Heteronuclear Dipolar Recoupling Research Articles

Through-space 11 B-27 Al correlation: Influence of the recoupling channel.

Through-space heteronuclear correlation (D-HETCOR) experiments based on heteronuclear multiple-quantum correlation (D-HMQC) and refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT) sequences have been proven to be useful approaches for the detection of the spatial proximity between half-integer quadrupolar nuclei in solids under magic-angle spinning (MAS) conditions. The corresponding pulse sequences employ coherence transfers mediated by heteronuclear dipolar interactions, which are reintroduced under MAS by radiofrequency irradiation of only one of the two correlated nuclei. We investigate herein using numerical simulations of spin dynamics and solid-state NMR experiments on magnesium aluminoborate glass how the choice of the channel to which the heteronuclear dipolar recoupling is applied affects the transfer efficiency of D-HMQC and D-RINEPT sequences between 11 B and 27 Al nuclei. Experimental results show that maximum transfer efficiency is achieved when the recoupling scheme is applied to the channel, for which the spin magnetization is parallel to the B0 axis in average.

Enhanced 1H-X D-HMQC performance through improved 1H homonuclear decoupling

The sensitivity of solid-state NMR experiments that utilize 1H zero-quantum heteronuclear dipolar recoupling, such as D-HMQC, is compromised by poor homonuclear decoupling. This leads to a rapid decay of recoupled magnetization and an inefficient recoupling of long-range dipolar interactions, especially for nuclides with low gyromagnetic ratios. We investigated the use, in symmetry-based 1H heteronuclear recoupling sequences, of a basic R element that was principally designed for efficient homonuclear decoupling. By shortening the time required to suppress the effects of homonuclear dipolar interactions to the duration of a single inversion pulse, spin diffusion was effectively quenched and long-lived recoupled coherence lifetimes could be obtained. We show, both theoretically and experimentally, that these modified sequences can yield considerable sensitivity improvements over the current state-of-the-art methods and applied them to the indirect detection of 89Y in a metal-organic framework.

Observation of proximities between spin-1/2 and quadrupolar nuclei: Which heteronuclear dipolar recoupling method is preferable?

We have recently shown that the dipolar-mediated heteronuclear multiple-quantum coherence (D-HMQC) method allows observing through-space proximities between spin-1/2 ((1)H, (13)C, (31)P...) and quadrupolar ((23)Na, (27)Al...) nuclei. However, the D-HMQC effectiveness depends on the choice of the heteronuclear dipolar recoupling sequence. Here, we compare the efficiency and the robustness of four rotor-synchronized sequences: the symmetry-based ones, R4(1)(2)R4(1)(-2) and its super-cycled version, SR4(1)(2), and two schemes based on simultaneous amplitude and frequency modulations, denoted SFAM-1 and SFAM-2. For the SFAM methods, we point out efficient recoupling conditions that facilitate their experimental optimization and we introduce analytical expressions for the buildup of D-HMQC signal in the case of an isolated spin pair. We show that the main differences between these four sequences lie in the number of adjustable parameters and in their robustness with respect to chemical shift and homonuclear dipolar interactions. The relative performances of these four recoupling sequences are analyzed using average Hamiltonian theory, numerical simulations, and (27)Al-{(31)P} D-HMQC experiments on crystalline aluminophosphate.

Heteronuclear dipolar recoupling of half-integer quadrupole nuclei under fast magic angle spinning

An experimental method for the heteronuclear dipolar recoupling of half-integer quadrupole nuclei is proposed. The idea is to manipulate the central transition based on the recoupling technique of spin-polarization-inversion rotary resonance. This method allows the extraction of structural parameters under fast magic-angle spinning. Its validity has been examined by the average Hamiltonian theory and numerical simulations. The initial rotational-echo dephasing arising from the dipolar evolution can be approximated by a parabolic function, from which the heteronuclear van Vleck second moment can be estimated. A factor, estimated from two-spin simulations, is required to account for the effects of the quadrupolar coupling and is rather independent of the geometry and the orders of the spin systems. Our method can facilitate the structural characterization of materials containing half-integer quadrupole nuclei under high-resolution condition. Experimental verification has been carried out on two aluminophosphate systems, namely, AlPO 4-5 and AlPO 4-11.

Effect of phase inversion of radiofrequency pulses on the recoupling of anisotropic NMR interactions under magic-angle spinning

The effect of phase inversion of radiofrequency pulses on the recoupling of anisotropic chemical shift and heteronuclear magnetic dipole–dipole interactions has been investigated by a simple analytical treatment. For the recoupling of chemical shift anisotropy, the evolution of the spin system will be different if the sign of all rf phases is inverted. This phenomenon is readily explained by the higher-order effect of average Hamiltonian theory. On the contrary, such dramatic phase inversion effect is not observed for heteronuclear dipolar recoupling, which can also be rationalized in our treatment. Numerical simulations are carried out to corroborate our analytical results and experimental verification is obtained for [1- 13C]-alanine.

High-resolution 14N-edited 1H– 13C correlation NMR experiment to study biological solids

It was recently shown that nuclear magnetic resonance (NMR) spectra of nitrogen-14 (spin I = 1) can be obtained by indirect detection via spin S = 1/2 nuclei in powders spinning at the magic angle. An increased number of solid-state NMR methods are now available to tailor sequences for specific purposes, e.g., hetero-nuclear dipolar recoupling or homo-nuclear dipolar decoupling schemes. Here, we combine the latest recoupling and decoupling techniques to obtain high-resolution 1H– 13C through-space correlation spectra, where only the correlation peaks of those carbons close to nitrogen nuclei are observed. The experiment is demonstrated on a 13C enriched l-histidine. HCl·H 2O powder sample.

Comparison of several hetero-nuclear dipolar recoupling NMR methods to be used in MAS HMQC/HSQC

We compare several hetero-nuclear dipolar recoupling sequences available for HMQC or HSQC experiments applied to spin-1/2 and quadrupolar nuclei. These sequences, which are applied to a single channel, are based either on the rotary resonance recoupling ( R 3) irradiation, or on two continuous rotor-synchronized modulations (SFAM 1 and SFAM 2), or on four symmetry-based sequences ( R 2 1 1 , SR 4 1 2 , R 12 3 5 , R 20 5 9 ) , or on the REDOR scheme. We analyze systems exhibiting purely hetero-nuclear dipolar interactions as well as systems where homo-nuclear dipolar interactions need to be canceled. A special attention is given to the behavior of these sequences at very fast MAS. It is shown that R 3 methods behave poorly due to the narrowness of their rf-matching curves, and that the best methods are SR 4 1 2 and SFAM (SFAM 1 or SFAM 2 if homo-nuclear interactions are not negligible). REDOR can also recouple efficiently hetero-nuclear dipolar interactions, provided the sequence is sent on the non-observed channel and homo-nuclear dipolar interactions are negligible. We anticipate that at ultra-fast spinning speed, SFAM 1 and SFAM 2 will be the most efficient methods.

Proton-Selective 17O−H Distance Measurements in Fast Magic-Angle-Spinning Solid-State NMR Spectroscopy for the Determination of Hydrogen Bond Lengths

We present a proton-selective method to determine 17O-1H distances in organic, biological, and biomimetic materials by fast magic-angle-spinning solid-state NMR spectroscopy. This method allows the determination of internuclear distances between specific (17O, 1H) spin pairs selectively. It enables the estimation of medium-range 17O...1H distances across hydrogen bonds in the presence of short-range 17O-1H contacts sharing the same 17O site. The method employs the newly developed symmetry-based radiofrequency pulse sequence SR%@mt;sys@%4%@sx@%1%@be@%2%@sxx@%%@mx@% applied to the protons to achieve heteronuclear dipolar recoupling, while simultaneously decoupling the homonuclear proton dipolar interactions. Fast MAS (50 kHz) and high static magnetic fields (18.8 T) achieve the required proton spectral resolution.

Heteronuclear dipolar recoupling in solid-state nuclear magnetic resonance by amplitude-, phase-, and frequency-modulated Lee–Goldburg cross-polarization

This paper presents a theoretical, numerical, and experimental study of phase- and frequency-switched Lee-Goldburg cross-polarization (FSLG-CP) under magic-angle spinning conditions. It is shown that a well-defined amplitude modulation of one of the two radio-frequency (rf) fields in the FSLG-CP sequence results in highly efficient heteronuclear dipolar recoupling. The recoupled dipolar interaction is gamma-encoded and, under ideal conditions, the effective spin Hamiltonian is equivalent to that in continuous-wave Lee-Goldburg CP. In practice, however, FSLG-CP is less susceptible to rf field mismatch and inhomogeneity, and provides better suppression of (1)H spin diffusion. The performance of FSLG-CP is experimentally demonstrated on liquid-crystalline samples exhibiting motionally averaged dipolar couplings.

Improving solid-state NMR dipolar recoupling by optimal control.

We present the first solid-state NMR experiments developed using optimal control theory. Taking heteronuclear dipolar recoupling in magic-angle-spinning NMR as an example, it proves possible to significantly improve the efficiency of the experiments while introducing robustness toward instrumental imperfections such as radio frequency inhomogeneity. The improvements are demonstrated by numerical simulations as well as practical experiments on a 13Calpha,15N-labeled powder of glycine. The experiments demonstrate a gain of 53% in the efficiency for 15N to 13Calpha coherence transfer relative to the typically double-cross-polarization experiments.

Solid-state NMR heteronuclear coherence transfer using phase and amplitude modulated rf irradiation at the Hartmann–Hahn sideband conditions

An improved variant of the popular double cross-polarization (DCP) experiment for heteronuclear dipolar recoupling in solid-state NMR spectroscopy under magic-angle-spinning is introduced. By simple phase and amplitude modulation of the rf irradiation at the Hartman-Hahn sideband conditions, the new pulse sequence, dubbed iDCP, enables broadband excitation with the high efficiency of γ-encoded coherence transfer. The efficiency and robustness of iDCP toward isotropic chemical shift variations and chemical shift anisotropies, in the order typically applying for the backbone atoms in uniformly 13C, 15N-labeled proteins, is demonstrated numerically and experimentally by 15N to 13C coherence transfer for 15N-labeled N-Ac- l-valyl- l-leucine and 13C, 15N-labeled- l-threonine.

Heteronuclear dipolar recoupling in liquid crystals and solids by PISEMA-type pulse sequences

A pulse sequence is described for the recoupling of heteronuclear dipolar interactions under MAS. The method is similar to the PISEMA experiment, but employs a well-defined amplitude modulation of one of the two radio-frequency fields. The technique is used for measurements of 1H– 13C dipolar couplings in unoriented solid and liquid-crystalline samples.

Solid-state NMR heteronuclear dipolar recoupling using off-resonance symmetry-based pulse sequences

A new approach to heteronuclear dipolar recoupling in solid-state NMR which combines symmetry-based, γ-encoded dipolar recoupling with off-resonance rf irradiation is described. By irradiating the two spin species with offsets of equal magnitude but opposite sign, the dipolar scaling factor may be increased while using lower rf field strengths than under on-resonance conditions. Larger scaling makes the experiment less susceptible to relaxation, while released rf field requirements facilitate combination with fast magic-angle spinning while maintaining efficient 1 H decoupling. Furthermore, homonuclear dipolar couplings may be suppressed by adjusting the off-resonance irradiation to the Lee–Goldburg condition.

Soft-triple resonance solid-state NMR experiments for assignments of U- 13C, 15N labeled peptides and proteins

The process of obtaining sequential resonance assignments for heterogeneous polypeptides and large proteins by solid-state NMR (ssNMR) is impeded by extensive spectral degeneracy in these systems. Even in these challenging cases, the cross peaks are not distributed uniformly over the entire spectral width. Instead, there exist both well-resolved single resonances and distinct groups of resonances well separated from the most crowded region of the spectrum. Here, we present a series of new triple resonance experiments that exploit the non-uniform clustering of resonances in heteronuclear correlation spectra to obtain additional resolution in the more crowded regions of a spectrum. Homonuclear and heteronuclear dipolar recoupling sequences are arranged to achieve directional transfer of coherence between neighboring residues in the peptide sequence. A frequency-selective ( soft) pulse is applied to select initial polarization from a limited (and potentially) well-resolved region of the spectrum. The pre-existing resolution of one or more spins is thus utilized to obtain additional resolution in the more crowded regions of the spectrum. A new protocol to utilize these experiments for sequential resonance assignments in peptides and proteins is also demonstrated.

REDOR: an assessment of the efficacy of dipolar recoupling with adiabatic inversion pulses.

A numerical assessment of the efficacy of REDOR recoupling of heteronuclear dipolar interactions employing adiabatic dephasing pulses has been carried out by considering an isolated dipolar coupled spin 1/2 I-S system. At moderate magic angle spinning frequencies in the range of 3-6 kHz and when the CSA of the dephased spins is large, it is shown that efficient broadband heteronuclear dipolar recoupling and reliable distance estimates can be achieved even under conditions where a significant fraction of the rotor period is occupied by the adiabatic pulse. The efficacy of REDOR with adiabatic inversion pulses has been demonstrated experimentally in two model 15N-13C spin systems, (13C',15N) Aib-(15N) Aib-NH2 (Aib = alpha-aminoisobutyric acid) and (1-13C,15N) glycine.

The Effect of RF Inhomogeneity on Heteronuclear Dipolar Recoupling in Solid State NMR: Practical Performance of SFAM and REDOR

Abstract Practical heteronuclear dipolar recoupling performances under magic angle spinning for SFAM and REDOR have been investigated under well-defined rf inhomogeneity environments with variation of resonance offsets for the irradiated nucleus. The heteronuclear dipolar recoupling efficiencies were quantitatively determined based on the experimentally obtained rf homogeneity. As a result, SFAM retains higher recoupling efficiency (>95%) at an 85% effective nutation frequency, and its recoupling efficiency is gradually reduced at lower effective nutation frequencies. On the other hand, although REDOR retains higher recoupling (>95%) efficiency at high (>92%) effective nutation frequency with an XY-8 compensation pulse sequence, the recoupling efficiency is dramatically decreased when the effective nutation frequency is below 90%. Over all, SFAM has significant advantages for insensitivity to carrier frequency offset and rf inhomogeneity.

Recoupling of heteronuclear dipolar interactions in solid-state NMR using symmetry-based pulse sequences

We apply symmetry theorems to the problem of heteronuclear dipolar recoupling in the presence of magic-angle spinning in solid-state NMR. Examples are shown in which the 13C NMR signal is acquired while rotor-synchronized pulse sequences with the symmetry R18 1 7 or R18 2 5 are applied to the 1H spins. This allows recoupling of heteronuclear dipolar interactions combined with homonuclear decoupling of the irradiated 1H spins. The structure of the 13C NMR spectrum is sensitive to bond lengths and bond angles. A two-dimensional procedure is described for applications to multiply isotopically labelled systems.

Frequency selective heteronuclear dipolar recoupling in rotating solids: accurate (13)C-(15)N distance measurements in uniformly (13)C,(15)N-labeled peptides.

We describe a magic-angle spinning NMR experiment for selective (13)C-(15)N distance measurements in uniformly (13)C,(15)N-labeled solids, where multiple (13)C-(15)N and (13)C-(13)C interactions complicate the accurate measurement of structurally interesting, weak (13)C-(15)N dipolar couplings. The new experiment, termed FSR (frequency selective REDOR), combines the REDOR pulse sequence with a frequency selective spin-echo to recouple a single (13)C-(15)N dipolar interaction in a multiple spin system. Concurrently the remaining (13)C-(15)N dipolar couplings and all (13)C-(13)C scalar couplings to the selected (13)C are suppressed. The (13)C-(15)N coupling of interest is extracted by a least-squares fit of the experimentally observed modulation of the (13)C spin-echo intensity to the analytical expression describing the dipolar dephasing in an isolated heteronuclear spin pair under conventional REDOR. The experiment is demonstrated in three uniformly (13)C,(15)N-labeled model systems: asparagine, N-acetyl-L-Val-L-Leu and N-formyl-L-Met-L-Leu-L-Phe; in N-formyl-[U-(13)C,(15)N]L-Met-L-Leu-L-Phe we have determined a total of 16 internuclear distances in the 2.5-6 A range.

Tilted n-fold symmetric radio frequency pulse sequences: Applications to CSA and heteronuclear dipolar recoupling in homonuclear dipolar coupled spin networks

We describe new solid-state NMR methods for measuring the magnitude of the chemical shift anisotropy (CSA) and/or the heteronuclear dipolar coupling in the presence of homonuclear dipolar coupled spin networks under magic-angle spinning (MAS). The techniques employ 2π/5 phase shifts of a frequency switched Lee–Goldburg irradiation scheme which recouples the CSA and/or the heteronuclear dipolar coupling while simultaneously decoupling the homonuclear dipolar interactions. The induced spin dynamics are sensitive to the magnitude of the CSA and its asymmetry and may be implemented under conditions where the MAS rate exceeds the size of the anisotropic interactions. These approaches could find use in measurements of the magnitudes and relative orientations of CSA and/or heteronuclear dipolar couplings to extract torsion angles in uniformly labeled systems.