Sensitivity- and Gradient-Enhanced Hetero (ω1) Half-Filtered TOCSY Experiment for Measuring Long-Range Heteronuclear Coupling Constants

An enhanced version of the X(ω₁) half-filtered TOCSY experiment for measurement of long-range heteronuclear coupling constants is proposed which yields high-quality spectra with substantially increased sensitivity and resolution. The modified method features gradient-enhanced X filtering sequences, broadband homonuclear decoupling duringt₁, optional¹J_XHscaling in theF₁domain, and gradient coherence selection in combination with the sensitivity-enhanced protocol for the TOCSY transfer. These modifications extend the applicability of the method—coupling constants can be measured accurately for natural abundance samples at low concentrations and for compounds yielding complex spectra. Computer-aided analysis of E.COSY-type multiplets is applied for the determination of heteronuclear long-range coupling constants.     

Sensitivity- and Gradient-Enhanced Hetero (ω1) Half-Filtered TOCSY Experiment for Measuring Long-Range Heteronuclear Coupling Constants

An enhanced version of the X(ω₁) half-filtered TOCSY experiment for measurement of long-range heteronuclear coupling constants is proposed which yields high-quality spectra with substantially increased sensitivity and resolution. The modified method features gradient-enhanced X filtering sequences, broadband homonuclear decoupling duringt₁, optional¹J_XHscaling in theF₁domain, and gradient coherence selection in combination with the sensitivity-enhanced protocol for the TOCSY transfer. These modifications extend the applicability of the method—coupling constants can be measured accurately for natural abundance samples at low concentrations and for compounds yielding complex spectra. Computer-aided analysis of E.COSY-type multiplets is applied for the determination of heteronuclear long-range coupling constants.     

1D Determination of Long-Range Heteronuclear Coupling Constants by Gradient Enhanced SIMBA

Two¹H-detected one-dimensional gradient enhanced experiments and corresponding reference experiments are proposed for the measurement of heteronuclear long-rangeⁿJ_CHcoupling constants in organic molecules. These experiments use inverse detection and selective carbon excitation, provide excellent suppression of protons not coupled to¹³C, and are able to measure couplings from regions that are crowded in both the proton and carbon frequency ranges. The experiments are applied to determine the long-rangeⁿJ_CHcouplings in the sucrose molecule, including coupling across the glycosidic linkage. These pulse sequences are shown to provide accurate and efficient measurements of coupling constants that would be difficult or time-consuming to measure by other techniques.     

Measurement of Magnitude and Sign of Heteronuclear Coupling Constants in Transition Metal Complexes

Sets of specifically tailored E.COSY-type correlation experiments and double-quantum/zero-quantum (DQ/ZQ) experiments are presented which enable the determination of sign and size of small heteronuclear coupling constants across the metal center of transition metal complexes. For the octahedrally coordinated complexes, [Ru(TPM)(H)(CO)(PPh₃)]⁺[BF₄]⁻(1) and [Ir(TPM)(H)(CO)(CO₂CH₃)]⁺[BF₄]⁻(2), 14 of 15 and 15 of 15 possible two-bond scalar coupling constants across the metal center were measured, respectively, using¹⁵N and¹⁵N/¹³C enriched samples (TPM = tris(1-pyrazolyl)methane)). The reduced coupling constants²K_X-M-Y= 4π²²J/(hγ_Xγ_Y) were found to be positive when the coupled nuclei X and Y weretranswith respect to the metal center, and negative when the coupled nuclei were incisposition. The validity of this sign rule was verified forJ_CC,J_NN,J_PN,J_PC,J_CN,J_HP,J_HC, andJ_HNcouplings. Idiosyncracies associated with 2D NMR spectra for the sign determination of coupling constants with¹⁵N which lead to corrections for the signs ofJ_HN,J_PN, andJ_CNcouplings reported previously are discussed.     

HECADE: HMQC- and HSQC-Based 2D NMR Experiments for Accurate and Sensitive Determination of Heteronuclear Coupling Constants from E.COSY-Type Cross Peaks

Two-dimensional pulse sequences for the determination of heteronuclear long-range coupling constants are presented. The sequences are based on the HMQC/HMBC or HSQC technique with subsequent optional homonuclear I-spin transfer. However, they yield tilted cross-peak patterns displaying antiphase heteronuclear coupling constants in the projections of both dimensions, which allow accurate determination of the couplings even in cases where the linewidth is of comparable magnitude. Two characteristic pulse-sequence elements were implemented to shape theF₁domain: the first element allows an arbitrary scaling of the heteronuclear coupling splittings relative to S-spin chemical-shift differences, whereas the second element achieves homonuclear broadband decoupling among the I spins in the HMQC/HMBC experiments and thus allows purely absorptive representations of such spectra. In comparison with established (ω₁) X-half-filtered TOCSY spectra, the signal dispersion inF₁is significantly improved and largely under experimental control. Furthermore, heteronuclear couplings of (I₁, S) pairs where S is either quaternary or carries one or more I spins that do not belong to the same I-coupling network as I₁can also be measured. The implementation of pulsed field gradients results in good suppression of spectral artifacts.     

Accurate Measurements of Multiple-Bond C–H Coupling Constants from Phase-Sensitive 2D INEPT Spectra

Measurements of multiple-bond ¹³C–¹H coupling constants are of great interest for the assignment of nonprotonated ¹³C resonances and the elucidation of molecular conformation in solution. Usually, the heteronuclear multiple-bond coupling constants were measured either by the J_CH splittings mostly in selective 2D spectra or in 3D spectra, which are time consuming, or by the cross peak intensity analysis in 2D quantitative heteronuclear J correlation spectra (1994, G. Zhu, A. Renwick, and A. Bax, J. Magn. Reson. A 110, 257; 1994, A. Bax, G. W. Vuister, S. Grzesiek, F. Delaglio, A. C. Wang, R. Tschudin, and G. Zhu, Methods Enzymol. 239, 79.), which suffer from the accuracy problem caused by the signal-to-noise ratio and the nonpure absorptive peak patterns. Concerted incrementation of the duration for developing proton antiphase magnetization with respect to carbon-13 and the evolution time for proton chemical shift in different steps in a modified INEPT pulse sequence provides a new method for accurate measurements of heteronuclear multiple-bond coupling constants in a single 2D experiment.     

Recoupled Polarization-Transfer Methods for Solid-State H–C Heteronuclear Correlation in the Limit of Fast MAS

An in-depth account of the effects of homonuclear couplings and multiple heteronuclear couplings is given for a recently published technique for ¹H–¹³C dipolar correlation in solids under very fast MAS, where the heteronuclear dipolar coupling is recoupled by means of REDOR π-pulse trains. The method bears similarities to well-known solution-state NMR techniques, which form the framework of a heteronuclear multiple-quantum experiment. The so-called recoupled polarization-transfer (REPT) technique is versatile in that rotor-synchronized ¹H–¹³C shift correlation spectra can be recorded. In addition, weak heteronuclear dipolar coupling constants can be extracted by means of spinning sideband analysis in the indirect dimension of the experiment. These sidebands are generated by rotor encoding of the reconversion Hamiltonian. We present generalized variants of the initially described heteronuclear multiple-quantum correlation (HMQC) experiment, which are better suited for certain applications. Using these techniques, measurements on model compounds with ¹³C in natural abundance, as well as simulations, confirm the very weak effect of ¹H–¹H homonuclear couplings on the spectra recorded with spinning frequencies of 25–30 kHz. The effect of remote heteronuclear couplings on the spinning-sideband patterns of CH_n groups is discussed, and ¹³C spectral editing of rigid organic solids is shown to be practicable with these techniques.     

Accurate and Precise Measurement of Heteronuclear Long-Range Couplings by a Gradient-Enhanced Two-Dimensional Multiple-Bond Correlation Experiment

We propose a phase-sensitive gradient-enhanced two-dimensional heteronuclear multiple-bond correlation (psge-2D HMBC) experiment for speedy, accurate, and precise measurement of²J_CHand³J_CH. The experiment does not suppress one-bond correlations. Rather, the value of a desired long-rangeJ_CHis obtained from the pertinent cross-peak pattern in the HMBC spectrum, using the corresponding¹J_CHcorrelation pattern as reference. The application of the proposed experiment is illustrated for the trisaccharide raffinose.     

More line narrowing in TROSY by decoupling of long-range couplings: shift correlation and JNC′ coupling constant measurements

Since the introduction of RDCs in high-resolution NMR studies of macromolecules, there is a growing interest in the development of accurate, and sensitive methods for determining coupling constants. Most methods for extracting these couplings are based on the measurement of the splitting between multiplet components in J-coupled spectra. However, these methods are often unreliable since undesired multiple-bond couplings can considerably broaden the multiplet components and consequently make accurate determination of their position difficult. To demonstrate one approach to this problem, G-BIRD^(r) decoupled TROSY sequences are proposed for the measurement of ¹J_NH and ¹J_NC′ coupling constants. Resolved or unresolved splittings due to remote protons are removed by a G-BIRD^(r) module employed during t₁ and as a result, spectra with narrow, well-resolved peaks are obtained from which heteronuclear one-bond couplings can be accurately measured. Moreover, introduction of a spin-state-selective α/β-filter in the TROSY sequence allows the separation of the ¹J_NC′ doublet components into two subspectra which contain the same number of peaks as the regular TROSY spectrum. The ¹J_NC′ couplings are obtained from the displacement between the corresponding peaks in the subspectra.     

Conformational Equilibria in Polypeptides. I. Determination of AccurateJHCCoupling Constants in Antamanide by 2D NMR Multiplet Simulation

High-precision heteronuclear three-bond coupling constants including³J(H^N, C′),³J(H^N, C^β),³J(H^α,C^γ), and³J(H^β, C′) are determined for the nonproline residues in uniformly¹³C-enrichedantamanide,cyclo-(-Val¹-Pro²-Pro³-Ala⁴-Phe⁵-Phe⁶-Pro⁷-Pro⁸-Phe⁹- Phe¹⁰-), using quantitative numerical 2D NMR spectrum evaluation based on the product-operator formalism. The experimental basis comprises two-dimensional¹H,¹³C-heteronuclear relayed E.COSY spectra [J. M. Schmidt, R. R. Ernst, S. Aimoto, and M. Kainosho,J. Biomol. NMR6,95 (1995)], the multiplet patterns of which are subjected to iterative least-squares 2D multiplet-simulation procedures. Accuracy and precision of the spectrum fit are assessed byFstatistics and analysis of variances (ANOVA) leading to confidence intervals for the optimized spin-system parameters. The long-range J coupling constants obtained and their standard deviations provide the experimental foundation for a later detailed analysis of φ and χ₁dihedral-angle equilibrium conformations contributing to the flexible peptide structure.     

苯及其取代物中孪位偶合常数2JCCH的 最大键级杂化轨道研究

在前文工作的基础上,本文结合MNDO分子轨道方法和最大键级杂化轨道方法研究得到了计算苯及其取代物中孪位偶合常数     

A hydrogen bonded complex C2H2…HBr isolated and characterized in the gas phase using pulsed-jet,Fourier transform microwave spectroscopy

A complex formed from ethyne and hydrogen bromide has been isolated and characterized by using a fast-mixing nozzle in conjunction with a pulsed-jet, Fourier transform microwave spectrometer. Any possible chemical reaction between the two components when mixed in the usual way was thereby precluded. The rotational constants A, B and C, the quartic centrifugal distortion constants δ_J, δ_Jk and δ_J and the Br nuclear hyperfine coupling constants X_aa and X_bb—X_cc (nuclear quadrupole) and M_bb (spin-rotation) were determined for each of the five isotopomers C₂H₂…H⁷⁹Br, C₂H₂…H⁸¹ Br, C₂H₂…D⁷⁹Br, C₂H₂…D⁸¹Br and C₂D₂…H⁷⁹Br. Interpretations of the spectroscopic constants show that the complex is planar and T shaped in the equilibrium conformation, with HCCH internuclear axis forming the cross of the T and the HBr internuclear axis lying along the C₂ axis of C₂H₂. The H of HBr is closer to the centre (?) of the π bond of ethyne, and therefore HBr is involved in a hydrogen bond to the π system, in which the distance of H from ? is r (?…H) = 2.469(1) A. The intermolecular stretching force constant is estimated as kσ = 5.38(2) Nm^?1 for the species involving a hydrogen bond (C₂H₂…H⁷⁹Br, C₂H₂…H⁸¹Br and C₂D₂…H⁷⁹Br) while this quantity increases to kσ = 5.68(2)Nm^?1 for those complexes bound through a deuterium bond (C₂H₂…D⁷⁹Br and C₂H₂…D⁷⁹Br). The opportunity is taken to consider similarities in the properties of complexes within the two series B…HBr and B…HC1 for a range of Lewis bases B, including B = C₂H₂. Some family relationships are identified in the two series.     

Use of the satellites of the 13C and 125Te obtained in the P.M.R. spectra of nematic liquid crystal mesophases

The complete r ^α structure of tellurophene has been determined from proton magnetic resonance spectra, including ¹³C and ¹²⁵Te satellites, obtained in three thermotropic nematic mesophases. The molecular structures observed in the different solvents have been compared both to each other and with microwave data, and discussed with respect to possible solute-solvent interactions. Isotropic P.M.R. spectra and the relative heteronuclear sub-spectra have also been analysed in order to obtain all the J_ij indirect coupling constants. Absolute signs were determined for ² J _TeH and ³ J _TeH. The anisotropic contribution to the Te-H indirect couplings was found to be negligible.     

High resolution proton magnetic resonance of selenophene

The high resolution proton magnetic resonance of selenophene has been studied at 100 Mc/s. The spectrum was analysed as an A₂X₂ system and the following coupling constants were obtained: J ₂₃ = 5·35, J ₂₄ = 1·05, J ₃₄ = 3·56, J ₂₅ = 2·47 c.p.s. Coupling was also observed between ⁷⁷Se and the α and β protons and coupling constants of 48 c.p.s. (α) and 9·5 c.p.s. (β) were obtained. The τ values were 2·30 (α) and 2·88 (β). The results are compared with values available for furan, pyrrole and thiophene and the conclusion is drawn that selenophene is probably a planar molecule.     

J Coupling between C and H across Hydrogen Bonds in Proteins

Two new two- or three-dimensional NMR methods for measuring ^3hJ_C′N and ^2hJ_C′H coupling constants across hydrogen bonds in proteins are presented. They are tailored to suit the size of the TROSY effect, i.e., the degree of interference between dipolar and chemical shift anisotropy relaxation mechanisms. The methods edit 2D or 3D spectra into two separate subspectra corresponding to the two possible spin states of the ¹H^N spin during evolution of ¹³CO coherences. This allows ^2hJ_C′H to be measured in an E.COSY-type way while ^3hJ_C′N can be measured in the so-called quantitative way provided a reference spectrum is also recorded. A demonstration of the new methods is shown for the ¹⁵N,¹³C-labeled protein chymotrypsin inhibitor 2.     

Nuclear magnetic double resonance for a spin system subject to scalar relaxation of the second kind

The equation of motion of a reduced spin density operator characterizing the state of a group of spins scalar coupled to a second group of spins which are either relaxing rapidly and/or irradiated with a strong radiofrequency field is derived within the framework of the non-viscous liquid approximation. An analytic expression for the spectral densities entering this equation is derived within the extreme narrowing approximation for the case in which the relaxation rates of the nuclei in the second group of spins are much greater than any scalar couplings between them. The resultant equation is used to discuss the conditions needed in a high resolution nuclear magnetic double resonance experiment to decouple a spin-½ nucleus (A) from a rapidly relaxing quadrupolar nucleus (X) in the fast relaxation limit, defined by |2πJ_AX T _1X | ? 1, in which the A resonance consists of a single broadened line. It is found that, if (ω _A - ω _X )² ? T _1X ^-2 so that the x and y components of the J_AX coupling can be neglected, decoupling, in the sense that all broadening of the A resonance is removed, occurs only when the strength of the B ₂ field be such that γ _X ² B ₂ ² ? T _1X ^-2 and that the conventional decoupling criterion γ _X ² B ₂ ² ? 4π² J_AX ² is not applicable. However, if the x and y components of the A-X scalar coupling cannot be neglected because the condition (ω _A - ω _X )² ? T _1X ^-2 is not satisfied, it is found that the X spin can never be entirely decoupled. It is shown that such double resonance experiments enable both the coupling constants between spin-½ and quadrupolar nuclei and the relaxation times of the latter nuclei to be extracted in situations in which measurement of the half-widths of spin-1/2 transitions can only yield the product J_AX ² T _1X in the absence of irradiation of the quadrupolar nuclei.     

Measurement of Small One-Bond Proton–Carbon Residual Dipolar Coupling Constants in Partially Oriented C Natural Abundance Oligosaccharide Samples: Analysis of Heteronuclear JCH-Modulated Spectra with the BIRD Inversion Pulse

Two 2D J-modulated HSQC-based experiments were designed for precise determination of small residual dipolar one-bond carbon–proton coupling constants in ¹³C natural abundance carbohydrates. Crucial to the precision of a few hundredths of Hz achieved by these methods was the use of long modulation intervals and BIRD pulses, which acted as semiselective inversion pulses. The BIRD pulses eliminated effective evolution of all but ¹J_CH couplings, resulting in signal modulation that can be described by simple modulation functions. A thorough analysis of such modulation functions for a typical four-spin carbohydrate spin system was performed for both experiments. The results showed that the evolution of the ¹H–¹H and long-range ¹H–¹³C couplings during the BIRD pulses did not necessitate the introduction of more complicated modulation functions. The effects of pulse imperfections were also inspected. While weakly coupled spin systems can be analyzed by simple fitting of cross peak intensities, in strongly coupled spin systems the evolution of the density matrix needs to be considered in order to analyse data accurately. However, if strong coupling effects are modest the errors in coupling constants determined by the “weak coupling” analysis are of similar magnitudes in oriented and isotropic samples and are partially cancelled during dipolar coupling calculation. Simple criteria have been established as to when the strong coupling treatment needs to be invoked.     

Molecular orbital calculations of hydrogen-hydrogen coupling constants in substituted cyclopropanes

The SCF finite perturbation theory of indirect spin-spin coupling constants was applied to ² J _HH and ³ J _HH in a series of mono-substituted cyclopropanes, C₃H₅X. where X=Li, H, CN, CO₂H, COCH₃, NH₂, OH and F. The calculated results are qualitatively consistent with the experimental patterns exhibited by ² J _HH and ³ J _HH. Problems were encountered for substituents for which through-space interactions may be important, particularly if X is a -I^- substitutent.     

Mean energies of hybridized valence states

A method of determining the relative signs of J _AB and J _BX in nuclear spin systems of the type AB₂X _q is described and applied to the AB₂X₃ proton spin system of 2-butyn-1-ol. Variation of the relative chemical shift v _A—v _B (through changes in temperature or in the composition of the solvent) causes certain pairs of ‘labile’ transitions in the X spectrum to coalesce on one side or other of the X spectrum. The information about the relative signs of the coupling constants is obtained by noting the two different critical values of v _A-v _B at which these coalescences occur. Since the method remains applicable even if the third coupling constant J _AX is vanishingly small, it is particularly useful in the determination of the signs of long-range spin coupling constants where the usual double resonance methods often break down. It is concluded that the long-range coupling J(H-C-C≡C-C-H) of 2-butyn-1-ol has a positive sign.     

Proton Strong Coupling in Heteronuclear Systems. Theoretical and Experimental Evaluation in Quantitative Analysis of SQC NOESY Spectra of Biopolymers

Hydrogen-detected NOE-relayed heteronuclear correlation via single-quantum coherence spectra (SQC-NOESY) may be affected by second-order anomalies when |(ω_I ± πJ_IS) − ω_I′| ≤ 20πJ_II′, where I and I′ are protons and S the heterospin coupled only to I. When the above condition applies, coherences of type S_αI_β (α, β = x, y, z) undergo oscillatory transfer to S_αI′_β coherences without the need for any pulse perturbation. Thus the INEPT transfers, as well as the t₁ precession step of the SQC-NOESY scheme, will no longer be effective in sorting out only antiphase or transverse coherences of the proton spin directly coupled to the heteronucleus S. In practice the process leads to measurable amplitude contributions to both auto- and cross-peak volumes, despite the fact that the effects developed during the INEPT steps are often negligible and the theoretical net transfer expected from the t₁ evolution is null. Since during t_m and t₂ (provided heteronuclear decoupling is applied) no effect is expected from the direct-heteronuclear-coupling operator, any strong-coupling contribution arising in these conditions can be computed using the specific parameters of the system under investigation. Thus auto- and cross-peak volumes can be corrected before internuclear distances are evaluated, In natural-abundance or slightly enriched ¹H-¹³C biopolymer systems, assuming J_II′/J_IS = 0.06 and t₁ = 10-20 ms, a correction amounting to 0-7% of the auto- and cross-peak volume sum should be applied to the connectivities of the strongly coupled pair, depending on Δω_I.