Sensitivity- and Gradient-Enhanced Heteronuclear Coupled/Decoupled HSQC–TOCSY Experiments for Measuring Long-Range Heteronuclear Coupling Constants

A pulsed field gradient version of the sensitivity-enhanced 2D HSQC–TOCSY experiment is proposed for measurement of long-range heteronuclear coupling constants. The coupling constants are obtained by computer-aided analysis of mixed-phase multiplets with and without the heteronuclear splitting. Generation of pure phase data is not required. Since large¹J_XHandJ_HHcouplings are used for coherence transfer, smallⁿJ_XHcan be measured accurately, which could be difficult to obtain from purely heteronuclear polarization transfer experiments.     

Spatial localization in the heteronuclear coherence transfer from scalar order

The spatial localization of the heteronuclear coherence transfer from scalar order was analyzed for a rectangular and an adiabatic remagnetization contact pulses. The transferred coherence amplitudes with and without localization were evaluated for different experimental conditions and an AX-1/2 spin-pair system. The use of this method for¹³C localized spectroscopy is discussed.     

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.     

Spatially localized heteronuclear scalar order

The spatial localization of the heteronuclear scalar order produced for an AX _N spin-1/2 system by adiabatic demagnetization in the rotating frame is discussed. The density operator formalism is used for an exact evaluation of the produced scalar order in the presence of the main field gradient. A pulse sequence to record the scalar order profile is presented.     

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.     

Anisotropic Rayleigh scattering

Coherence transfer between heteronuclear spin species in the rotating frame is analysed in view of application to heteronuclear two-dimensional cross-correlation spectroscopy in liquids. The general formalism and applications to simple spin systems are presented. In particular, the longitudinal and transverse coherence transfer functions are evaluated and discussed in terms of transfer efficiency, oscillation frequencies, phase anomalies and multiplet effects.     

Slice selection in solids by localized double-quantum coherence transfer

Double-quantum heteronuclear coherence transfer in solids shows a strong spatial dependence when performed in the presence of a magnetic field gradient. This is a direct consequence of the off-resonance sensitivity of the coherence transfer process and represents a new principle for localized NMR spectroscopy of quadrupole nuclei in solids. Since the slice-selective excitation is achieved simultaneously to the cross-polarization, the suggested pulse sequences avoid the use of shaped pulses, the application of which is problematic in solids. In the present work, the localization efficiency of this new slice-selection principle was analyzed in dependence on the experimental parameters for a spin system consisting of abundant spin-1/2 and rare spin-1 nuclei. The resulting slice profiles and the calculated dependences of the slice thickness for the basic coherence transfer procedures are discussed on the example of¹H?²H in monodeuterated benzene. The proposed method opens the possibility of volume-selective investigations of the structure and dynamics of materials using the well-established methodology of deuteron-NMR spectroscopy.     

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.     

Comparison of the Effects of Different <Superscript>19</Superscript>F <Emphasis Type="Italic">π</Emphasis> Pulses on the Sensitivity and Phaseability of the <Superscript>19</Superscript>F-<Superscript>13</Superscript>C HSQC

The ¹⁹F-¹³C heteronuclear single quantum coherence (HSQC) experiment is vital for the structural elucidation of polyfluorinated organic species, yet its sensitivity and phaseability are limited by difficulties in uniform excitation of the widely disperse ¹⁹F spectral window. Adiabatic pulses of different types have previously been employed to generate effective π pulses for inversion and refocussing, but a systematic comparison of various adiabatic and other inversion pulses has not been published. In this work, it was observed that the use of a broadband inversion pulse (BIP) during the t ₁ evolution period resulted in properly phaseable spectra for experiments optimized to detect ¹ J _CF, in contrast to CHIRP or WURST adiabatic pulses. For the INEPT and reverse-INEPT transfer segments of the HSQC, optimal sensitivity for resonances distant from the transmitter frequency was afforded by optimized universal rotation (BURBOP) or CHIRP pulses. In HSQC experiments with delays optimized for two-bond correlations, only the use of BURBOP pulses in INEPT and reverse-INEPT sequences afforded spectra cleanly phaseable across the F ₂ and F ₁ spectral windows. This observation is supported by off-resonance pulsed field gradient spin-echo experiments.     

Modified BIRD Pulses and Design of Heteronuclear Pulse Sequences

A simple modification of BIRD pulses, which removes the inversion effect on an X nucleus and retains a chemical-shift evolution of the heterospin, is proposed. This modification extends the possibilities for manipulation of heteronuclear spin systems. The concept of group-selective π pulses was shown to be useful in systematic design of pulse sequences in heteronuclear NMR spectroscopy. This approach was illustrated for the proton-carbon chemical-shift-correlated experiment, a new method for measurement of ¹J(C, H) from correlated spectra.     

Evidence for Quantum Beats generated by a photo-induced charge transfer reaction

Photo-initiated charge transfer reactions in solution proceed via a radical pair intermediate which in most cases will separate on a time scale of several nanoseconds. The non-adiabicity of this process is the source of coherences in the spin system. A quantitative discussion of the observability of the resulting quantum beats using Fourier-transform EPR is presented. We also show experimental evidence for the predicted quantum beats using the photo-initiated charge transfer reaction of Zn-Tetraphenylporphyrin and benzoquinone in 2-propanol. The analysis of the beat frequency leads to an estimate of the exchamge couplingJ≈2.6·10⁶ s^?1 of the Coulomb-coupled radical pair.     

New methods for the measurement of NHCαH coupling constants in 15N-labeled proteins

Several new techniques, requiring ¹⁵N incorporation, are described for measuring NHCαH J couplings in proteins. ¹H-detected heteronuclear ¹H¹⁵N multiple-quantum correlation spectra retain the homonuclear J coupling information. Because of the favorable relaxation properties of ¹⁵N¹H zero- and double-quantum coherences, significant line narrowing occurs in the F₁, dimension compared to the regular NH ¹H linewidth, permitting high accuracy measurements of J splittings, even for medium sized proteins. Methods for convenient analysis of such coupling information are described, correcting for linewidth and dispersion mode contributions. The new approach is demonstrated for the protein staphylococcal nuclease (18 kDa), complexed with pdTp and calcium.     

Resolution enhancement in heteronuclear two-dimensional spectroscopy by realignment of coherence transfer echoes

The resolution in heteronuclear two-dimensional NMR spectroscopy is affected by coherence transfer echoes. By introducing a delay between the transfer of coherence and the signal observation, the resolution in the crucial ω₁ dimension can be enhanced. This procedure removes the symmetry of the multiplet structure, and an additional pulse is required to realign the signals in the two-dimensional spectra. The methods are illustrated by applications to the indirect observation of scalar proton-proton couplings through phosphorus-31 in phosphothreonine.     

Dynamics of resonant energy transfer in a cold Rydberg gas

We investigate excitation transfer and migration processes in a cold gas of rubidium Rydberg atoms. Density-dependent measurements of the resonant population exchange for atoms initially excited into the 32P_3/2(|m_J|=3/2) state are compared with a Monte Carlo model for coherent energy transfer. The model is based on simulations of small atom subensembles involving up to ten atoms interacting via coherent pair processes. The role of interatomic mechanical forces due to the resonant dipole-dipole interaction is investigated. Good agreement is found between the experimental data and the predictions of the model, from which we infer that atomic motion has negligible influence on the energy transfer up to Rydberg densities of 10⁸ cm^-3, that the system has to be described in terms of many-body dynamics, and that the energy transfer preserves coherence on microsecond timescales.     

Cyclic and Slice-Selective J Cross Polarization for Heteronuclear Editing and Localized NMR

This work has two objectives. First, heteronuclear editing was considered, with particular aim at proton-detected NMR spectroscopy of rare nuclei. The second aspect refers to localized heteronuclear NMR spectroscopy, implying the editing option. The common basis of both techniques is J cross polarization (or coherent rotating-frame polarization transfer). In the presence of magnetic field gradients this process is slice selective and is introduced as a principle for localized liquid-state NMR, particularly of heteronuclear spins. Heteronuclear editing of NMR lines is performed by the aid of a cyclic cross-polarization pathway. Both methods can be combined to a technique for localized proton-detected NMR spectroscopy of rare nuclei. Test experiments demonstrating the slice selectivity of the transfer process and the efficiency of the cyclic cross-polarization editing procedure are reported. Applications to localized homo- or heteronuclear spectroscopy and imaging are discussed.     

Study of 170,171,173Ta isotopes at high spinṡ

High-spin data are presented for ¹⁷⁰Ta and ¹⁷¹Ta for the first time, and the known level scheme of ¹⁷³Ta is extended to higher spins. Anomalies found in the crossing frequencies for the alignment of a pair of neutrons are considered. At high spin a large and constant value of J⁽²⁾, with J⁽²⁾ > J⁽¹⁾, is found for two ¹⁷¹Ta bands and the implications of this are discussed. The πi_13/2 [660]1/2⁺, α = + 1/2 orbital is identified as it crosses the rotational band built on the [402]5/2⁺, α = + 1/2 orbital in ¹⁷¹Ta. Magnetic transition probabilities obtained from measured branching ratios show the effects of the aligning pair of neutrons and the spectator orbital. The data are interpreted using cranked shell-model calculations and a semiclassical vector-coupling scheme developed by Donau and Frauendorf.     

1,2-二氢-4-苯基(2H)二氮杂萘酮的NMR谱研究

报道了1,2-二氢-2-(4-氨基苯基)-4-[4-(4-氨基苯氧基)苯基](2H)二氮杂萘-1-酮的合成,并用2D同核化学位移相关谱(¹H,¹H-COSY),2DROESY谱,2D异核¹³C-¹H相关谱和2D异核¹³C-¹H多键相关谱(HMBC)对该化合物的¹H和¹³C NMR谱进行了完整归属,证明了此化合物是含二氮杂萘酮结构,而不是二氮杂萘醚结构.     

Complete NMR fingerprints of proteins in H2O solution without solvent presaturation. An application of two-dimensional longitudinal two-spin-order spectroscopy

A method for obtaining complete two-dimensional ¹H NMR amide proton-C_α proton J-connectivity maps for proteins (the “fingerprint”) is describe. The method relies on the conversion of antiphase single-quantum coherence into longitudinal two-spin order which is made observable by utilizing a selective solvent-suppression readout pulse. Since irradiation of the solvent resonance is unnecessary in this experiment, J connectivities are observable even for protons resonating exactly at the solvent frequency. Thus, a complete protein fingerprint is obtained at a single set of experimental conditions. The method is demonstrated for a 75-amino-acid protein.     

PRAWN: mixing sequences for selective heteronuclear J cross polarization

In this work, we present a family of pulse sequences for selective heteronuclear J cross-polarization (JCP), which we have developed especially for indirect 13C imaging using JCP, for example in the CYCLCROP environment. The sequences are straightforward to implement and operate reliably. Results of an average Hamiltonian analysis are given for the basic sequence, which we term PRAWN (pulsed rotating frame transfer sequence with windows). It is shown experimentally that the pulse sequence, which operates efficiently with low RF duty cycles down to a few percent, has a useful tolerance range to absolute Hartmann-Hahn mismatch and generates coherence transfer spectra in close correspondence with the JCP average Hamiltonian. Computer simulation of the performance of the basic sequence on a heteronuclear spin-(1/2) AX system is also presented. The mismatch compensation of PRAWN may be markedly enhanced further by issuing a pi pulse to each spin halfway through the basic PRAWN train and in phase quadrature to it. A simple analysis of this modified sequence, PRAWN-pi, is given under conditions of mismatch and off-resonance irradiation.