HMQC-TOCSY技术在植物环肽和配糖体中的应用

核磁共振HMQC-TOCSY二维技术可在氢谱方向和碳谱方向分别得到独立的自旋系统内氢核和碳核的全相关信息.该技术对于分子内具有多个自旋系统的植物环肽和配糖体的结构解析尤为适用.本文以环肽annosquamosin A和三萜皂甙prostratoside A为例说明该技术在这两类化合物中的应用.     

二蕊荷莲豆环肽B的NMR应用研究

植物环肽的¹H 和¹³C NMR图谱, 由于各种氨基酸自旋系统质子和碳的化学位移非常接近，谱峰高度重叠，结构解析比较困难. 文中以二蕊荷莲豆环肽B为例讨论了
现代2D NMR新技术，在植物环肽结构解析中的应用. HMQC-TOCSY图谱在氢谱方向和碳谱方向分别提供每一个氨基酸自旋系统内的氢和除季碳外碳的全相关信息，从而将每个氨基酸残基的NMR信号相互区分开来；结合¹H-¹H COSY 和 HMQC或HSQC图谱，就可以准确归属每个氨基酸的氢和碳的化学位移. 氨基酸残基之间的连接顺序可用HMBC、NOESY或ROESY图谱获得.     

胸腺五肽的NMR研究

应用改进的DPFGSE 1D-TOCSY 和1D-NOESY 核磁共振方法测定胸腺五肽的氨基酸残基,从重叠的¹H谱中分离出各自旋系统内氢的亚谱信号;继而由DPFGSE 1D-NOE 技术测定肽链氨基酸残基的连接顺序. 结合二维相关谱对胸腺五肽的碳、氢信号做了全归属. 并进行了计算机分子模拟,结果与NOESY 实验相符.     

应用二维谱学技术归属泰妥拉唑的1H和13C NMR

应用DEPT（无畸变极化转移增强谱）、HMQC（异核多量子相干谱）、HMBC（异核多键相干谱）、¹H-¹H COSY（相关谱）、NOESY（核欧沃豪斯谱）和 TOCSY（全相关谱）谱学技术,对泰妥拉唑所有的氢进行了归属. 不对称硫原子引起邻接CH₂ 2个氢化学位移不等价;该2个氢相互偶合,其偶合常数为12.9 Hz;借助DEPT谱,2个甲氧基被区别;借助DEPT和TOCSY谱,另2个与碳相连的甲基被区分;利用HMBC谱,所有的碳峰可以归属.      

应用二维谱学技术归属泰妥拉唑的1H和13C NMR

应用DEPT（无畸变极化转移增强谱）、HMQC（异核多量子相干谱）、HMBC（异核多键相干谱）、¹H-¹H COSY（相关谱）、NOESY（核欧沃豪斯谱）和 TOCSY（全相关谱）谱学技术,对泰妥拉唑所有的氢进行了归属. 不对称硫原子引起邻接CH₂ 2个氢化学位移不等价;该2个氢相互偶合,其偶合常数为12.9 Hz;借助DEPT谱,2个甲氧基被区别;借助DEPT和TOCSY谱,另2个与碳相连的甲基被区分;利用HMBC谱,所有的碳峰可以归属.      

二维核磁共振谱测定植物新环肽-太子参环肽A和B中氨基酸序列

在植物新环肽[太子参环肽A(HA)和B(HB)]的结构研究中,应用COLOC谱较成功地解决了其氨基酸序列。结果表明当J值选择在6Hz-15Hz时,COLOC谱能满意地给出氮上含有活泼氢的氨基酸序列。     

利用Raman磁共振研究四极核对AX体系弛豫行为的影响

本文利用一维多量子Raman磁共振谱线的线宽测定了氯仿(CHCl₃)中碳氢体系的多量子的弛豫时间,由此得到第二类标量耦合弛豫起主导作用时的碳氢核自旋弛豫的交叉相关系数,氯核与碳和氢核的标量耦合常数,以及它们的相对符号。     

三自旋体系核自旋单重态的制备与单重态二维谱的实现

核自旋单重态是一种特殊的自旋状态,其寿命远长于相应自旋的横向和纵向弛豫时间,能够被用于研究分子的慢扩散、慢运动、特征信号选择等过程.目前单重态的研究主要集中于孤立的两自旋体系.而本文以N-乙酰基天冬氨酸（NAA）分子中由亚甲基和次甲基的三个氢原子核构成的三自旋体系为研究对象,将亚甲基中的两个氢核制备成单重态.利用优化控制和数值计算方法,分别设计了包含和不包含次甲基氢核耦合的单重态制备脉冲,结果发现,不考虑次甲基氢耦合设计的优化脉冲,其在实际三自旋体系中的单重态制备效率会显著下降.另外,我们以单重态为起点,实现了针对次甲基和亚甲基的信号选择COSY谱和NOESY谱,结果表明基于单重态的二维谱能够有效避免谱峰重叠现象,提高谱图分辨率,并有助于提高分子结构解析的准确性.     

固体中半整数四极核的自旋扩散

首先介绍固态中自旋扩散的一般理论，包括半经典描述和建立在投影算子理论上的密度矩阵描述. 接着以丰核环境中相互偶合的自旋-1/2系统以及自旋-3/2系统为典型列举了自旋扩散速率的计算. 最近藉助多量子魔角旋转(MQMAS)方法实现半整数四极核的多量子谱自旋扩散实验，可以测量固体粉末中半整数四极核体系四极张量相对方向. 结合作者最近的计算机模拟和实验测量结果对这一新兴方向作了重点介绍，尤其指出了射频
脉冲强度、宽度及样品旋转速度对交叉峰线型的影响.     

原子核高自旋超形变谱学研究

本刊1988年第三、四期“原子核高自旋超形变的研究”一文对高自旋超形变谱学作了初步介绍。本文综合分析近两年来该领域的新进展。两年来,A～190Hg-Pb 超形变区的确认;A～150区奇质子核及更多超形变核的发现;A～130—140区奇质子超形变核和Sm 超形变核的研究;同一个核中多重超形变带的发现;各超形变区超形变带丰富的谱学现象,超形变组态-高j 侵入轨道的不同占据对超形变带谱学性质的成功描述,及对理论模型的检验和修正;A～190和A～150区相邻(同位素和同中素)核中超形变带一系列性质的极端相似—超形变带中内禀自旋排列和质子激发的发现;超形变、高自旋下的对关联研究;超形变带退激馈入正常形变带的时间延迟的实验测定;超形变退激机制的研究;应用于Hg-Pb区超形变带自旋指定的一种有效方法等,都是原子核超形变研究的主要成果。此外,锕系区超形变核中与同质异能裂变竞争的、由位能面第二极小向第一极小退激的“返回”γ衰变(E1跃迁)的实验测定,是原子核在零自旋和低自旋下超形变研究的一个重要进展。     

3D HCCH3-TOCSY for Resonance Assignment of Methyl-Containing Side Chains in C-Labeled Proteins

Two 3D experiments, (H)CCH₃-TOCSY and H(C)CH₃-TOCSY, are proposed for resonance assignment of methyl-containing amino acid side chains. After the initial proton–carbon INEPT step, during which either carbon or proton chemical shift labeling is achieved (t₁), the magnetization is spread along the amino acid side chains by a carbon spin lock. The chemical shifts of methyl carbons are labeled (t₂) during the following constant time interval. Finally the magnetization is transferred, in a reversed INEPT step, to methyl protons for detection (t₃). The proposed experiments are characterized by high digital resolution in the methyl carbon dimension (t_2max = 28.6 ms), optimum sensitivity due to the use of proton decoupling during the long constant time interval, and an optional removal of CH₂, or CH₂ and CH, resonances from the F₂F₃ planes. The building blocks used in these experiments can be implemented in a range of heteronuclear experiments focusing on methyl resonances in proteins. The techniques are illustrated using a ¹⁵N, ¹³C-labeled E93D mutant of Schizosacharomyces pombe phosphoglycerate mutase (23.7 kDa).     

High-Speed Magic-Angle SpinningC MAS NMR Spectra of Adamantane: Self-Decoupling of the Heteronuclear Scalar Interaction and Proton Spin Diffusion

We have investigated the carbon line shape of solid adamantane under high-speed magic-angle sample spinning (MAS) acquired without proton decoupling. The CH-group shows a spinning-speed-dependent line broadening while the CH₂-group consists of a spinning-speed-independent sharp component and a spinning-speed-dependent broader part. These phenomena can be explained by self-decoupling of theJ-interaction due to proton spin diffusion. Such a self-decoupling process can be described by a magnetization exchange process between the multiplet lines. Changing the spin-diffusion rate constant by off-resonance irradiation of the protons allows us to observe the full range from slow exchange to coalescence to fast exchange of the carbon spectra. One of the multiplet components in the CH₂-group corresponds to a group spin of the protons of zero and therefore does not couple to the other protons. This gives rise to the sharp central line. The magnetization exchange rate constant between the different multiplet lines can be determined from the spectra and is a measure for the spinning-speed-dependent proton spin-diffusion rate constant. Even at an MAS speed of 30 kHz, proton spin diffusion is still observable despite the relatively weak intermolecular proton dipolar-coupling network in adamantane which results in a static proton line width of only 14 kHz (full width at half height).     

Muon spin relaxation investigation of frustrated antiferromagnetic pyrochlores A2B2O7

In a system where magnetic ions occupy the vertices of edge or corner sharing triangular units, the natural antiferromagnetic coupling between ions is geometrically frustrated. A wide variety of interesting magnetic behaviour has been observed in pyrochlores, where magnetic ions form a network of corner sharing tetrahedra. The low temperature spin dynamics of a number of pyrochlores A₂B₂O₇ have been investigated using the technique of μ SR. For example, Y₂Mo₂O₇ shows a transition to a disordered magnetic state similar to a spin glass at T_F=22 K. However, unlike conventional metallic spin glasses, a non‐zero muon spin depolarization rate is observed to persist well below 0.1\ T_F. These results suggest that there is a finite density of states for magnetic excitations in this system near zero energy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Recent results from HERA

Results of the HERA experiments HERMES, HI and ZEUS obtained in 1997 are presented. They cover recent measurements on the proton structure functionF ₂, the extraction of the gluon density, which is compared with open charm production, diffractive scattering and results on the proton spin structure. We further give an update on the NC events at very highQ ² and report on an observation of events with an isolated lepton and missingp _t.     

Double resonance experiments in low magnetic field: Dynamic polarization of protons by N and measurement of low NQR frequencies

The possibilities of dynamically polarizing proton spin system via the quadrupole ¹⁴N spin system in low magnetic field are analyzed. The increase of the proton magnetization is calculated. The polarization rate of the proton spin system is related to the transition probabilities per unit time between the ¹⁴N quadrupole energy levels and proton energy levels. The experiments performed in 1,3,5-triazine confirm the results of the theoretical analysis. A new double resonance technique is proposed for the measurement of nuclear quadrupole resonance frequencies ν_Q of the order of 100 kHz and lower. The technique is based on magnetic field cycling between a high and a low static magnetic field and observation of the proton NMR signal in the high magnetic field. In the low magnetic field the quadrupole nuclei and protons resonantly interact at the proton Larmor frequency ν_H = ν_Q/2. The quadrupole nuclei are simultaneously excited by a resonant rf magnetic field oriented along the direction of the low static magnetic field. The experimental procedure is described and the sensitivity of the new technique is estimated. Some examples of the measurement of low ¹⁴N and ²H nuclear quadrupole resonance frequencies are presented.     

An electron spin resonance and polarographic study of the sulphone group

The electron spin resonance spectra of the radical anions of diphenyl sulphone, dibenzothiophene S, S-dioxide and thianthrene S, S, S′, S′-tetroxide have been measured and the proton hyperfine splitting constants assigned to the various protons by measurement of the spectra of deuterio and methyl derivatives.

The experimentally determined spin populations were correlated by molecular orbital calculations, using the formalism of Koch and Moffit to describe the sulphone group, and it is shown that irrespective of the mutual orientation of this group and the conjugated system it may be described as far as the electron spin resonance spectra are concerned as contributing a vacant symmetric orbital to the conjugated system whose coulomb integral (α_SO2 ) and sulphur carbon resonance integral (β_C-SO2 ) are given by α_SO2 =α_C – 2·0β, β_C-SO2 =0·8β where α and β are the carbon coulomb integral and carbon-carbon resonance integrals respectively. The polarographic half wave reduction potentials of these compounds and some of their methyl derivatives dissolved in anhydrous dimethylformamide were also determined and the effect of methyl substitution on this quanttiy is also well described by the above parameters for the sulphone group. Finally a difference between the results obtained here and those previously obtained by Vincow from the electron spin resonance spectrum of thioxanthone S, S-dioxide radical anion is noted and discussed.     

Two dimensional NQR separation of inhomogeneous and homogeneous lineshapes in disordered solids

A two-dimensional (2D) zero field NQR separation of inhomogeneous and homogeneous lineshapes technique is described. The nuclear spin Hamiltonian for spinsI>1/2 in zero magnetic field consists to a good approximation only of the electric quadrupole term. The 2D separation technique enables a separate spectroscopic observation of a static and a randomly time-fluctuating dynamic part of the quadrupole interaction. The separation is based on the fact that nuclear spin precession under a static quadrupolar Hamiltonian can be time-reversed whereas this can not be achieved under the action of a randomly time-fluctuaing Hamiltonian. The 2D spectrum displays in theω ₂-domain the inhomogeneously broadened lineshape, which is a convolution of the inhomogeneous static frequency distribution function and the homogeneous (adiabatic) lineshape. Theω ₁-domain shows the pure homogeneous lineshape. A deconvolution of the inhomogeneous lineshape with the homogeneous one yields a pure static inhomogeneous frequency distribution function which is characteristic and theoretically known for many different models of disordered solids like glasses and incommensurate systems. This technique is important in studies where both lineshapes have comparable widths. The 2D NQR separation technique has been applied to⁷⁵As in a proton glass Rb_0.98(NH₄)_0.02H₂AsO₄.     

Smooth mass corrections to massless theories

We analyze the transition from massive to massless theories in cases in which mass singularities are present. In massless theories these singularities are absorbed into densities or fragmentation functions. We define a subtraction procedure for massive theories which is a canonical generalization of the “minimal subtraction” in massless theories. We use this procedure to calculate smooth mass corrections to the structure functionsF ₂ andG ₁ of deep inelastic scattering. In particular we show how, in our scheme, heavy quarks affect the spin of the proton.     

Muon spectroscopy for studying magnetism and protons and lithium dynamics in spinel manganese oxides

Polarised positive muons can be implanted into any type of material and rapidly thermalize, then the local magnetic environment dictates the evolution of muon spin vectors and provokes the muon depolarisation. The muon spin relaxation (μSR) technique provides interesting information on magnetism and spin dynamics in spinel lithium manganates insertion compounds. In this work, we compare the behaviour of muons into a lithium-rich spinel manganese oxide and its lithium extracted product. The chemical extraction of lithium from Li_1.33Mn_1.67O₄, where all the manganese is Mn^IV, is essentially a lithium by proton ion exchange process to give a protonated manganese oxide with spinel structure, H⁺–MnO₂. Muons clearly have showed the presence of protons in H⁺–MnO₂, and the movement of lithium ions or protons at increasing temperatures in both samples. Muons are quasi-static in these compounds, and they are located both in ‘regular’ lithium and proton sites and also in interstitial sites of the spinel structure, these latter being used during diffusion of lithium ions. Below 50 K, static muons behave as in a paramagnet, where Mn magnetic spins are slowing down and ordering near 6 and 14 K in the protonated and lithiated spinels, respectively.