Heteronuclear Decoupling Research Articles

Heteronuclear decoupling in rotating solids: Improving the efficacy of [formula omitted] symmetry-based tanh/tan adiabatic RF pulse schemes

Heteronuclear dipolar decoupling in MAS solid state NMR studies of biological systems has been investigated at moderate spinning speeds. It is shown that the efficacy of C N n ν symmetry-based decoupling schemes with tanh/tan adiabatic pulses can be significantly improved by optimising the amplitude and frequency modulation characteristics of the inversion pulses employed. The decoupling performance of the symmetry-based schemes is compared with that of the commonly used TPPM technique.

Swept-frequency two-pulse phase modulation for heteronuclear dipolar decoupling in solid-state NMR

We introduce a heteronuclear dipolar decoupling sequence for application in solid-state nuclear magnetic resonance. The sequence, called swept-frequency two-pulse phase modulation (SW f-TPPM), is based on one of the decoupling sequences, TPPM. The sequence is robust in performance with respect to various experimental parameters, such as, the pulse flip angle, pulse phase, and offset and a comparison is made with other decoupling schemes, namely TPPM, SPINAL, and XiX, on a sample of U– 13C-labelled tyrosine for magic-angle spinning speeds up to 14 kHz.

Fer expansion for effective propagators and Hamiltonians in NMR

Solving a time-dependent linear differential equation towards obtaining propagators is a central problem to NMR in general and solid-state NMR in particular. Designing of various pulse sequences and understanding of different experiments are based on the form of effective Hamiltonians and/or effective propagators. The commonly used methods for this are average Hamiltonian theory based on Magnus expansion and Floquet theory where use of van Vleck perturbation is employed. Here, we dwell upon an alternative expansion method to solving time-dependent linear differential equations called Fer expansion. We highlight the basics of this scheme and hint at its potential by considering Bloch–Siegert shift and heteronuclear dipolar decoupling in solid-state NMR.

Floquet–van Vleck analysis of heteronuclear spin decoupling in solids: The effect of spinning and decoupling sidebands on the spectrum

We investigate the effect of magic angle spinning on heteronuclear spin decoupling in solids. We use an analytical Floquet–van Vleck formalism to derive expressions for the powder-averaged signal as a function of time. These expressions show that the spectrum consists of a centerband at the isotropic frequency of the observed spin, ω 0 , and rotational decoupling sidebands at ω 0± ω 1± mω r , where ω 1 is the decoupling field strength and ω r is the rotation frequency. Rotary resonance occurs when the rotational decoupling sidebands overlap with the centerband. Away from the rotary resonance conditions, the intensity of the centerband as a function of ω r / ω 1 is simply related to the total intensity of the rotational decoupling sidebands. Notably, in the absence of offset terms it is shown that as ω 1 decreases, the centerband intensity can decrease without any associated broadening. Furthermore, the centerband width is shown to be independent of spinning speed, to second order for the effects we consider. The effects of I spin chemical shift anisotropy and homonuclear dipolar couplings are also investigated. The analytical results are compared to simulations and experiments.

Amplitude-modulated decoupling in rotating solids: A bimodal Floquet approach

This paper centers on a theoretical study of amplitude-modulated heteronuclear decoupling in solid-state NMR under magic-angle spinning (MAS). A spin system with a single isolated rare spin coupled to a large number of abundant spins is used in the analysis. The phase-alternating decoupling scheme (XiX decoupling) is analyzed using bimodal Floquet theory and the operator-based perturbation method developed by van Vleck. An effective Hamiltonian correct to second order is calculated for the spin system under XiX decoupling. The results of these calculations indicate that under XiX decoupling the main contribution to the residual line width comes from a cross-term between the heteronuclear and the homonuclear dipolar couplings. This is in contrast to continuous-wave decoupling, where the residual line width is dominated by the cross-term between the heteronuclear dipolar coupling and the chemical-shielding tensor of the irradiated spin. For high-power decoupling the method results in very good decoupling provided that certain unfavorable recoupling conditions, imposed by specific ratios of the amplitude modulation frequency and the MAS frequency, are avoided. For low-power decoupling, the method leads to acceptable decoupling when the pulse length corresponds to an integer multiple of a 2 π rotation and the rf-field amplitude is less than a quarter of the MAS frequency. The performance of the XiX scheme is analyzed over a range of values of the rf power, and numerical results that agree well with the most recent experimental observations are presented.

Heteronuclear decoupling interference during symmetry-based homonuclear recoupling in solid-state NMR

We examine the influence of continuous-wave heteronuclear decoupling on symmetry-based double-quantum homonuclear dipolar recoupling, using experimental measurements, numerical simulations, and average Hamiltonian theory. There are two distinct regimes in which the heteronuclear interference effects are minimized. The first regime utilizes a moderate homonuclear recoupling field and a strong heteronuclear decoupling field; the second regime utilizes a strong homonuclear recoupling field and a weak or absent heteronuclear decoupling field. The second regime is experimentally accessible at moderate or high magic-angle-spinning frequencies and is particularly relevant for many realistic applications of solid-state NMR recoupling experiments to organic or biological materials.

Decoupling and recoupling using continuous-wave irradiation in magic-angle-spinning solid-state NMR: A unified description using bimodal Floquet theory

The application of two or more different time-dependent coherent perturbations with, in general, incommensurable frequencies occurs quite commonly in NMR experiments. Here we develop a unified description of the entire class of experiments using bimodal Floquet theory and van Vleck-Primas perturbation theory. This treatment leads to a time-independent effective Hamiltonian in Hilbert space and can be looked at as a generalization of average Hamiltonian theory to several incommensurate time dependencies. As a prototype experiment we treat the application of continuous-wave (cw) radio-frequency irradiation in combination with magic-angle sample spinning. Practically relevant examples of this type of experiments are heteronuclear spin decoupling and recoupling experiments using cw irradiation, e.g., rotary-resonance recoupling. Perturbations up to the third order must be taken into account to explain all experimentally observed resonance conditions.

Heteronuclear decoupling under fast MAS by a rotor-synchronized Hahn-echo pulse train

A new heteronuclear decoupling mechanism under fast magic-angle spinning MAS is introduced. It is based on refocusing the coherences responsible for the dephase of low-γ nuclei ( 13C, 15N) transverse spin-polarization in the presence of strongly dipolar-coupled protons, and has the advantage that can be implemented by pulsed techniques, with all the benefits resulting from a reduced duty cycle compared with conventional decoupling by continuous rf irradiation. The decoupling efficiency of a simple rotor-synchronized Hahn-echo pulse train is analyzed both theoretically and experimentally. It was found that a substantial improvement in sensitivity and resolution can be achieved in compounds with small 1H chemical shielding parameters even at moderate sample spinning, and some interesting applications are shown. It is also shown that much faster spinning frequencies, or alternative refocusing sequences, are needed for applications on rigid organic solids, i.e., in systems with larger 1H chemical shifts.

Studies of Organically Functionalized Mesoporous Silicas Using Heteronuclear Solid-State Correlation NMR Spectroscopy under Fast Magic Angle Spinning

Highly resolved solid-state HETCOR NMR spectra between protons and low gamma nuclei ((13)C and (29)Si) can be suitably obtained on surfaces using a "brute force" (1)H-(1)H decoupling by MAS at rates > or =40 kHz. Despite a small rotor volume (<10 microL), a (1)H-(13)C HETCOR spectrum of allyl groups (AL, -CH(2)-CH=CH(2)) covalently anchored to the surface of MCM-41 silica was acquired without using isotope enrichment. The advantages of using fast MAS in such studies include easy setup, robustness, and the opportunity of using low RF power for decoupling. In the case of the (1)H-(29)Si HETCOR experiment, the sensitivity can be dramatically increased, in some samples by more than 1 order of magnitude, through implementing into the pulse sequence a Carr-Purcell-Meiboom-Gill train of pi pulses at the (29)Si spin frequency. The use of low-power heteronuclear decoupling is essential in the (1)H-(29)Si CPMG-HETCOR experiment, due to unusually long acquisition periods. These methods provided detailed structural characterization of the surface of AL-MCM mesoporous silica.

Theory of heteronuclear decoupling in solid-state nuclear magnetic resonance using multipole-multimode Floquet theory

A formal theory for heteronuclear decoupling in solid-state magic angle spinning (MAS) nuclear magnetic resonance experiments is presented as a first application of multipole-multimode Floquet theory. The method permits a straightforward construction of the multispin basis and describes the spin dynamics via effective Floquet Hamiltonians obtained using the van Vleck transformation method in the Floquet-Liouville space. As a test case, we consider a model three-spin system (I2S) under asynchronous time modulations (both MAS and rf irradiation) and derive effective Hamiltonians for describing the spin dynamics in the Floquet-Liouville space during heteronuclear decoupling. Furthermore, we describe and evaluate the origin of cross terms between the various anisotropic interactions and illustrate their exact contributions to the spin dynamics. The theory presented herein should be applicable to the design and understanding of pulse sequences for heteronuclear and homonuclear recoupling and decoupling.

Heteronuclear Two-Bond Correlation: Suppressing Heteronuclear Three-Bond or Higher NMR Correlations while Enhancing Two-Bond Correlations Even for Vanishing 2JCH

A novel two-dimensional NMR pulse sequence, H2BC, for long-range correlation of 1H and 13C nuclei is presented. The experiment has several attractive features compared to the widely used HMBC experiment, for example, (a) typically strong enhancement of correlations over two bonds while suppressing those over more bonds, that is, resolving ambiguities in standard HMBC spectra and showing two-bond correlations not present in HMBC spectra, (b) independence of long-range 1H-13C coupling constants, (c) full homo- and heteronuclear decoupling in the indirect dimension and heteronuclear decoupling in the acquisition dimension, (d) pure 2D absorption peak shapes, and (e) a pulse sequence duration significantly shorter than that of HMBC. The experiment is quite complementary to HMBC and does not effect correlations to quaternary carbons that must be obtained by HMBC.

Measurement of eight scalar and dipolar couplings for methine?methylene pairs in proteins and nucleic acids

A new 3D, spin-state-selective coherence transfer NMR experiment is described that yields accurate measurements for eight scalar or dipolar couplings within a spin system composed of a methylene adjacent to a methine group. Implementations of the experiment have been optimized for proteins and for nucleic acids. The experiments are demonstrated for Cbeta-Calpha moieties of the third IgG-binding domain from Streptococcal Protein G (GB3) and for C5'-C4' groups in a 24-nt RNA oligomer. Chemical shifts of Calpha, Cbeta and Hbeta (respectively C4', C5' and H5') are dispersed in the three orthogonal dimensions, and the absence of heteronuclear decoupling leads to distinct and well-resolved E.COSY multiplet patterns. In an isotropic sample, the E.COSY displacements correspond to 1J(CalphaHalpha), 2J(CalphaHbeta2)+2J(CalphaHbeta3), 2J(CbetaHalpha), 1J(CbetaHbeta2)+1J(CbetaHbeta3), 1J(CbetaHbeta2)-2J(Hbeta2Hbeta3), 1J(CbetaHbeta3)-2J(Hbeta2Hbeta3), 3J(HalphaHbeta2) and 3J(HalphaHbeta3) for proteins, and 1J(C4'H4'), 2J(C4'H5')+2J(C4'H5"), 2J(C5'H4'), 1J(C5'H5')+1J(C5'H5"), 1J(C5'H5')-2J(H5'H5"), 1J(C5'H5")-2J(H5'H5"), 3J(H4'H5') and 3J(H4'H5") in nucleic acids. The experiment, based on relaxation-optimized spectroscopy, yields best results when applied to residues where the methine-methylene group corresponds to a reasonably isolated spin system, as applies for C, F, Y, W, D, N and H residues in proteins, or the C5'-C4' groups in nucleic acids. Splittings can be measured under either isotropic or weakly aligned conditions, yielding valuable structural information both through the 3J couplings and the one-, two- and three-bond dipolar interactions. Dipolar couplings for 10 out of 13 sidechains in GB3 are found to be in excellent agreement with its X-ray structure, whereas one residue adopts a different backbone geometry, and two residues are subject to extensive chi1 rotamer averaging. The abundance of dipolar couplings can also yield stereospecific assignments of the non-equivalent methylene protons. For the RNA oligomer, dipolar data yielded stereospecific assignments for six out of the eight C5'H2 groups in the loop region of the oligomer, in all cases confirmed by 1J(C5'H5')>1J(C5'H5"), and H5' resonating downfield of H5".

1H?15N correlation spectroscopy of nanocrystalline proteins

The limits of resolution that can be obtained in 1H-15N 2D NMR spectroscopy of isotopically enriched nanocrystalline proteins are explored. Combinations of frequency switched Lee-Goldburg (FSLG) decoupling, fast magic angle sample spinning (MAS), and isotopic dilution via deuteration are investigated as methods for narrowing the amide 1H resonances. Heteronuclear decoupling of 15N from the 1H resonances is also studied. Using human ubiquitin as a model system, the best resolution is most easily obtained with uniformly 2H and 15N enriched protein where the amides have been exchanged in normal water, MAS at approximately 20 kHz, and WALTZ-16 decoupling of the 15N nuclei. The combination of these techniques results in average 1H lines of only approximately 0.26 ppm full width at half maximum. Techniques for optimizing instrument stability and 15N decoupling are described for achieving the best possible performance in these experiments.

Heating caused by radiofrequency irradiation and sample rotation in 13C magic angle spinning NMR studies of lipid membranes.

Application of rapid sample rotation and radiofrequency irradiation in magic angle spinning (MAS) NMR of lipid bilayers can significantly increase the sample temperature. In this work, we studied the extent of heating during the acquisition of 1H-decoupled 13C MAS spectra of hydrated dimyristoylphosphatidylcholine (DMPC) in the L(alpha) phase. First, we describe a simple procedure for determining the increase in temperature by observing the shift of the 1H water signal. The method is then used to identify and assess the various factors that contribute to the sample heating. The important factors discussed in this paper include: (i) the spinning speed, (ii) the variable-temperature gas pressure, (iii) the rotor geometry, (iv) the power, duration and frequency of the radiofrequency irradiation and (v) the hydration level. A comparison of different heteronuclear decoupling schemes in terms of their ability to produce highly resolved 13C spectra of DMPC is also reported.

Heteronuclear decoupling in rotating solids via symmetry-based adiabatic RF pulse schemes

The performance of symmetry-based adiabatic RF pulse schemes for 1H decoupling at moderate magic angle spinning (MAS) frequencies has been evaluated via numerical simulations and experimentally compared with other decoupling techniques. Our studies reveal that in MAS solid state NMR studies of polycrystalline specimens, in contrast to symmetry-based RF pulse schemes involving rectangular RF pulses, adiabatic decoupling sequences can be effectively implemented to make use of all the available decoupling RF field strength and to eliminate the deleterious effects of H 1 inhomogeneity and resonance offset. Symmetry-based adiabatic decoupling sequences lead to improved decoupling performance compared to a continuous train of adiabatic inversion pulses that are phase cycled as in solution state NMR studies.

Characterization of heteronuclear decoupling through proton spin dynamics in solid-state nuclear magnetic resonance spectroscopy.

The work presented here aims at understanding the performance of phase modulated heteronuclear decoupling sequences such as Cosine Modulation or Two Pulse Phase Modulation. To that end we provide an analytical description of the intrinsic behavior of Cosine Modulation decoupling with respect to radio-frequency-inhomogeneity and the proton-proton dipolar coupling network. We discover through a Modulation Frame average Hamiltonian analysis that best decoupling is obtained under conditions where the heteronuclear interactions are removed but notably where homonuclear couplings are recoupled at a homonuclear Rotary Resonance (HORROR) condition in the Modulation Frame. These conclusions are supported by extensive experimental investigations, and notably through the introduction of proton nutation experiments to characterize spin dynamics in solids under decoupling conditions. The theoretical framework presented in this paper allows the prediction of the optimum parameters for a given set of experimental conditions.

Transverse dephasing optimised NMR spectroscopy in solids: natural-abundance 13C correlation spectra.

AbstractThe transverse dephasing optimized (TDOP) approach can be used to obtain heteronuclear decoupling sequences that increase significantly coherence lifetimes in solid‐state NMR spectroscopy, leading to a dramatic improvement in the sensitivity of correlation experiments. Herein we demonstrate experimentally on several model organic compounds that, by applying this procedure, solid‐state 13C‐refocused INADEQUATE experiments are feasible at natural abundance on medium‐sized molecules. Examples are provided for powdered crystalline cholesteryl acetate as well as for a sample of brucine, a less ordered compound. Effects of molecular dynamics on the sensitivity of the experiment are also discussed for a sample of tyrosine ethyl ester.

Radio-frequency driven polarization transfer without heteronuclear decoupling in rotating solids

This Letter reports significant excitation of 13C homonuclear double-quantum coherence in a solid-state magic-angle-spinning NMR experiment, acquired without heteronuclear decoupling of abundant 1H spins. The heteronuclear decoupling efficiency of a group of existing homonuclear polarization transfer sequences are investigated and optimal schemes are identified. Since the approach permits the application of fast MAS and significantly reduces the total radio-frequency power used, it is particularly well suited to biomolecular applications at ultra-high magnetic fields.

The performance of phase modulated heteronuclear dipolar decoupling schemes in fast magic-angle-spinning nuclear magnetic resonance experiments

The experimental performance of the phase modulated decoupling schemes SDROOPY-1 and SPINAL64 is investigated for heteronuclear dipolar decoupling in solid-state magic-angle-spinning nuclear magnetic resonance (NMR), in comparison with existing decoupling methods. It is found that both sequences perform well over a range of spinning rates from 0 to 35 kHz, with linewidths that are virtually independent of the spinning rate. The signal intensity is found to diminish with increasing spinning rate, and this behavior is compared qualitatively with theory. It is also demonstrated that decoupling sequences strongly influence the refocused linewidths observed in solid-state NMR, and that SDROOPY-1 and SPINAL64 both yield particularly long dephasing times.

Improved heteronuclear decoupling schemes for solid-state magic angle spinning NMR by direct spectral optimization

It is shown that a direct spectral optimization scheme can be implemented to obtain improved heteronuclear dipolar decoupling schemes for solid-state magic-angle-spinning NMR experiments. The resulting schemes, which turn out to have a particularly simple form, are shown to be applicable over the whole range of commercially available spinning speeds (from 5 to 35 kHz), and are shown to improve on the performance of the best existing sequences.