Measurement Of Coupling Constants Research Articles

Sensitivity-enhanced IPAP-SOFAST-HMQC for fast-pulsing 2D NMR with reduced radiofrequency load

The SOFAST-HMQC experiment [P. Schanda, B. Brutscher, Very fast two-dimensional NMR spectroscopy for real-time investigation of dynamic events in proteins on the time scale of seconds, J. Am. Chem. Soc. 127 (2005) 8014–8015] allows recording two-dimensional correlation spectra of macromolecules such as proteins in only a few seconds acquisition time. To achieve the highest possible sensitivity, SOFAST-HMQC experiments are preferably performed on high-field NMR spectrometers equipped with cryogenically cooled probes. The duty cycle of over 80% in fast-pulsing SOFAST-HMQC experiments, however, may cause problems when using a cryogenic probe. Here we introduce SE-IPAP-SOFAST-HMQC, a new pulse sequence that provides comparable sensitivity to standard SOFAST-HMQC, while avoiding heteronuclear decoupling during 1H detection, and thus significantly reducing the radiofrequency load of the probe during the experiment. The experiment is also attractive for fast and sensitive measurement of heteronuclear one-bond spin coupling constants.

Density-dependent correlations between properties of nuclear matter and neutron stars in a nonlinear [formula omitted] mean-field approximation

Properties of nuclear matter and neutron stars and their density-dependent correlations are discussed in a nonlinear σ – ω – ρ mean-field approximation with self-interactions and mixed-interactions of mesons. The nonlinear interactions are renormalized as effective masses ( M N * , m σ * , m ω * , m ρ * ) and effective coupling constants ( g σ * , g ω * , g ρ * ) required by the theory of conserving approximations, which generates a thermodynamically consistent approximation essential to examine density-dependent correlations. The nonlinear σ – ω – ρ mean-field approximation becomes equivalent to Hartree approximation with effective masses and effective coupling constants, and self-consistent structure is clarified. The current nonlinear mean-field approximation is applied to neutron-rich nuclear matter which is defined as isospin asymmetric and beta-equilibrium nuclear matter. The density-dependent correlations between properties of isospin symmetric and asymmetric nuclear matter, the saturation of symmetry energy in high densities are discussed in terms of density-dependent, effective masses and effective coupling constants induced by nonlinear interactions. The accumulating data and accurate measurements of effective masses and effective coupling constants, observables in high energy density will supply significant information in order to understand and testify theoretical consistency of nuclear models.

1H-Detected IPAP DEPT-INADEQUATE and IPAP RINEPT-INADEQUATE for the measurement of long-range carbon–carbon coupling constants

The sensitivity of cryoprobes, which are rapidly becoming available, have brought about the possibility of measurement of 13C, 13C coupling constants at the natural abundance of 13C using tens rather than hundreds of milligrams of compounds. This relatively recent development lays the foundation for a more routine use of the 13C, 13C long-range coupling constants in the conformational analysis of molecules. We have designed novel 1H-detected INADEQUATE experiments optimized for long-range 13C, 13C correlations and the measurement of long-range coupling constants. These experiments incorporate refocusing of 1 J CH coupling constants prior to the formation of DQ coherences and 1H-decoupling during the long carbon–carbon evolution intervals. Such modifications significantly enhance their performance over 1H-detected INADEQUATE experiments currently in use for mapping the one-bond 13C, 13C correlations. 1H or 13C polarization is used a starting point in long-range correlation 1H-detected IPAP DEPT-INADEQUATE or RINEPT-INADEQUATE experiments. These correlation experiments were modified yielding in-phase (IP) or antiphase (AP) 13C, 13C doublets in F 1. Procedures were developed for their editing yielding accurate values of small 13C, 13C coupling constants. The methods are illustrated using mono- and disaccharide samples and compared with related 13C-detected experiments by means of the measurement of interglycosidic 13C, 13C coupling constants of a disaccharide.

Complete assignments of 1H and 13C NMR spectral data for arylnaphthalene lignan lactones

In this work we describe the complete (1)H and (13)C NMR analyses of three arylnaphtalene lignan lactones (taiwanin C, 4-methyl dehydroretrodendrin and justicidin B) using modern NMR techniques such as gCOSY, nonedited gHSQC, gHMBC and NOE experiments. Complete assignment and homonuclear hydrogen coupling constant measurements were performed.

Activation of Chlorosilanes at Ruthenium: A Route to Silyl σ-Dihydrogen Complexes

The hydrido σ-dihydrogen complex RuClH(η2-H2)(PCy3)2 (2) reacts with the chlorosilanes HSiMe3-nCln (n = 1−3) to form the corresponding silyl σ-dihydrogen complexes RuCl(SiMe3-nCln)(η2-H2)(PCy3)2 (3Me3-nCln). These complexes display a 16-electron configuration, as shown by NMR, by X-ray data in the case of 3MeCl2, and by theoretical calculations. The σ-H2 ligand in 3MeCl2 has been located by X-ray diffraction, and the H−H distance of 1.05(3) Å compares well with the value obtained by DFT/B3PW91 (1.073 Å) as well as with the value of 1.08 ± 0.01 Å derived from the measurement of the JHD coupling constant of 17.5 Hz for the deuterated isotopomer RuCl(SiMeCl2)(η2-HD)(PCy3)2. The series of model complexes RuCl(SiMe3-nCln)(η2-H2)(PMe3)2 (S3Me3-nCln) was investigated by DFT at the B3PW91 level. The most stable isomers have a structure that resembles the X-ray structure of 3MeCl2: i.e., a silyl σ-dihydrogen formulation. In the case of S3Me2Cl and S3MeCl2 a second minimum very close in energy was optimized and formulated as a hydrido σ-silane species. The influence of the number of Cl substituents on Si and their location have been analyzed. The difference between 3Me2Cl on one side and 3MeCl2 and 3Cl3 on the other side is highlighted both by NMR and DFT data and by their reactivity toward ethylene. No reaction was observed for the latter complexes, whereas reaction with 3Me2Cl produces the hydrido η2-ethylene complex RuClH(C2H4)(PCy3)2 (4). In the case of styrene, the arene complex RuCl(SiMe2Cl)(η6-C8H10)(PCy3) (5) was isolated.

13C‐detected IPAP‐INADEQUATE for simultaneous measurement of one‐bond and long‐range scalar or residual dipolar coupling constants

The sensitivity of cryoprobes, which are rapidly becoming available, means that the measurement of coupling constants involving 13C, 13C pairs at the natural abundance of 13C can now, in principle, be done by using tens rather then hundreds of milligrams of compounds. However, a robust method that would yield reliable values of small long-range carbon--carbon coupling constants is still missing. In this Communication, we describe a novel 13C-detected incredible natural-abundance double-quantum transfer experiment (INADEQUATE) experiment for simultaneous correlation of one-bond and long-range 13C- 13C pairs and the measurement of both types of coupling constants in 13C natural abundance samples. This method yields accurate values of one-bond and long-range coupling constants by manipulation of pure phase in-phase (IP) and antiphase (AP) doublets, and is referred to as 13C-detected IPAP-INADEQUATE. It is illustrated by the measurement of interglycosidic (3)J(CCOC) coupling constants in a disaccharide molecule providing important information about the conformation of the glycosidic linkage. Owing to the simplicity of INADEQUATE spectra the carbon-carbon coupling constants are particularly suitable for studies of partially oriented molecules through the measurement of carbon-carbon residual dipolar couplings (RDCs). An example of this approach is presented. We expect the method to find a variety of applications in the conformational analysis of small molecules, determination of diastereoisomers and enantiomers, and studies of molecules in aligned media.

New Karplus equations for 2JHH, 3JHH, 2JCH, 3JCH, 3JCOCH, 3JCSCH, and 3JCCCH in some aldohexopyranoside derivatives as determined using NMR spectroscopy and density functional theory calculations

The 1H– 13C coupling constants of methyl α- and β-pyranosides of d-glucose and d-galactose have been measured by one-dimensional and two-dimensional 1H– 13C heteronuclear zero and double quantum, phase sensitive J-HMBC spectra to determine a complete set of coupling constants ( 1 J CH, 2 J CH, 3 J CH, 2 J HH, and 3 J HH) within the exocyclic hydroxymethyl group (CH 2OH) for each compound. In parallel with these experimental studies, structure, energy, and potential energy surfaces of the hydroxymethyl group for these compounds were determined employing quantum mechanical calculations at the B3LYP level using the 6-311++G ∗∗ basis set. Values of the vicinal coupling constants involving 1H and 13C in the C5–C6 ( ω) and C6–O6 ( θ) torsion angles in the aldohexopyranoside model compounds were calculated with water as the solvent using the PCM method. To test the relationship between 3 J CXCH (X = C, O, S) and torsion angle C1–X ( φ) around the anomeric center, the conformations of 24 derivatives of glucose and galactose, which represent sequences of atoms at the anomeric center of C-glycosides (C–C bond), O-glycosides (C–O bond), thioglycosides (C–S bond), glycosylamines (C–N bond), and glycosyl halides (C–halogen (F/Cl) bond) have been calculated. Nonlinear regression analysis of the coupling constants 1 J C1,H1, 2 J C5,H6 R , 2 J C5,H6 S , 2 J C6,H5, 3 J C4,H6 R , 3 J C4,H6 S , 2 J H6 R,H5 , and 3 J H5,H6 R as well as 3 J CXCH (X = C, O, S) on the dihedral angles ω, θ, and φ have yielded new Karplus equations. Good agreement between calculated and experimentally measured coupling constants revealed that the DFT method was able to accurately predict J-couplings in aqueous solutions.

Experimental and theoretical investigation of NMR 2JHH coupling constant on six‐membered ring systems containing oxygen or sulfur atoms

Theoretical and experimental 2JHH coupling constants for six-membered rings containing oxygen or sulfur atoms were studied to investigate whether the 2JHH coupling constant can be used for stereoelectronic studies in heterocyclohexanes, instead of 1JCH, because it is well known that experimental measurements of 2JHH coupling constants at low temperature are much easier to determine than the corresponding 1JCH couplings. For all compounds studied here, the 2JHH coupling constants are affected by sigma*C-H antibonding occupancy together with bond angle effects. For cyclohexane and oxygen-containing compounds, the influence on the geminal coupling for Hax-C2-Heq and for X1-C2-X3 (X=O and C), bond angles are more pronounced than for the sulfur derivatives.

Synthesis, Structure, and Properties of 9-Phospha-10-silatriptycenes and their Derivatives

A variety of 9-phospha-10-silatriptycenes (PSiT) and some derivatives, such as the phosphine selenides and cis-platinum complexes, have been prepared in order to study their structures and characteristics as a phosphine ligand. The 1J(P−Se) NMR coupling constant of the phosphine selenides demonstrate the large s character of the lone pair orbital on the phosphorus atom, attributable to the small C−P−C bond angles of the PSiT framework, as confirmed by X-ray crystallography. The strong trans influence of PSiT has been determined from the 1J(Pt−C) coupling constant measurements of the cis-[PtMe2(psit)2], which is due to the increase in the P−Pt−P angle in the complexes derived from the steric repulsion between the two bulky PSiT ligands. Both the substituent on the silicon and the bridging benzene rings affect the electronic character. Electron-withdrawing substituents on the silicon atom slightly increase the s character of the phosphorus lone pair orbital, and the isopropoxy groups on the bridging benzene rings increase the trans influence.

Long‐range JCH heteronuclear coupling constants in cyclopentane derivatives. Part II

Here we report the detailed measurement of long-range heteronuclear spin-spin coupling constants, especially 2, 3JCH spin-spin couplings for eight different cyclopentane derivatives. These 2, 3JCH constants were shown to be a useful tool in the determination of the relative stereochemistry in these rings. The coupling constant measurements reported here are based on two different experiments: a 2D heteronuclear correlation experiment named G-BIRDR, X-CPMG-HSQMBC and the 2D-coupled gHSQC {1H-13C} experiment

Halogen – metal exchange of 3‐substituted 1,2‐dibromoarenes: the use of long‐range JCH coupling constants to determine regiochemistry

Regioselective halogen/metal exchange reactions were carried out on a series of 3-substituted- 1,2-dibromoarenes. Product mixtures were quenched with CO2 to form the corresponding benzoic acid analogs to facilitate HPLC and NMR analysis. Substitution at the 3-position could readily be assigned on the basis of 2D HMBC long-range correlations, while assignment at the 2-position was not as straightforward. The use of three-bond J(CH) coupling constant measurements, extracted from 1-D 1H coupled 13C experiments, were necessary to render unequivocal regio assignments.

Absence of Reverse Anomeric Effect in Furanosides

A series of conformationally restricted N-"furanosides" has been synthesized, where the carbons of the tetrahydrofuran ring are kept in one plane by a rigid norbornane skeleton, permitting only the ring oxygen to move above or below the tetrahydrofuran ring plane. This causes the substituents of the anomeric carbon to occupy a pseudoaxial or a pseudoequatorial position. On protonation of these "norbornane-furanosides" with trifluoromethanesulfonic acid, all three compounds exhibited decreasing coupling constants for the anomeric proton, indicating a shift toward the pseudoaxial conformation. The coupling constant measurements were supported by volume integration of NOESY cross-peaks, which also showed a change toward the pseudoaxial conformation upon protonation of the nitrogen. These results provide no evidence for the so-called reverse anomeric effect; on the contrary they are in full agreement with a small normal anomeric effect.

Simultaneous determination of one- and two-bond scalar and residual dipolar couplings between13C′,13Cαand15N spins in proteins

Four simple and sensitive HNCO-based methods for measurement of 1J(C'Calpha), 1J(NCalpha) and 2J(NCalpha) coupling constants in protein main chains are presented. Three of these experiments enable the simultaneous measurement of 1J(C'Calpha), 1J(NCalpha) and 2J(NCalpha) couplings. Exploitation of the E.COSY principle provides excellent dispersion of cross peaks in the resulting 3D spectra. The couplings can be retrieved with good accuracy from peak-to-peak separations. Karplus parameterizations are provided for 1J(NCalpha) and 2J(NCalpha), obtained from a nearly complete set of couplings of human ubiquitin. In addition, feasibility of the proposed methodology for measuring several residual dipolar couplings (RDCs) simultaneously is assessed.

Functional Characterization of the Flagellar Glycosylation Locus in Campylobacter jejuni 81–176 Using a Focused Metabolomics Approach

Bacterial genome sequencing has provided a wealth of genetic data. However, the definitive functional characterization of hypothetical open reading frames and novel biosynthetic genes remains challenging. This is particularly true for genes involved in protein glycosylation because the isolation of their glycan moieties is often problematic. We have developed a focused metabolomics approach to define the function of flagellin glycosylation genes in Campylobacter jejuni 81-176. A capillary electrophoresis-electrospray mass spectrometry and precursor ion scanning method was used to examine cell lysates of C. jejuni 81-176 for sugar nucleotides. Novel nucleotide-activated intermediates of the pseudaminic acid (Pse5NAc7NAc) pathway and its acetamidino derivative (PseAm) were found to accumulate within select isogenic mutants, and use of a hydrophilic interaction liquid chromatography-mass spectrometry method permitted large scale purifications of the intermediates. NMR with cryo probe (cold probe) technology was utilized to complete the structural characterization of microgram quantities of CMP-5-acetamido-7-acetamidino-3,5,7,9-tetradeoxy-L-glycero-alpha-L-manno-nonulosonic acid (CMP-Pse5NAc7Am), which is the first report of Pse modified at C7 with an acetamidino group in Campylobacter, and UDP-2,4-diacetamido-2,4,6-trideoxy-alpha-D-glucopyranose, which is a bacillosamine derivative found in the N-linked proteinglycan. Using this focused metabolomics approach, pseB, pseC, pseF, pseI, and for the first time pseA, pseG, and pseH were found to be directly involved in either the biosynthesis of CMP-Pse5NAc7NAc or CMP-Pse5NAc7Am. In contrast, it was shown that pseD, pseE, Cj1314c, Cj1315c, Cjb1301, Cj1334, Cj1341c, and Cj1342c have no role in the CMP-Pse5NAc7NAc or CMP-Pse5NAc7Am pathways. These results demonstrate the usefulness of this approach for targeting compounds within the bacterial metabolome to assign function to genes, identify metabolic intermediates, and elucidate novel biosynthetic pathways.

Selective J-resolved spectra: A double pulsed field gradient spin-echo approach

A simple method to obtain selective J-resolved spectra is presented, which relies on the refocusing properties of double pulsed field gradient spin-echoes and provides unambiguous assignment of the measured coupling constants. The proposed examples show how this method is of general applicability, and requires no more than a simple optimization strategy to produce artifact-free spectra. Examples of application include the determination of a small, long-range coupling constant (0.7 Hz) in trans-retinal and H α couplings in a tripeptide.

Accurate measurement of long-range heteronuclear coupling constants from undistorted multiplets of an enhanced CPMG-HSQMBC experiment

Here, we present a modified CPMG-HSQMBC experiment which is capable to reduce the detrimental phase twists in the “long-range” connectivity multiplets caused by proton–proton couplings. We demonstrate that concerted CPMG pulse trains applied on both nuclei in the starting CPMG-INEPT transfer step can considerably be improved by composite π pulses that compensate for pulse imperfections and off-resonance effects. Experimental optimization of the interpulse delay within the CPMG cycle was found to be crucial in order to achieve the best possible “decoupling” of homonuclear coupling modulation.

13C/15N distance determination by CPMAS NMR in uniformly13C labeled molecules

The REDOR and CPMAS techniques are applied for measuring 13C-15N dipolar coupling constants in glycine. It is shown that the selective CP or SPECIFIC CP technique removes the coherent evolution of the spin system under homonuclear 13C-13C J couplings. While the large coupling constant (approximately 900 Hz) is readily determined because of the presence of large oscillations in the CPMAS dynamics, their absence precludes the measurement of the small coupling constant (approximately 200 Hz). The experimental results and numerical simulations demonstrate that the determination of 13C-15N coupling constants of medium size (<1 kHz) by the CPMAS technique is mainly limited by the strength of the 1H decoupling field and the size of the 13C and 15N chemical shift anisotropies.

Detailed 1H and 13C NMR structural assignment of three biologically active lignan lactones

In this paper we present a complete 1H and 13C NMR spectral analysis of three lignan lactones (methylpluviatolide, dimethylmatairesinol and hinokinin) by the use of techniques such as COSY, HMQC, HMBC and J-resolved. Complete assignment and all homonuclear hydrogen coupling constant measurements were performed, providing enough data also to the confirmation of the relative stereochemistry.

Polyproline II propensities from GGXGG peptides reveal an anticorrelation with β-sheet scales

There is growing appreciation of the functional relevance of unfolded proteins in biology. However, unfolded states of proteins have proven inaccessible to the usual techniques for high-resolution structural and energetic characterization. Unfolded states are still generally conceived of as statistical coils, based on the pioneering work of Flory [(1969) Statistical Mechanics of Chain Molecules (Wiley, New York)] and Tanford [(1968) Adv. Protein Chem. 23, 121-282]. Recently, several lines of independent evidence have raised doubts about the random coil model and offer support for alternative views. Here, we show that polyproline II conformation is dominant in a host-guest peptide model AcGGXGGNH(2) (X not equal glycine), in equilibrium predominantly with beta-structure. This result is inconsistent with a random coil model and the general view that these peptides are unstructured. By calculating a set of apparent DeltaG values from the measured coupling constants of the backbone amides, we can construct a polyproline II scale that correlates negatively with beta-sheet scales.

Long‐range JCH heteronuclear coupling constants in cyclopentane derivatives

Detailed measurements of long-range heteronuclear spin-spin coupling constants, especially (2, 3)J(CH) spin-spin couplings for various cyclopentane derivatives, are reported. The measurements are based on a 2D heteronuclear correlation experiment named G-BIRD(R, X)-CPMG-HSQMBC.