核磁共振中多量子相干扩散行为的理论表述和计算机模拟

改进了Warren所提出的CRAZED脉冲序列以研究分子间多量子相干的扩散过程 ,讨论了利用核磁共振测量分子内和分子间多量子相干表观自扩散系数的理论表述 ,采用粒子的随机行走模型模拟其扩散行为 .在短脉冲近似和长脉冲梯度场两种实验条件下 ,分别获得了因扩散引起的不同相干阶数的相对信号衰减强度随梯度场脉冲间隔时间的变化曲线 ,由此得到分子内多量子相干和分子间多量子相干的表观扩散率与溶液分子扩散系数的关系 .还将计算机模拟结果与理论预测进行分析和比较 ,发现二者能很好地吻合 .研究结果表明 ,分子间多量子相干的表观扩散率与常规的分子内多量子相干的表观扩散率明显不同 ,因此 ,分子间多量子相干的表观扩散率可能提供一种新的核磁共振成像的对比度机理     

液体核磁共振中由分子间偶极耦合和分子内标量耦合共同作用后的谱图裂分模式

在高极化多自旋液体样品中,同时存在着分子间偶极（D）耦合和分子内标量（J）耦合,它们的共同作用产生了一些原来观测不到的分子间多量子相干信号。而且,信号的裂分模式与只存在J耦合的多自旋体系中观测到的多量子相干信号的裂分模式不同。本文从理论和实验上研究了这些禁阻的共振峰及其独特的裂分模式。为了比较验证,我们以I2S3+X自旋体系为例,结合使用选择和非选择性的射频脉冲序列来获得分子间双量子相干信号的五种裂分模式。进而归纳出对IpSq+Xk (p, q, k = 1, 2, 3,…)自旋体系普适的裂分模式规则。并指出,它们中如(1:0:-1)的裂分模式会放大J耦合裂分,使得J耦合常数的测量更精确,特别在J耦合常数很小或不均匀场中的J耦合常数的测量中具有诱人的应用前景。结果表明理论预测,计算机模拟和实验观测结果三者吻合的很好。     

基于碰撞感应激发产生的量子相干效应研究

近年来，量子相干在诸如高灵敏高精度激光光谱和新型量子频标、分子运动和物理化学反应的操控、原子分子激发态结构信息、量子态绝热布居转移、量子计算和量子信息处理等方面具有广泛的应用而受到人们的关注。基于量子干涉效应而产生的量子相干可分为频域和时域内的两种：频域内的量子相干来源于两个或多个同时的不同跃迁路径间的干涉，时域内的量子相干来源于两个或多个具有一定延时的同一跃迁路径间的干涉。本报告对以往较少涉及的原子及原子——分子混合体系中基于非相干碰撞而产生的频域和时域内多能级系统量子相干效应进行研究，并着重探索量子相干产生的条件及其对外界条件的依赖关系等。     

基于分子间多量子相干的矢量场磁共振成像模拟

对基于分子间多量子相干的矢量场成像进行了系统的模拟研究. 模拟结果表明：这类成像能够对几十微米量级的异质结构进行成像,并较好地抑制来自均匀介质的信号. 即使不存在梯度磁场情况下,通过相位循环该方法亦可实现对异质结构的成像. 这将有助于开拓基于分子间多量子相干成像的应用领域,深化对其成像机制的理解.     

非对称自旋-轨道耦合系统的多体量子相干含时演化

本文以具有非对称性自旋相互作用的三体自旋系统为研究对象,重点研究了三体量子相干含时演化规律.采用精确量子对角化和基于量子主方程的数值模拟方法,讨论了三体量子系统中多种量子相干组分及其退相干.研究发现,量子相干组分的含时演化与整个系统的初态量子特性紧密相关.当初态为可分离纯态时,在较短时间内,非对称相互作用有利于增加多体量子相干度.这些量子相干度因受噪声影响而逐渐衰减.当初态为类Werner态时,量子相干度的分布满足加和性,即三体量子相干度等于所有两体量子相干度之和.自旋之间非对称相互作用和环境噪声都会引起三体量子相干度大于所有两体量子相干度之和.这些结论有助于多体量子资源的制备.     

极性分子摆动态的三体量子关联

极性分子具有较长的相干时间和较强的偶极-偶极相互作用,因此它被视为量子信息处理的有效量子载体.基于分子摆动态作为量子比特,研究了处于热平衡状态下三极性分子线性链系统的三体量子关联特性,分析了三体负性熵纠缠度、测量诱导扰动和三体量子失协随与电场强度、分子电偶极矩、分子转动常数、偶极-偶极相互作用和温度等参数有关的三个无量纲变量之间的变化关系.研究表明:在其他参数给定的情况下,随电场强度的增加,三体量子关联均变小;随偶极-偶极相互作用强度的增大,三体量子关联先增加到峰值再逐渐变小;温度越高,负性熵纠缠度和三体量子失协越小,但测量诱导扰动随温度的改变在电场强度和偶极-偶极相互作用影响下呈现不同的变化趋势.此外,通过调节电场强度、偶极-偶极相互作用和温度,可改变与操控极性分子摆动态的三体量子关联.     

量子测量问题的研究及应用

文章结合最近完成的量子测量实验（如用冷原子Ｂｒａｇｇ散射实现的“ｗｈｉｃｈ－ｗａｙ”实验和用Ｃ６０分子完成的小系统量子干涉实验）,比较系统地介绍了量子测量问题及其相关的基本概念和基本思想,如量子相干、量子退相干和量子纠缠,从理论和实验结合的角度,讨论了外部环境和内部运动怎样诱导量子退相干和量子耗散,由此探讨了“薛定谔猫佯谬”和“宏观物体空间局域化描述”的物理解释,最后,文章简单地描述了量子计算和量     

分子磁体与环境的纠缠和退相干

本文研究了双轴分子磁体在耗散环境中的相干量子隧穿,作为环境的声子库抑制了相干量子隧穿,从而引起分子磁体中薛定谔猫态的退相干. 而环境内部声子之间的相互作用会导致分子磁体与热库之间退耦合,于是对退相干有一定的抑制作用. 在绝热近似和非绝热近似下,借助于约化密度矩阵计算了超Ohmo耗散中分子磁体与环境之间的纠缠度,当纠缠达到最大时,相干隧穿被完全抑制.     

原子分子光子系统的耗散相互作用和退相干

原子分子系统与量子化的电磁场或光子模式耦合的系统是非相对论量子力学理论研究和实验研究的主要对象和模型. 现实系统必然与外界环境耦合，且即便原子隔绝较好、光学腔壁品质因子足够高，原子系统也不等价于少数几个能级构成的简单模型：它仍然有不为零的几率跃迁到不可控的能级空间、与原子相互作用的自由空间真空场的量子效应也必须考虑. 本文将结合开放量子系统理论的基本要素与原子光子的基本模型，对原子分子系统在电磁场中发生的耗散以及量子退相干过程做简单综述，并重点介绍描述量子系统退相干过程的主流理论工具——主方程.     

量子相干

量子相干不仅是量子力学中的一个基本概念,同时也是重要的量子信息处理的物理资源.随着基于资源理论框架的量子相干度量方案的提出,量子相干度的量化研究成为近年来人们关注的一个热点问题.量子相干作为一种物理资源也十分脆弱,极容易受到环境噪声的影响而产生退相干,因此开放系统中的量子相干演化和保持也是人们广泛关注的课题.另外,量子相干在量子多体系统、量子热动力学、量子生物学等领域也有着潜在的应用价值.本文介绍量子相干度量的资源理论框架和基于该框架定义的相对熵相干性、l1范数相干性、基于量子纠缠的相干性、基于凸顶结构的相干性和相干鲁棒性等量子相干度量函数,概述开放系统中量子相干演化的动力学行为、典型信道的量子相干产生和破坏能力以及量子相干的冻结等现象,同时例举量子相干在Deutsch-Jozsa算法、Grover算法以及量子多体系统相变问题研究等方面的重要应用.量子相干研究仍处于快速发展之中,期望本综述能为该领域的发展带来启示.     

NMR技术在蛋白质-蛋白质相互作用研究中的应用——泛素-蛋白水解酶体通路研究实例介绍

蛋白质-蛋白质相互作用在多种细胞内生理活动中发挥关键性作用,而蛋白质复合物结构信息的获得主要依赖于X-射线衍射技术和核磁共振技术2种主要技术手段的使用. 需要指出的是,虽然大部分蛋白质复合物的结构解析使用了X-射线衍射技术,然而在包括无法获得蛋白质复合物晶体、 蛋白质与蛋白质结合强度较弱以及蛋白质复合物系统具有复杂的动力学行为等几种情况下,核磁共振技术是可用于蛋白质复合物结构测定的唯一手段. 用于蛋白质-蛋白质相互作用研究的NMR技术主要有化学位移扰动分析、分子间NOE的检测、顺磁弛豫增强技术、残余偶极耦合检测技术等几种. 该文将结合这几种技术在泛素-蛋白水解酶体通路领域的应用实例对它们的工作原理以及可提供的信息做出总结介绍.     

Anisotropic formation mechanism and nanomechanics for the self-assembly process of cross-β peptides

Nanostructures self-assembled by cross-β peptides with ordered structures and advantageous mechanical properties have many potential applications in biomaterials and nanotechnologies. Quantifying the intra-and inter-molecular driving forces for peptide self-assembly at the atomistic level is essential for understanding the formation mechanism and nanomechanics of various morphologies of self-assembled peptides. We investigate the thermodynamics of the intra-and inter-sheet structure formations in the self-assembly process of cross-β peptide KIIIIK by means of steered molecular dynamics simulation combined with umbrella sampling. It is found that the mechanical properties of the intra-and inter-sheet structures are highly anisotropic with their intermolecular bond stiffness at the temperature of 300 K being 5.58 N/m and0.32 N/m, respectively. This mechanical anisotropy comes from the fact that the intra-sheet structure is stabilized by enthalpy but the inter-sheet structure is stabilized by entropy. Moreover, the formation process of KIIIIK intra-sheet structure is cooperatively driven by the van der Waals(VDW) interaction between the hydrophobic side chains and the electrostatic interaction between the hydrophilic backbones, but that of the inter-sheet structure is primarily driven by the VDW interaction between the hydrophobic side chains. Although only peptide KIIIIK is studied, the qualitative conclusions on the formation mechanism should also apply to other cross-β peptides.     

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.     

有机稀土铕的螯合物的发光和能量传递的研究

本文应用荧光光谱和瞬时时间分辨光谱研究了稀土铕有机螯合物,以及在二苯酮做为三线态敏化剂存在时,对能量的吸收、传递和发光的过程,探讨了有机配位体与稀土离子的能级、三线态敏化剂和配位体分子的性质对发光的影响。     

(1)H-(13)C INEPT MAS NMR correlation experiments with (1)H-(1)H mediated magnetization exchange to probe organization in lipid biomembranes

Two-dimensional (1)H-(13)C INEPT MAS NMR experiments utilizing a (1)H-(1)H magnetization exchange mixing period are presented for characterization of lipid systems. The introduction of the exchange period allows for structural information to be obtained via (1)H-(1)H dipolar couplings but with (13)C chemical shift resolution. It is shown that utilizing a RFDR recoupling sequence with short mixing times in place of the more standard NOE cross-relaxation for magnetization exchange during the mixing period allowed for the identification and separation of close (1)H-(1)H dipolar contacts versus longer-range inter-molecular (1)H-(1)H dipolar cross-relaxation. These 2D INEPT experiments were used to address both intra- and inter-molecular contacts in lipid and lipid/cholesterol mixtures.     

Frequency (250 MHz to 9.2 GHz) and viscosity dependence of electron spin relaxation of triarylmethyl radicals at room temperature

Electron spin relaxation times for four triarylmethyl (trityl) radicals at room temperature were measured by long-pulse saturation recovery, inversion recovery, and electron spin echo at 250 MHz, 1.5, 3.1, and 9.2 GHz in mixtures of water and glycerol. At 250 MHz T(1) is shorter than at X-band and more strongly dependent on viscosity. The enhanced relaxation at 250 MHz is attributed to modulation of electron-proton dipolar coupling by tumbling of the trityl radicals at rates that are comparable to the reciprocal of the resonance frequency. Deuteration of the solvent was used to distinguish relaxation due to solvent protons from the relaxation due to intra-molecular electron-proton interactions at 250 MHz. For trityl-CD(3), which contains no protons, modulation of dipolar interaction with solvent protons dominates T(1). For proton-containing radicals the relative importance of modulation of intra- and inter-molecular proton interactions varies with solution viscosity. The viscosity and frequency dependence of T(1) was modeled based on dipolar interaction with a defined number of protons at specified distances from the unpaired electron. At each of the frequencies examined T(2) decreases with increasing viscosity consistent with contributions from T(1) and from incomplete motional averaging of anisotropic hyperfine interaction.     

A simulation algorithm based on Bloch equations and product operator matrix: application to dipolar and scalar couplings

A product operator matrix is proposed to describe scalar couplings in liquid NMR. Combination of the product operator matrix and non-linear Bloch equations is employed to describe effects of chemical shift, translational diffusion, dipolar field, radiation damping, and relaxation in multiple spin systems with both scalar and dipolar couplings. A new simulation algorithm based on this approach is used to simulate NMR signals from dipolar field effects in the presence of scalar couplings. Several typical coupled spin systems with both intra-molecular scalar couplings and inter-molecular dipolar couplings are simulated. Monte Carlo methods are incorporated into simulations as well to analyze diffusion process in these complicated spin systems. The simulated results of diffusion and relaxation parameters and 2D NMR spectra are coincident with the experimental measurements, and agree with theoretical predictions as well. The simulation algorithm presented herein therefore provides a convenient means for designing pulse sequences and quantifying experimental results in complex coupled spin systems.     

Atomic refinement with correlated solid-state NMR restraints

The orientation data provided by solid-state NMR can provide a great deal of structural information about membrane proteins. The quality of the information provided is, however, somewhat degraded by sign degeneracies in measurements of the dipolar coupling tensor. This is reflected in the dipolar coupling penalty function used in atomic refinement, which is less capable of properly restraining atoms when dipolar sign degeneracies are present. In this report we generate simulated solid-state NMR data using a variety of procedures, including back-calculation from crystal structures of alpha-helical and beta-sheet membrane proteins. We demonstrate that a large fraction of the dipolar sign degeneracies are resolved if anisotropic dipolar coupling measurements are correlated with anisotropic chemical shift measurements, and that all sign degeneracies can be resolved if three data types are correlated. The advantages of correlating data are demonstrated with atomic refinement of two test membrane proteins. When refinement is performed using correlated dipolar couplings and chemical shifts, perturbed structures converge to conformations with a larger fraction of correct dipolar signs than when data are uncorrelated. In addition, the final structures are closer to the original unperturbed structures when correlated data are used in the refinement. Thus, refinement with correlated data leads to improved atomic structures. The software used to correlate dipolar coupling and chemical shift data and to set up energy functions and their derivatives for refinement, CNS-SS02, is available at our web site.     

Ordering in nematic liquid crystals from NMR cross-polarization studies

The measurement of dipolar couplings between nuclei is a convenient way of obtatining directly liquid crystalline ordering through NMR since the coupling is dependent on the average orientation of the dipolar vector in the magnetic field which also aligns the liquid crystal. However, measurement of the dipolar coupling between a pair of selected nuclei is beset with problems that require special solutions. In this article the use of cross polarization for measuring dipolar couplings in liquid crystals is illustrated. Transient oscillations observed during cross polarization provide the dipolar couplings between essentially isolated nearest neighbour spins which can be extracted for several sites simultaneously by employing two-dimensional NMR techniques. The use of the method for obtaining heteronuclear dipolar couplings and hence the order parameters of liquid crystals is presented. Several modifications to the basic experiment are considered and their utility illustrated. A method for obtaining proton-proton dipolar couplings, by utilizing cross polarization from the dipolar reservoir, is also presented.