and do little to explain the connection between NMR data and emulsion properties. Transformation of the NMR decays into a discreet domain with non-negative least squares permits the use of multilinear regression (MLR) on the resulting amplitudes as predictors and hardness or water drainage as responses. The MLR coefficients show that hardness is highly correlated with the components that have T2 distributions of about 20 and 200 ms whereas water drainage is correlated with components that have T2 distributions around 400 and 1800 ms. These T2 distributions very likely correlate with water populations present in pores with different sizes and/or wall mobility. The results for the emulsions studied demonstrate that NMR time-domain decays can be employed to predict properties and to provide insight in the underlying microstructural features.     

Diffusion exchange NMR spectroscopy in inhomogeneous magnetic fields

Two-dimensional diffusion exchange experiments in the presence of a strong, static magnetic field gradient are presented. The experiments are performed in the stray field of a single sided NMR sensor with a proton Larmor frequency of 11.7 MHz. As a consequence of the strong and static magnetic field gradient the magnetization has contributions from different coherence pathways. In order to select the desired coherence pathways, a suitable phase cycling scheme is introduced. The pulse sequence is applied to study diffusion as well as the molecular exchange properties of organic solvents embedded in a mesoporous matrix consisting of a sieve of zeolites with a pore size of 0.8 nm and grain size of 2 μm. This pulse sequence extends the possibilities of the study of transport properties in porous media, with satisfying sensitivity in measurement times of a few hours, in a new generation of relatively inexpensive low-field NMR mobile devices.     

Study on corrosion properties of pipelines in simulated produced water saturated with supercritical CO2

This work aims at systematically investigating the corrosion properties of three pipeline steels in static simulated produced water (SPW) saturated with supercritical carbon dioxide using weight-loss tests. SEM, XRD and XPS were employed to study the chemical composition and structure of the corroded surface. The results showed that the corrosion rates of the tested steels significantly decreased with increasing the exposure temperature and time in static SPW saturated with SC-CO₂. The surface film on the corroded surface, which markedly influenced the CO₂ corrosion behavior of the samples, was mainly composed of (Fe, Ca)CO₃ and α-FeOOH. Inhomogeneous element distribution of carbon, oxygen, calcium and iron in the surface film was observed. (Fe, Ca)CO₃ formed at a lower temperature was more stable than that formed at elevated temperatures.     

Role of Supercritical Water and Pyrite in Transformations of Propylene

The effect of supercritical water and pyrite on the transformations of propylene upon the uniform heating (1.5 K/min) of reagents to 718 K is studied. The products are analyzed by IR and ¹H NMR spectroscopy, mass-spectrometry and gas chromatography/mass-spectrometry. It is established based on the temperature dependences of the pressure of reagents that the addition of pyrite in the absence of water gives rise to a decrease in the starting temperature of propylene oligomerization. In the absence of pyrite, the addition of water suppresses propylene oligomerization. A synergetic effect of supercritical water and pyrite on the degree of conversion of propylene is revealed. It is shown that hydrogen sulfide, thiols, and methyl-derivatives of thiophene are formed in the presence of pyrite, as well as the yield of aromatic and polyaromatic hydrocarbons increases. The mechanisms of the observed processes are discussed.