Magic-Angle Spinning NMR and Molecular Mobility in Heterogeneous Systems

Magic-angle sample spinning (MAS) nuclear magnetic resonance (NMR) measurements are treated for heterogeneous systems with nanometer dimensions. An appreciable line narrowing in the MAS NMR spectra of the embedded molecules may be achieved also in the cases when the molecules still possess an appreciable local mobility. It appears that the MAS frequencies are of comparable order of magnitude as the frequencies which characterize the random molecular motional processes and which compete with MAS. It will be shown that this behavior may occur if inhomogeneous local magnetic fields due to susceptibility effects have a dominating influence on the widths and shapes of the resonance NMR lines. Properties of these local fields are described. Spectra simulations are carried for molecules embedded in these heterogeneous systems when the coherent averaging by MAS is superimposed by random local motions. This situation may occur for molecules contained in nanoporous solids and also for heterogeneous systems like membranes and biological tissues with flexible components like water, lipids, and small peptides. Several examples are treated which reveal advantages and limitations of these experiments and their theoretical interpretation.     

Anomalous Rotational Resonance Spectra in Magic-Angle Spinning NMR

Magic-angle spinning NMR spectra of samples containing dilute spin-1/2 pairs display broadenings or splittings when a rotational resonance condition is satisfied, meaning that a small integer multiple of the spinning frequency matches the difference in the two isotropic shift frequencies. We show experimental rotational resonance NMR spectra of a 13C2-labeled retinal which are in qualitative disagreement with existing theory. We propose an explanation of these anomalous rotational spectra involving residual heteronuclear couplings between the 13C nuclei and the neighboring 1H nuclei. These couplings strongly influence the rotational resonance 13C spectrum, despite the presence of a strong radiofrequency decoupling field at the 1H Larmor frequency. We model the residual heteronuclear couplings by differential transverse relaxation of the 13C single-quantum coherences. We present a superoperator theory of the phenomenon and describe a numerical algorithm for rapid Liouville space simulations in periodic systems. Good agreement with experimental results is obtained by using a biexponential transverse relaxation model for each spin site.     

Broadband Polarization Transfer under Magic-Angle Spinning: Application to Total Through-Space-Correlation NMR Spectroscopy

A pulse sequence is described that leads to a broadband recoupling of the dipolar interaction in magic-angle-spinning solid-state NMR experiments of¹³C spins. The sequence is based on a combination of rotating frame and laboratory frame transfer periods. The recovered dipolar interaction is only weakly dependent on spectral parameters but is a faithful measure for the internuclear distances. Furthermore, a pure zero-quantum term is recovered (of the type found in static “spin-diffusion” experiments). This makes the pulse sequence particularly suited for incorporation into two-dimensional total through-space correlation experiments that deliver simultaneous information about all dipolar couplings in a single 2D experiment. It is found that the necessary decoupling from abundant protons is best performed in two steps: first, the strong homonuclear couplings between the high-γ spins are averaged by Lee–Goldburg irradiation and, second, the heteronuclear dipolar interaction is averaged by the combined application of an RF field to the low-γ spins and magic-angle sample spinning. Phase-inversion and amplitude attenuation in the rotating frame and refocusing pulses in the laboratory frame part of the pulse sequence are introduced to achieve an optimum chemical-shift offset-independence and for the suppression of unwanted double-quantum transitions. The design principles are explained in detail. Finally, the pulse scheme is applied to total-correlation spectroscopy of a uniformly labeled amino acid. The experimental cross-peak intensities are in qualitative agreement with the known crystal structure of the model compound.     

Application of Rotor-Synchronized Amplitude-Modulated Cross-Polarization in aC–H Spin Pair under Fast Magic-Angle Spinning

Rotor-synchronized amplitude-modulated Hartmann–Hahn cross-polarization has been applied under fast magic-angle spinning to a powder sample of ferrocene. The influence on the cross-polarization process of the heteronuclear double quantum (flop–flop) transitions induced by the amplitude modulation for low ratios of the radiofrequency-field strength to the spinning speed is studied in details. The experimental data are in good agreement with theoretical calculations for a two-spinsystem, although the intensity of the double quantum transitions is observed to be significantly smaller than expected. Moreover, it is shown that an efficient polarization transfer at the Hartmann–Hahn centerband matching condition as well as a broadening of the matching conditions is obtained by an appropriate partial scaling of the effective radiofrequency fields which minimizes the destructive effect of double quantum cross-polarization.     

Influence of Proton Chemical-Shift Anisotropy on Magic-Angle Spinning Spectra of Hydrate Crystals

It is shown that the proton chemical-shift anisotropy of hydrate crystals affect dipolar powder pattern at 7.0 T. This may be clearly observed via an asymmetric envelope of dipolar spinning sidebands in magic-angle spinning proton spectra.     

电场催陈米酒核磁共振分析

以自行研制的设备对新产广东玉冰烧米酒进行催陈 (处理条件 :4 0kV·m- 1 的电场强度处理 180min ,5 0Hz) ;然后采用1 HNMR对新酒、电场催陈酒及成品酒进行了分析 ,研究发现新酒中甲基和亚甲基峰出现了 4个杂峰而其他样品没有 ,说明新酒中单分子与各种状态的缔合结构共存 ,其他各主要有机物质亦参与成峰 ;而关于羟基质子峰 ,新酒为平头峰 ,推断出溶液中有 2种羟基质子存在 ,一种参与形成缔合 ,另一种未参与缔合 ;成品酒为单一峰 ,说明溶液中形成了单一稳定的大分子缔合结构 ;电场处理酒样为不规则峰 ,反映出多种缔合状态共存 ,其主要缔合结构接近成品酒缔合结构     

关于斑蝥素~1H NMR和UV测定结果的商榷

斑蝥素是一种从药用昆虫中提取的天然药物, 由于可用于治疗各种癌症而得到逐步深入的研究. 作者用核磁共振波谱法测定从贵州短翅豆芫菁体内分泌的斑蝥素时, 发现与参考文献[1]的解释不同; 在用紫外吸收光谱分析法测定斑蝥素的最大吸收波长时,发现斑蝥素的最大吸收波长应该是212nm, 此也与参考文献[2]的结果不一样.该文对实验结果进行了讨论,希望能得到斑蝥素的准确分析方法.     

Mobility and structure of human serum albumin powders in water-acetonitrile mixtures by1H NMR relaxation and FTIR spectroscopy

The effect of acetonitrile on protein dynamics was investigated for solid human serum albumin samples at various hydration levels. Temperature dependences of¹H nonselective nuclear magnetic resonanceT ₁ andT ₂ relaxation times at 27 MHz have been measured and data were interpreted in terms of three kinds of internal motions in the protein. Microdynamic parameters of the motions were obtained within a “model-free” approach. It was found that acetonitrile hardly affects the fast motions but noticeably influences the slow motion of side chain groups, shortening the correlation time and increasing the amplitude of the motion. The acetonitrile effect on dynamics is likely based on the appearance of additional free volume as a result of the formation of rigid helical parts in the protein structure. Water, plasticizing the protein structure, promotes the action of organic solvent. A definite part of side chain groups, slowly moving in the same frequency window as the rest of protein side chain groups, performs less constrained “liquidlike” motion. The relative population of these highly movable protons is closely correlated with the increment of the helical structure induced by water and acetonitrile.     

Protonation Sequence Study of the Solution Structure of DTPA by 1H NMR

The protonation sequence of DTPA is evaluated using a 300 MHz H NMR instrument. A terminal nitrogen is protonated first (pK_a1,2=10.02, 8.45). Upon the first protonation a dynamic hydrogen bonded five membered ring is formed between the terminal nitrogen and the central nitrogen. The second equivalent of protons protonates the opposite terminal amine and causes the ring to open. The third protonation (pK_a3=4.65) takes place on the central acetate and forms a ring with the central amine. The fourth protonation is assigned to a terminal acetate. The formation of five membered rings involving hydrogen bonding gives a better quantitative and qualitative account for the unusual shifts observed in the H NMR spectra than previous explanations.     

1H NMR Study of Some Thiosalicylic Acid Complexes

Five thiosalicylic acid (TSA) complexes were prepared. The ¹H NMR of these complexes were recorded and analysed. It was found that molybdenum, arsenic, selenium, silver and tellurium metals coordinates through the sulfur atom. The ¹H NMR chemical shift of ring protons of As, Se and Te shift to low field upon complexation, while it shifts to high field when TSA complex with Ag and Mo     

原油降粘剂组分的红外光谱及核磁共振氢谱的分析研究

为了明确原油降粘剂的主要组分,采取蒸馏、溶解一沉淀法及柱色谱法对原油降粘剂进行分离与纯化,用红外光谱法对其主要组分官能团的结构进行了鉴定,推断其主要组分是丙烯酸乙酯/甲基丙烯酸甲酯/丙烯酸三元共聚物,用核磁共振氢谱及质谱进行了结构确认及定量分析,共聚物中3种单体:丙烯酸乙酯、甲基丙烯酸甲酯、丙烯酸的摩尔数比为37.1:25.8:37.1;重量百分比为41.1:28.8:29.8.用红外光谱对甲醇溶液部分进行了结构分析,结果表明,非离子表面活性剂为聚氧乙烯醚,分子量范围是800～1 600,阴离子表面活性剂为烷基苯磺酸钠,其余为助剂和水.     

Molecular dynamics of n-dodecylammonium chloride in aqueous solutions investigated by 2H NMR and 1H NMR relaxometry

Molecular dynamics in n-dodecylammonium chloride/water solutions for concentrations of 34 and 45 wt% was studied by 2H NMR and by 1H NMR dispersion of spin-lattice relaxation in the 2 kHz-90 MHz frequency range. The system exhibits a number of lyotropic liquid crystalline phases, which differ in symmetry and involve motions characterized by a wide frequency scale. The analysis of 2H NMR lineshapes of selectively deuterated DDACl molecules gave us an evidence for local trans-gauche conformational changes in the chains, whereas the dispersion of spin-lattice relaxation times T1 explored by fast field cycling method revealed fast local motions, translational diffusion and collective molecular dynamics of the chains. In particular, we have found that the order director fluctuation mechanism in smectic and nematic phases dominates spin-lattice relaxation below 1 MHz and that local motions and translational diffusion are responsible for the spin-lattice relaxation in the higher Larmor frequency range.     

Pore-size determination of mesoporous materials by1H NMR spectroscopy

The pore-size distributions of a series of mesoporous silica materials were determined by measuring the¹H nuclear magnetic resonance (NMR) signal from the nonfrozen fraction of organic probe molecules as a function of temperature. The melting point distribution curves of confined benzene reveal 2–3 transition points. The high-temperature transition point, corresponding to the temperature at the first maximum of the melting point distribution curve, is interpreted as the average depressed melting point of the confined substance. However, the intensity data reveal that a measurable portion of the confined benzene apparently remains nonfrozen even 120 K below the bulk melting point in the 4–10 nm pore systems. The component at lowest temperature is largely attributed to the liquidlike molecules at the pore wall, while the component at the intermediate temperature might result from pockets in the solid matrix or even a bimodal pore-size distribution. The average pore-size distributions obtained by NMR agree fairly well with those obtained by N₂ sorption. However, NMR gives a more detailed picture of the distribution, revealing two or three well-defined peaks. The peak at the smallest pore size, however, reflects the surface layer rather than a pore-size distribution.     

Mobile Low-Field 1H NMR Spectroscopy Desktop Analysis of Biodiesel Production

Biodiesel produced mainly by the base-catalyzed transesterification of vegetable oils or animal fats with a short chain alcohol, has become an attractive alternative to petroleum-based diesel fuel. Even though high-field ¹H nuclear magnetic resonance (NMR) is a reliable method for biodiesel quality control, it is restricted by its poor mobility and expensive superconducting coils. As an alternative, this study presents a mobile low-field ¹H NMR spectrometer for the analysis of biodiesel samples derived from different feedstock oils. The low-field ¹H NMR spectra of all the compounds coexisting in a typical transesterification reaction such as rapeseed oil, rapeseed biodiesel, methanol, and glycerol, could be clearly differentiated. Field-dependent characteristic parameters such as relaxation times are provided. The degree of saturation of the different biofuels samples could be reliably estimated via integration of the resolved signals of the spectra. The obtained results agreed well with those measured at high-field ¹H NMR. Since this compositional information is directly related to the biodiesel properties, the presented mobile low-field ¹H NMR device built from permanent magnets arrayed in a Halbach geometry, constitutes an excellent alternative tool for biodiesel quality control.     

Study of molecular reorientation and quantum rotational tunneling in tetramethylammonium selenate by 1H NMR

1H NMR spin-lattice relaxation time measurements have been carried out in [(CH3)4N]2SeO4 in the temperature range 389-6.6 K to understand the possible phase transitions, internal motions and quantum rotational tunneling. A broad T1 minimum observed around 280 K is attributed to the simultaneous motions of CH3 and (CH3)4N groups. Magnetization recovery is found to be stretched exponential below 72 K with varying stretched exponent. Low-temperature T1 behavior is interpreted in terms of methyl groups undergoing quantum rotational tunneling.     

Molecular dynamics in solid riboflavin as studied by 1H NMR

Spin–lattice relaxation times T₁ and T_1d as well as NMR second moment were employed to study the molecular dynamics of riboflavin (vitamin B₂) in the temperature range 55–350 K. The broad and flat T₁ minimum observed at low temperatures is attributed to the motion of two nonequivalent methyl groups. The motion of the methyl groups is interpreted in terms of Haupt's theory, which takes into account the tunneling assisted relaxation. An additional mechanism of relaxation in the high temperature region is provided by the motion of a proton in one of the hydroxyl groups. The Davidson–Cole distribution of correlation times for this motion is assumed.     

Molecular dynamics in solid pyridoxine as studied by 1H NMR

Spin-lattice relaxation times T1 and T1d as well as NMR second moment were employed to study molecular dynamics of pyridoxine (vitamin B6) in the temperature range 10-350 K. The T1 minimum observed at low temperatures at 200 MHz is attributed to a motion of methyl group. The motion is interpreted in terms of Haupt's theory, which takes into account the tunneling assisted relaxation. At low temperatures, where T1 is temperature independent, occupation of the ground state only is assumed. A motion of proton of the hydroxyl groups or CH2OH groups probably provides additional mechanism of relaxation, in the high-temperature region.     

Determination of membrane Peptide orientation by 1H-detected 2H NMR spectroscopy

We demonstrate the application of the proton inverse detected deuteron (PRIDE) NMR technique to the measurement of the orientation of membrane-bound peptides with enhanced sensitivity. Gramicidin D, a transmembrane peptide, and ovispirin, a surface-bound peptide, were used as model systems. The peptides were 2H-labeled by 1H/2H exchange and oriented uniaxially on glass plates. The directly detected 2H spectra of both peptides showed only a strong D(2)O signal and no large quadrupolar splittings. In contrast, the PRIDE spectrum of gramicidin exhibited quadrupolar splittings as large as 281 kHz, consistent with its transmembrane orientation. Moreover, the large D(2)O signal in the directly detected 2H spectra was cleanly suppressed in the PRIDE spectrum. For ovispirin, the 1H indirectly detected 2H spectrum revealed a 104 kHz splitting and a zero-frequency peak. The former reflects the in-plane orientation of most of the helix axis, while the latter results from residues with a magic-angle orientation of the N-D bonds. These are consistent with previous 15N NMR results on ovispirin. The combination of PRIDE and exchange labeling provides an economical and sensitive method of studying membrane peptide orientations in lipid bilayers without the influence of D(2)O and with the ability to detect N-D bonds at the magic angle from the bilayer normal.