Long-range Heteronuclear Coupling Constants Research Articles

I-spin n-quantum coherences in I nS spin systems employed for E.COSY-type measurement of heteronuclear long-range coupling constants in NMR

We propose a general NMR pulse scheme for conversion of I-spin multiple-quantum coherence to I-spin single-quantum coherence in I nS spin 1 2 systems that exploit large single-bond J IS coupling constants and employs rf pulses on the S spin which, however, is fully restored to its original state after the process. This is essential for application in multidimensional pulse experiments for E.COSY-type extraction of heteronuclear long-range coupling constants. The new pulse scheme is derived in a systematic way, by first considering an energy level diagram in which selective rotations are required to achieve the desired effect and then deriving nonselective pulse sequences that produce the same effect.

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.

Excitation-Sculptured Indirect-Detection Experiment (EXSIDE) for Long-Range CH Coupling-Constant Measurement

A new pulse sequence (excitation-sculpturedindirect-detectionexperiment, EXSIDE) for measuring long-range heteronuclear coupling constants has been proposed. The experiment is a band-selective variant of the gradient–HSQC experiment. The initial polarization transfer from1H to13C and the refocusing of the antiphase1H magnetization after the back-transfer from13C to1H is unmodulated by any passive homonuclear couplings. This is achieved by a variation of the double pulse-field-gradient spin-echo technique. The sequence provides pure absorptive lineshapes, and long-range heteronuclear coupling constants areJ-scaled and measured along theF1(carbon) dimension. Unlike methods where the heteronuclear couplings are extracted along theF2(proton) dimension with overlapping homonuclear couplings, in the EXSIDE spectrum the active coupling constant is measured with no interference from any passive couplings. As long as one can group subsets of resonances that are not coupled to each other, the method can be used with multifrequency selection. The applicability of the experiment to measurenJCHis demonstrated using strychnine. The unique chemical-shift region of anomeric protons in oligosaccharides and the H1′ protons in oligonucleotides makes this method easily applicable for measuring long-range CH coupling constants across the glycosidic bond.

A Method for Measurement of Long-Range Heteronuclear Coupling Constants from 2D HMQC Spectra

A Method for Measurement of Long-Range Heteronuclear Coupling Constants from 2D HMQC Spectra

A Sensitivity-Enhanced Method for Measuring Heteronuclear Long-Range Coupling Constants from the Displacement of Signals in Two 1D Subspectra

A Sensitivity-Enhanced Method for Measuring Heteronuclear Long-Range Coupling Constants from the Displacement of Signals in Two 1D Subspectra

Accurate Measurement of Heteronuclear Long-Range Coupling Constants from 1D Subspectra in Crowded Spectral Regions

A general review of novel NMR methods to measure heteronuclear long-range proton–carbon coupling constants (nJCH; n > 1) in small molecules is made. NMR experiments are classified in terms of NMR pulse scheme and cross-peak nature. A discussion about simplicity, general applicability and accuracy for each particular NMR experiment is presented and exemplified. Important aspects such as the sign determination and measurement of very small coupling values involving protonated and non-protonated carbons as well as the complementarity between different experiments are also discussed. Finally, a compilation of applications in structural and conformational analysis of different types of molecules since 2000 is surveyed.

One-Dimensional Inverse-Detected Methods for Measurement of Long-Range Proton-Carbon Coupling Constants. Application to Saccharides

Abstract Several inverse-detected one-dimensional methods for measurement of long-range heteronuclear coupling constants are proposed. A double-INEPT-type polarization transfer is used in these sequences. An initial transfer via one-bond couplings is followed by modulation of the single-quantum carbon magnetization by long-range heteronuclear couplings and a final back-transfer of the carbon magnetization to protons. Multiplets in pure antiphase with respect to the heteronuclear long-range couplings and additional in-phase splitting from proton-proton couplings are observed. Selection of a particular isotopomer, the subspectrum of which is coadded constructively during the course of the experiment, is achieved by a combined use of selective pulses and chemical-shift-selective filters, both applied in the 13C or the 1H frequency domains. Modifications are proposed which remove the undesirable effect of one-bond and passive long-range proton-carbon coupling constants. Signal intensities are analyzed theoretically and compared experimentally for various experimental schemes. The methods are illustrated on a peracetylated β-cyclodextrin (Mr = 2016 g/mol). The emphasis is placed on the measurement of interglycosidic heteronuclear long-range couplings, which proved to be useful in the conformational analysis of saccharides.

Characterization of the extent of internal motions in oligosaccharides.

A detailed investigation has been undertaken on the extent and nature of torsional fluctuations about the glycosidic linkage of the model disaccharide Man alpha 1-3Man alpha 1-OMe. In particular, we sought to determine whether the three nuclear Overhauser effects and the two long-range heteronuclear 3JCH spin coupling constants measurable across the glycosidic linkage were consistent with a single conformation or multiple conformations about that linkage. Within experimental error, we have found that these five parameters can be interpreted in terms of a single, rigid geometry. Alternatively, the data are also consistent with a model in which the glycosidic torsional angles exhibit significant but restricted fluctuations about the global minimum energy conformation. Evidence from restrained molecular dynamics simulations both in vacuo and with explicit inclusion of solvent water and from 13C relaxation measurements upon an oligomannose glycan in covalent association with protein suggests that the latter model is the most accurate representation of the conformational behavior of oligosaccharides in solution.

Heteronuclear Long-Range Coupling Constants from Inverse 2D NMR Spectra Dispersed by the Heteronuclear Chemical Shift

Heteronuclear Long-Range Coupling Constants from Inverse 2D NMR Spectra Dispersed by the Heteronuclear Chemical Shift

Z-Filtered Heteronuclear Coupled-HSQC-TOCSY Experiment as a Means for Measuring Long-Range Heteronuclear Coupling Constants

z-Filtered Heteronuclear Coupled-HSQC-TOCSY Experiment as a Means for Measuring Long-Range Heteronuclear Coupling Constants

Measurements of heteronuclear long-range coupling constants from inverse homonuclear 2D NMR spectra

Measurements of heteronuclear long-range coupling constants from inverse homonuclear 2D NMR spectra

New methods for measurement of long-range proton-carbon coupling constants in oligosaccharides

The geometry about the glycosidic l inkage plays a crucial role in the conformational analysis of oligosaccharides. Currently, most interglycosidic constraints are obtained from proton-proton NOE measurements. However, as has been pointed out ( 1, 2)) these spectral parameters can often be interpreted in terms of more than one set of conformer populations. Additional experimental constraints are therefore needed in order to allow for a more accurate description of saccharide conformations. The use of interglycosidic heteronuclear coupling constants ( 3Jn,.coc. and 3Jn,,coc,.) to determine the values of torsion angles Cp and + about the glycosidic l inkage is well known (3)) but their measurement (4, 5) has been lim ited by the low natural abundance of 13C and its low sensitivity. The introduction of inverse experiments, with the corresponding gain in sensitivity, has resulted in much wider interest in the measurement and interpretation of “J cn coupling constants (6-13). Most of the proposed experiments are, however, designed for the structural analysis of proteins (8-13) and usually performed on isotopically enriched samples. Recently, two different one-dimensional inverse-detected schemes for the evaluation of nJCH coupling constants of saccharides with natural abundance 13C have been proposed ( 14, 15). In the present Communicat ion we describe a novel two-dimensional inverse-detected NMR technique for the measurement of long-range heteronuclear coupling constants. A convenient strategy for the determination of interglycosidic 3JHcin saccharides has been developed. The methodology is based upon the pulse sequences shown in F ig. 2. The method is illustrated on a mode l disaccharide, methyl-b-xylobioside, 1, the ‘H NMR spectrum of which is shown in F ig. 1. The proposed experiment can be characterized as a heterocorrelated experiment optimized for nJCH in which single-quantum 13C magnetization ( 16) evolves during the evolution period t, of a two-dimensional experiment. The initial building block of the pulse sequence (Fig. 2a) is a refocused INEPT ( 17) optimized for ’ JCH. The 13C magnetization further evolves during the period A2 only under the effect of “JCH.

Band-selective heteronuclear decoupling using shaped pulses as an aid in measuring long-range heteronuclear coupling constants

Band-selective heteronuclear decoupling using shaped pulses as an aid in measuring long-range heteronuclear coupling constants

Assessment of a method for the measurement of long-range heteronuclear coupling constants

Recently ( J. Magn. Reson. 85, 111–131, 1989) we described a method for the convenient measurement of long-range heteronuclear coupling constants using a combination of a proton-detected two-dimensional shift-correlation spectrum and a simple data-fitting procedure. This method enables accurate values of the long-range couplings to be extracted from the highly distorted cross peaks in the two-dimensional spectrum. In this paper the reliability of the method is investigated to establish likely error limits on the values of the couplings. The effects of strong coupling, isotopic labeling, phase variations, thermal noise, and t 1 noise are all considered. It is concluded that the procedure is robust with respect to deviations of the spectra from the ideal and, for typical experimental data, yields values of coupling constants which have errors of less than 10%.

1H-detected measurements of long-range heteronuclear coupling constants. Application to a trisaccharide

1H-detected measurements of long-range heteronuclear coupling constants. Application to a trisaccharide

Selective inverse multiple bond analysis. A simple 1D experiment for the measurement of long-range heteronuclear coupling constants

The general concept of selective excitation is well established and the methodology has been utilized on numerous occasions in a wide range of applications. An interesting application of selective pulses has been in the development of selective analogues of two-dimensional NMR experiments. The first such effort germane to our work was that of Bax and Freeman ( 1)) in which they described the selective heteronuclear 2D J spectrum. Davis applied semiselective proton pulses to the Ha resonances of peptides (2) to establish long-range correlations to carbonyl resonances with high resolution in both frequency domains. In a paper published very shortly thereafter, Bermal and co-workers (3) described a group of two-dimensional pulse sequences intended to provide access to heteronuclear coupling constants that employed semiselective Gaussian 13C pulses. Kessler and colleagues have employed semiselective 13C pulses to improve the Fi resolution in proton-detected heteronuclear long-range correlation experiments (4). Supportive ancillary work has addressed the problem of extracting coupling constants from complex cross-peak multiplets observed in proton-detected heteronuclear correlation spectra (5, 6). The importance of measuring small heteronuclear coupling constants has not waned, as evidenced by recently reported heteronuclear two( 7, 8) and three-dimensional (9) studies designed to determine and utilize small heteronuclear coupling constants of biologically important molecules. In a departure from existing multidimensional methods, we report a simple, one-dimensional pulse sequence designed to allow the measurement of long-range heteronuclear coupling constants and to provide the means for establishing long-range heteronuclear connectivities. The earliest reports of one-dimensional analogs of two-dimensional NMR experiments were those of Berger ( IO, II), which were followed by the work of Kessler and colleagues (12, 13). In an extension of these efforts, we describe a one-dimensional analogue of the HMBC experiment ( 14)) for which we propose the acronym SIMBA (selective inverse multiple bond analysis). With this new experiment, each proton with a resolved long-range coupling to the selected carbon simultaneously appears in the resultant spectrum with an apparent

Measurement of long-range heteronuclear coupling constants

A method for the measurement and assignment of long-range heteronuclear coupling constants is described. The technique uses an established pulse sequence to generate a proton-detected two-dimensional correlation spectrum whose cross peaks are analyzed with a simple data fitting procedure. In this way, accurate values of the magnitudes of the couplings are extracted from highly distorted cross-peak multiplets in the correlation spectrum.

Accurate measurements of long-range heteronuclear coupling constants from homonuclear 2D NMR spectra of isotope-enriched proteins

Accurate measurements of long-range heteronuclear coupling constants from homonuclear 2D NMR spectra of isotope-enriched proteins

Determination and assignment of heteronuclear long-range coupling constants. Methods based on semiselective INEPT

One-dimensional methods for the determination and assignment of heteronuclear 1H-X (X = rare spin-z) coupling constants based on the semiselective polarization transfer via INEPT pulse sequence are proposed. Here the selectivity of the polarization transfer plays a positive role with respect to the sensitivity of the measurement and purity of the observed multiplets. In nonrefocused experiments the acquired antiphase multiplets enable an unambiguous assignment of long-range couplings of a preselected proton. The analysis of such multiplets is also discussed. In the refocused version the purging pulse (INEPT+) was used to provide pure in-phase multiplets. The spectral editing technique DISCO was applied to simplify the spectra and to extract the couplings from complex multiplets. Finally, the modified INEPT experiments which combine semiselective polarization transfer with selective proton decoupling are proposed.

Determination of heteronuclear long-range coupling constants by soft-H,C-COSY

Determination of heteronuclear long-range coupling constants by soft-H,C-COSY