Silicon-29 diffusion-ordered NMR spectroscopy (DOSY) as a tool for studying aqueous silicates

The first use of silicon-29 diffusion-ordered NMR spectroscopy (DOSY) is reported, in a study of the speciation of aqueous silicates.     

Proton to carbon-13 INEPT in solid-state NMR spectroscopy

A refocused INEPT through-bond coherence transfer technique is demonstrated for NMR of rigid organic solids and is shown to provide a valuable building block for the development of NMR correlation experiments in biological solids. The use of efficient proton homonuclear dipolar decoupling in combination with a direct spectral optimization procedure provides minimization of the transverse dephasing of coherences and leads to very efficient through-bond (1)H-(13)C INEPT transfer for crystalline organic compounds. Application of this technique to 2D heteronuclear correlation spectroscopy leads to up to a factor of 3 increase in sensitivity for a carbon-13 enriched sample in comparison to standard through-bond experiments and provides excellent selectivity for one-bond transfer. The method is demonstrated on a microcrystalline sample of the protein Crh (2 x 10.4 kDa).     

Natural products dereplication by diffusion ordered NMR spectroscopy (DOSY)

Diffusion-ordered NMR spectroscopy (DOSY) can be used to analyze mixtures of compounds since resonances deriving from different compounds are distinguished by their diffusion coefficients (D). Previously, DOSY has mostly been used for organometallic and polymer analysis, we have now applied DOSY to investigate diffusion coefficients of structurally diverse organic compounds such as natural products (NP). The experimental Ds derived from 55 diverse NPs has allowed us to establish a power law relationship between D and molecular weight (MW) and therefore predict MW from experimental D. We have shown that D is also affected by factors such as hydrogen bonding, molar density and molecular shape of the compound and we have generated new models that incorporate experimentally derived variables for these factors so that more accurate predictions of MW can be calculated from experimental D. The recognition that multiple physicochemical properties affect D has allowed us to generate a polynomial equation based on multiple linear regression analysis of eight calculated physicochemical properties from 63 compounds to accurately correlate predicted D with experimental D for any known organic compound. This equation has been used to calculate predicted D for 217 043 compounds present in a publicly available natural product database (DEREP-NP) and to dereplicate known NPs in a mixture based on matching of experimental D and structural features derived from NMR analysis with predicted D and calculated structural features in the database. These models have been validated by the dereplication of a mixture of two known sesquiterpenes obtained from Tasmannia xerophila and the identification of new alkaloids from the bryozoan Amathia lamourouxi. These new methodologies allow the MW of compounds in mixtures to be predicted without the need for MS analysis, the dereplication of known compounds and identification of new compounds based solely on parameters derived by DOSY NMR.

We report accurate DOSY NMR based molecular weight and diffusion coefficient prediction tools. These tools can be used to dereplicate known natural products from databases using structurally rich NMR data as a surrogate for mass spectrometric data.     

Tools and strategies for processing diffusion-ordered 2D NMR spectroscopy (DOSY) of a broad,featureless resonance: an application to methylaluminoxane (MAO)

DOSY has been extremely successful in many studies of molecular weight distributions, especially when the components are separable along the chemical shift axis. However, an unresolved NMR resonance yields the familiar problem of overlapping exponential decays. In a study of methylaluminoxane (MAO), a set of data processing and simulation tools were developed: read Bruker data files (Matlab); preliminary non-linear least-squares fit with f-test (Matlab); movie generation of the fits (Matlab); conversion of diffusion coefficients to molecular masses through molecular volumes (Gaussian-98); and simulation of DOSY data sets for various molecular mass distributions (Mathematica). These tools are presented here and briefly compared with other DOSY analysis methods.Electronic Supplementary Material Supplementary material is available in the online version of this article at . The following Matlab and Mathematica files are made available: Plot_Raw_DOSY.m, Fit_Two_Component.m (which calls One_Gaussian_LEASTSQ.m, Two_Gaussian_LEASTSQ.m, and pFTest.m), and DOSY_theory.nb. Note: Matlab v5.2 optimization toolbox, as supplied, lacked a confidence interval subroutine; upon our request, confint.m was provided to the authors by Mathworks, Inc. Newer versions of Matlab have a similar program already included in the optimization     

Particle size measurement of lipoprotein fractions using diffusion-ordered NMR spectroscopy

The sizes of certain types of lipoprotein particles have been associated with an increased risk of cardiovascular disease. However, there is currently no gold standard technique for the determination of this parameter. Here, we propose an analytical procedure to measure lipoprotein particles sizes using diffusion-ordered nuclear magnetic resonance spectroscopy (DOSY). The method was tested on six lipoprotein fractions, VLDL, IDL, LDL₁, LDL₂, HDL₂, and HDL₃, which were obtained by sequential ultracentrifugation from four patients. We performed a pulsed-field gradient experiment on each fraction to obtain a mean diffusion coefficient, and then determined the apparent hydrodynamic radius using the Stokes–Einstein equation. To validate the hydrodynamic radii obtained, the particle size distribution of these lipoprotein fractions was also measured using transmission electron microscopy (TEM). The standard errors of duplicate measurements of diffusion coefficient ranged from 0.5% to 1.3%, confirming the repeatability of the technique. The coefficient of determination between the hydrodynamic radii and the TEM-derived mean particle size was r ² = 0.96, and the agreement between the two techniques was 85%. Thus, DOSY experiments have proved to be accurate and reliable for estimating lipoprotein particle sizes.     

三取代环丙烷的^1^3C核磁共振谱研究

前文曾报道三取代环丙烷的质子核磁共振谱及其解析结果,讨论了取代基对化学位移和偶合常数的影响.环丙烷的~(13)C核磁共振谱研究报道极少。Monti等人研究了甲基、溴代和乙炔基环丙烷的~(13)C核磁共振谱,发现在多取代的环丙烷中,取代基相互之间的影响是很显著的.Clague等人亦报道了一系列环丙烷的~(13)C核磁共振谱,但未有详细的分析.本文报道10种尚未见载于文献的三取代环丙烷的~(13)C核磁共振谱,数据见表1. 在1,2,3-三取代环丙烷中,由于取代基的相互作用,影响环上碳的化学位移的因素是比较复杂的.从我们测定化合物的数据来看,取代基为CH_3的环上碳的平均化学位移为29.24     

Investigations by (13)C NMR spectroscopy of ethene-initiated catalytic CO hydrogenation

13C NMR spectroscopy shows that the n-alkene and n-alkane products from the catalytic hydrogenation of CO in the presence of (13)C(2)H(4) probes over Ru/150 degrees C, Co/180 degrees C, Fe/220 degrees C, or Rh/190 degrees C (1 atm, CO:H(2) 1:1, "mild conditions") contain terminal (13)CH(3)(13)CH(2)- units. This is consistent with their formation by a regiospecific polymerization of C(1) species derived from CO and initiated by (13)C(2)H(4). Although the activities toward individual products differed somewhat, similar distributions and similar product labeling patterns were obtained over all the four catalysts. 1-Butene and the higher 1-n-alkenes from all the catalysts were largely (13)CH(3)(13)CH(2)(CH(2))(n)()CH=CH(2) (n = 0-3), propene formed over Ru or Co was (13)CH(3)(13)CH=CH(2), while both (13)CH(3)(13)CH=CH(2) and (13)CH(2)=(13)CHCH(3) were formed over Fe or Rh. Comparison of the conclusions from these probe experiments with those from isotope transient experiments by other workers indicates that the ethene initiator does not significantly modify the course of the CO hydrogenation. The reaction products are largely kinetically determined, and the primary products are mainly linear 1-n-alkenes, while the n-alkanes and 2-n-alkenes largely arise via secondary processes. Since the distribution of products and the labeling in them is so similar, it is concluded that one basic primary mechanism applies over all the four metals. Several different reaction paths involving a polymerization of surface methylene, [CH(2(ad))], have been proposed. Although the predictions based on several of these mechanisms agree with many of the results, the alkenyl + [CH(2(ad))] mechanism, initiated by a surface vinyl [CH(2)=CH((ad))], most easily accommodates the experimental evidence. An alternative path involving sequential addition of surface methylidyne and hydride either to a growing alkylidene chain (alkylidene + [CH(ad) + H(ad)]) or to an alkyl chain (alkyl + [CH((ad)) + H(ad)]) has recently been proposed by van Santen and Ciobica. The [CH(2(ad))] mechanism offers an easier explanation for the formation of the various alkenes, the distribution of products, and of the initiation, while the [CH(ad) + H(ad)] mechanism can explain any n-alkanes formed as primary products and not derived from alkenes. At higher reaction temperatures over Ru and Co, considerable (13)C(1) incorporation (from natural abundance in the CO and from cleavage of the (13)C(2)H(4) probe) was found in all the hydrocarbons. Thus, at higher temperatures (13)C(1(ad)) in addition to (13)C(2(ad)) species participate in both chain growth and initiation. In summary, adsorbed CO is transformed very easily into surface C(1(ad)), probably [CH(2(ad))] in equilibrium with [CH((ad))+H(ad)], which act as the propagating species.     

13C NMR spectroscopy of coumarin derivatives

The ¹³C chemical shifts of 209 naturally occurring and synthetic coumarin derivatives are listed and a number of methods for signal assignments are explained. Substituent effects on ¹³C chemical shifts (SCS) in monosubstituted coumarins and non-additivities of SCS in coumarins with more than one substituent are discussed in detail.     

13C NMR spectroscopy of oleanane derivatives

The chemical shifts of the carbon atoms in the¹³C NMR spectrum of 188 natural and synthetic oleanane derivatives published up to 1988 are given. The possibilities of¹³C NMR spectroscopy for the identification and study of the structures of new derivatives are discussed.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Branch, Russian Academy of Sciences, Vladivostok. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 3–31, January–February, 1992.     

Structural study of three isomers of Tm@C(82) by (13)C NMR spectroscopy

The (13)C NMR spectra were measured for three isomers of Tm@C(82), which is one of the divalent metallofullerenes. The molecular symmetries were determined for each isomer: isomer I has C(s) symmetry, isomer II has C(2) symmetry, and isomer III has C(2v) symmetry. Moreover the cage structure of Tm@C(82)(III) was found to be C(82)(9). As a result, it was revealed that Tm@C(82)(III) has a cage identical to that of La@C(82), which is one of the trivalent metallofullerenes.     

A high-temperature thermometer for 13C NMR spectroscopy

The chemical shift difference between the carbon atoms in neat dichloroacetic acid is temperature dependent. The temperature dependence was calibrated against the melting points of pure organic compounds to provide an NMR thermometer for ¹³C spectroscopy.     

13C shielding scale for MAS NMR spectroscopy

We have performed the direct measurements of ¹³C magnetic shielding for pure liquid TMS, solution of 1% TMS in CDCl₃ and solid fullerene. The measurements were carried out in spherical ampoules exploring the relation between the resonance frequencies, shielding constants and magnetic moments of ¹³C and ³He nuclei. Next the ¹³C shielding constants of glycine, hexamethylbenzene and adamantane were established on the basis of appropriate chemical shifts measured in the solid state. All the new results are free from susceptibility effects and can be recommended as the reference standards of ¹³C shielding scale in the magic angle spinning NMR experiments. Copyright © 2011 John Wiley & Sons, Ltd.     

Supramolecular interactions probed by 13C-13C solid-state NMR spectroscopy

We present a robust solid-state NMR approach for the accurate determination of molecular interfaces in insoluble and noncrystalline protein-protein complexes. The method relies on the measurement of intermolecular (13)C-(13)C distances in mixtures of [1-(13)C]glucose- and [2-(13)C]glucose-labeled proteins. We have applied this method to Parkinson's disease-associated α-synuclein fibrils and found that they are stacked in a parallel in-register arrangement. Additionally, intermolecular distance restraints for the structure determination of the fibrils at atomic resolution were measured.     

Constant-time through-bond 13C correlation spectroscopy for assigning protein resonances with solid-state NMR spectroscopy

Even as available magnetic fields for NMR continue to increase, resolution remains one of the most critical limitations in assigning and solving structures of larger biomolecules. Here we present a novel constant-time through-bond correlation spectroscopy for solids that offers superior resolution for 13C chemical shift assignments in proteins. In this experiment, the indirect evolution and transfer periods are combined into a single constant time interval, offering increased resolution while not sacrificing sensitivity. In GB1, this allows us to resolve peaks that are otherwise unresolved and to make assignments in the absence of multibond transfers.     

13C NMR spectroscopy of new amino protective groups

The ¹³C data of several new amino protecting groups of the urethane structure are reported. The speeds of acidolytic cleavage are correlated with the ¹³C parameters.