Spin‐Polarized Structures and Solid‐State NMR Spectroscopy of Paramagnetic Compounds

On an atomic scale and with high sensitivity, solid‐state NMR spectroscopy can provide information about the electronic spin density and coupling mechanisms in paramagnetic compounds. The picture shows how the hyperfine splitting collapses through relaxation. Insights into which compounds are suitable and which approximations have to be made are given.

     

Thermodynamics and Kinetics of Electron Transfer of Electrode-Immobilized Small Laccase from Streptomyces coelicolor

The thermodynamic and kinetic properties for heterogeneous electron transfer (ET) were measured for the electrode-immobilized small laccase (SLAC) from Streptomyces coelicolor subjected to different electrostatic and covalent protein-electrode linkages, using cyclic voltammetry. Once immobilized electrostatically onto a gold electrode using mixed carboxyl- and hydroxy-terminated alkane-thiolate SAMs or covalently exploiting the same SAM subjected to N-hydroxysuccinimide+1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (NHS-EDC) chemistry, the SLAC-electrode electron flow occurs through the T1 center. The E°′ values (from +0.2 to +0.1 V vs. SHE at pH 7.0) are lower by more than 0.2 V compared to the protein either in solution or immobilized with different anchoring strategies using uncharged SAMs. For the present electrostatic and covalent binding, this effect can, respectively, be ascribed to the negative charge of the SAM surfaces and to deletion of the positive charge of Lys/Arg residues due to amide bond formation which both selectively stabilize the more positively charged oxidized SLAC. Observation of enthalpy/entropy compensation within the series indicates that the immobilized proteins experience different reduction-induced solvent reorganization effects. The E°′ values for the covalently attached SLAC are sensitive to three acid base equilibria, with apparent pK_a values of pK_a1ox = 5.1, pK_a1red = 7.5, pK_a2ox = 8.4, pK_a2red = 10.9, pK_a2ox = 8.9, pK_a2red = 11.3 possibly involving one residue close to the T1 center and two residues (Lys and/or Arg) along with moderate protein unfolding, respectively. Therefore, the E°′ value of immobilized SLAC turns out to be particularly sensitive to the anchoring mode and medium conditions.     

Solid‐State 17O NMR Spectroscopy of Paramagnetic Coordination Compounds

High‐quality solid‐state ¹⁷O (I=5/2) NMR spectra can be successfully obtained for paramagnetic coordination compounds in which oxygen atoms are directly bonded to the paramagnetic metal centers. For complexes containing V^III (S=1), Cu^II (S=1/2), and Mn^III (S=2) metal centers, the ¹⁷O isotropic paramagnetic shifts were found to span a range of more than 10 000 ppm. In several cases, high‐resolution ¹⁷O NMR spectra were recorded under very fast magic‐angle spinning (MAS) conditions at 21.1 T. Quantum‐chemical computations using density functional theory (DFT) qualitatively reproduced the experimental ¹⁷O hyperfine shift tensors.     

Unique Identification of Supramolecular Structures in Amyloid Fibrils by Solid‐State NMR Spectroscopy

The fibril structure formed by the amyloidogenic fragment SNNFGAILSS of the human islet amyloid polypeptide (hIAPP) is determined with 0.52 Å resolution. Symmetry information contained in the easily obtainable resonance assignments from solid‐state NMR spectra (see picture), along with long‐range constraints, can be applied to uniquely identify the supramolecular organization of fibrils.

     

Installation of a Rigid EDTA‐Like Motif into a Protein α‐Helix for Paramagnetic NMR Spectroscopy with Cobalt(II) Ions

Coupling two copies of an iminodiacetic acid–cysteine hybrid ligand to a pair of cysteine residues positioned in an i, i+4 arrangement within a protein α‐helix leads to generation of an EDTA‐like metal ion‐binding motif. Rigid binding of a Co^II ion by this motif produces pseudo‐contact shifts suitable for paramagnetic NMR structural studies.     

Structure of a Protein–RNA Complex by Solid‐State NMR Spectroscopy

Solid‐state NMR (ssNMR) is applicable to high molecular‐weight (MW) protein assemblies in a non‐amorphous precipitate. The technique yields atomic resolution structural information on both soluble and insoluble particles without limitations of MW or requirement of crystals. Herein, we propose and demonstrate an approach that yields the structure of protein–RNA complexes (RNP) solely from ssNMR data. Instead of using low‐sensitivity magnetization transfer steps between heteronuclei of the protein and the RNA, we measure paramagnetic relaxation enhancement effects elicited on the RNA by a paramagnetic tag coupled to the protein. We demonstrate that this data, together with chemical‐shift‐perturbation data, yields an accurate structure of an RNP complex, starting from the bound structures of its components. The possibility of characterizing protein–RNA interactions by ssNMR may enable applications to large RNP complexes, whose structures are not accessible by other methods.     

Multiple-Quantum Magic-Angle Spinning: High-Resolution Solid State NMR Spectroscopy of Half-Integer Quadrupolar Nuclei

Summary.  Experimental and theoretical aspects of the multiple-quantum magic-angle spinning experiment (MQMAS) are discussed in this review. The significance of this experiment, introduced by Frydman and Harwood, is in its ability to provide high-resolution NMR spectra of half-integer quadrupolar nuclei (I ≥ 3/2). This technique has proved to be useful in various systems ranging from inorganic materials to biological samples. This review addresses the development of various pulse schemes aimed at improving the signal-to-noise ratio and anisotropic lineshapes. Representative spectra are shown to underscore the importance and applications of the MQMAS experiment. Corresponding author. E-mail: madhu@soton.ac.uk Received April 16, 2002; accepted May 6, 2002     

Characterization of Surface Modified Silica Nanoparticles by 29Si Solid State NMR Spectroscopy

 Various aerosils surface modified with silane reagents were prepared and investigated by ²⁹Si solid state NMR spectroscopy. The mode of bonding onto the silanol surface (mono- or divalent) can be specified by comparison with chemical shifts from solution. A detailed analysis also leads to the detection of products formed via hydrosilylation reactions. A rough quantification of the surface loading can be obtained by a signal deconvolution process of the silanol resonances on the Aerosil surface.     

Probing the Motional Behavior of Eumelanin and Pheomelanin with Solid‐State NMR Spectroscopy: New Insights into the Pigment Properties

Melanin is the most widespread pigment in the animal kingdom. Despite its importance, its detailed structure and overall molecular architecture remain elusive. Both eumelanin (black) and pheomelanin (red) occur in the human body. These two melanin compounds show very different responses to UV‐radiation exposure, which could relate to their microscopic features. Herein, the structural properties and motional behavior of natural eu‐ and pheomelanin extracted from black and red human hair are investigated by means of solid‐state NMR spectroscopy. Several 1D and 2D NMR spectroscopic techniques were combined to highlight the differences between the two forms of the pigment. The quantitative analysis of the ¹H NMR wide‐line spectra extracted from 2D ¹H–¹³C LG‐WISE experiments revealed the presence of two dynamically distinguishable components in both forms. Remarkably, the more mobile fraction of the pigment showed a higher mobility with respect to the proteinaceous components that coexist in the melanosome, which is particularly evident for the red pigment. An explanation of the observed effects takes into account the different architecture of the proteinaceous matrix that constitutes the physical substrate onto which melanin polymerizes within the eu‐ and pheomelanosomes. Further insight into the molecular structure of the more mobile fraction of pheomelanin was also obtained by means of the analysis of 2D ¹H–¹³C INEPT experiments. Our view is that not only structural features inherent in the pure pigment, but also the role of the matrix structure in defining the overall melanin supramolecular arrangement and the resulting dynamic behavior of the two melanin compounds should be taken into account to explain their functions. The reported results could pave a new way toward the explanation of the molecular origin of the differences in the photoprotection activity displayed by black and red melanin pigments.     

An Investigation of Poly(dimethylsiloxane) Chain Dynamics and Order in Its Inclusion Compound with γ‐Cyclodextrin by Fast‐MAS Solid‐State NMR Spectroscopy

The dynamics of poly(dimethylsiloxane) in its inclusion compound with γ‐cyclodextrin are elucidated using modern fast‐MAS solid‐state NMR techniques. Measurements of methyl ¹H–¹H and ¹H–¹³C dipolar coupling constants indicate that the polymer undergoes a uniform motion, rendering all methyl groups equivalent. The dynamics of the Si—C bond is characterized by either a dynamic order parameter of S = 0.72, or, assuming a stably rotating helical structure, an inclination angle of 73° relative to the rotation axis.     

稻秆木素侧链13C同位素示踪及固体13CNMR分析

在水稻(Oryza sativa L.)生长过程中,向其茎秆部节间的空腔分别注入在侧链α,β,γ位带有¹³C标记的松柏醇葡萄糖甙,得到¹³C标记的稻秆木素,用高分辨率固体核磁¹³CNMR对其组织进行分析,发现外源性的松柏醇葡萄糖甙并不影响水稻中木素的正常合成.证明了β-O-4,β-β,β-5和β-1结构是稻秆原本木素中的主要结构,另外还有少量的松柏醇和阿魏酸类结构,并证实木素在α位与糖类等组分有共价键形成.     

3D-ASL灌注成像与静息态功能MRI在癫痫诊断及鉴别中的价值

选取123例癫痫患者作为研究组,同期69例健康体检者作为对照组,均行三维动脉自旋标记技术(3D-ASL)灌注成像与静息态功能MRI(rs-FMRI)检查,研究二者在癫痫病诊断及鉴别中的价值.结果发现,研究组全面发作脑血流量(CBF)、低频振幅(ALFF)＜部分发作和对照组,全面发作血清脑源性神经细胞营养因子(BDNF)...     

脑部磁共振结合静息态fMRI方法在卒中患者认知障碍中的应用

本研究主要探讨脑部磁共振结合静息态fMRI方法在卒中患者认知障碍中的应用。本研究将符合卒中认知障碍患者25例设为患者组,将25例健康者设为对照组。采用脑部静息态核磁共振对实验者进行扫描,以脑部双侧海马为种子点进行功能连接分析,并通过软件分析两组实验者静息态双侧海马差异。结果发现,与对照组相比,患者组右侧海马与左右侧额中回、左侧额下回等的连接减少;右侧海马与左侧颞上中回、双侧丘脑的连接增加;左侧海马与左侧颞额中回、右侧颞额上回、右侧额下回的连接减少;左侧海马与右侧顶下回连接增加。总之,本研究发现卒中患者海马脑区与颞叶皮层、额叶皮层之间的连接减少,可能是由卒中患者脑损害导致。     

Probing the Local Structure of Pure Ionic Liquid Salts with Solid‐ and Liquid‐State NMR

Room‐temperature ionic liquids (RTILs) are gaining increasing interest and are considered part of the green chemistry paradigm due to their negligible vapour pressure and ease of recycling. Evidence of liquid‐state order, observed by IR and Raman spectroscopy, diffraction studies, and simulated by ab initio methods, has been reported in the literature. Here, quadrupolar nuclei are used as NMR probes to extract information about the solid and possible residual order in the liquid state of RTILs. To this end, the anisotropic nature and field dependence of quadrupolar and chemical shift interactions are exploited. Relaxation time measurements and a search for residual second‐order quadrupolar coupling were employed to investigate the molecular motions present in the liquid state and infer what kind of order is present. The results obtained indicate that on a timescale of ～10^?8 sec or longer, RTILs behave as isotropic liquids without residual order.     

Insight into the Supramolecular Architecture of Intact Diatom Biosilica from DNP‐Supported Solid‐State NMR Spectroscopy

Diatom biosilica is an inorganic/organic hybrid with interesting properties. The molecular architecture of the organic material at the atomic and nanometer scale has so far remained unknown, in particular for intact biosilica. A DNP‐supported ssNMR approach assisted by microscopy, MS, and MD simulations was applied to study the structural organization of intact biosilica. For the first time, the secondary structure elements of tightly biosilica‐associated native proteins in diatom biosilica were characterized in situ. Our data suggest that these proteins are rich in a limited set of amino acids and adopt a mixture of random‐coil and β‐strand conformations. Furthermore, biosilica‐associated long‐chain polyamines and carbohydrates were characterized, thereby leading to a model for the supramolecular organization of intact biosilica.     

固体核磁共振弛豫在高分子研究中的应用

本文综述了近年来固体核磁弛豫方法在高分子研究中的应用，共分５个部分加以介绍：（１）自旋－晶格弛豫过程；（２）在旋转坐标系中的＾１３Ｃ自旋－晶格弛豫过程；（３）交叉极化速率和旋转坐标系中的＾１Ｈ自旋－晶格弛豫过程；（４）自旋－自旋弛豫过程；（５）动态结构导致的线形变化。本文主要讨论磁性核的各种弛豫过程以及它们与分子结构和分子运动的关系。     

Sc Solid State NMR studies of the silicides ScTSi (T=Co, Ni, Cu, Ru, Rh, Pd, Ir, Pt)

The silicides ScTSi (T=Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, Pt) were synthesized by arc-melting and characterized by X-ray powder diffraction. The structures of ScCoSi, ScRuSi, ScPdSi, and ScIrSi were refined from single crystal diffractometer data. These silicides crystallize with the TiNiSi type, space group Pnma. No systematic influences of the ⁴⁵Sc isotropic magnetic shift and nuclear electric quadrupolar coupling parameters on various structural distortion parameters calculated from the crystal structure data can be detected. ⁴⁵Sc MAS-NMR data suggest systematic trends in the local electronic structure probed by the scandium atoms: both the electric field gradients and the isotropic magnetic shifts relative to a 0.2 M aqueous Sc(NO₃)₃ solution decrease with increasing valence electron concentration and within each T group the isotropic magnetic shift decreases monotonically with increasing atomic number. The ⁴⁵Sc nuclear electric quadrupolar coupling constants are generally well reproduced by quantum mechanical electric field gradient calculations using the WIEN2k code.