Dipolar Coupling Networks Research Articles

Mapping Hydrogens in Hydrous Materials

Hydrous materials such as clays, cements, hydroxyapatite, and metal hydroxides have been known and extensively used since antiquity. However, experimental elucidation of hydrogen in hydrous materials remains challenging due to the intrinsic insensitivity, limited accessibility, sample damage, or poor spectral resolution of many characterization methods. 1H solid-state NMR spectroscopy has evolved into an ideal site-specific characterization tool of hydrogen-containing materials, but its current application in hydrous materials is hampered by the low resolution of 1H NMR spectra in the solid state due to the presence of intense dipolar coupling networks and a narrow 1H chemical shift range. Herein, a significant advance in the characterization of hydrogens in hydrous materials is reported. A combination of magic-angle spinning (MAS) NMR, moderate 2H substitution, and high magnetic fields has unlocked the capacity of identifying many chemically similar while crystallographically distinct hydrogen sites in a prototypical hydrous material Y4(OH)10Br2·3H2O (LYH-Br) by 1D 1H solid-state NMR experiments. 1H NMR peaks were first tentatively assigned in accordance with the density functional theory (DFT) calculation results, and the assignment was further refined by 2D 1H–89Y heteronuclear correlation (HETCOR) and 1H–1H double-quantum (DQ) MAS NMR data. The order of 1H chemical shift (δiso) values provides valuable information on the relative strength of hydrogen bond for ten hydroxide hydrogen sites. The power of very high spectral resolution is further described by observing all triple-quantum (TQ) coherences in the 2D 1H–1H TQ MAS NMR spectrum, in which all spatial proximities among three hydrogen sites are resolved. This demonstration of the very high spectral resolution obtained in 1D and 2D 1H solid-state NMR experiments illustrates how scientists in various communities can now study the multiple hydrous species of hydrous materials and obtain previously inaccessible fine structural information.

Can proton-proton recoupling in fully protonated solids provide quantitative, selective and efficient polarization transfer?

Dipolar recoupling sequences have been used to probe spatial proximity of nuclear spins and were traditionally designed to probe rare spins such as 13C and/or 15N nuclei. The multi-spin dipolar-coupling network of the rare spins is weak due to smaller couplings and large chemical shift dispersion. Therefore, the recoupling approaches were tailored to design offset compensated or broadband sequences. In contrast, protons have a substantially stronger dipolar-coupling network and much narrower chemical shift range. Broadband recoupling sequences such as radio-frequency driven recoupling (RFDR), back-to-back (BABA), and lab frame proton-proton spin diffusion have been routinely used to characterize the structures of protein/macromolecules and small molecules. Recently selective 1H-1H recoupling sequences have been proposed that combine chemical shift offset of the resolved proton spectrum (at fast MAS) with first- and second-order dipolar recoupling Hamiltonians to obtain quantitative and qualitative proton distances, respectively. Herein, we evaluate the performances of broadband and selective proton recoupling sequences such as finite pulse RFDR (fp-RFDR), band-selective spectral spin diffusion (BASS-SD), second-order cross-polarization (SOCP), and selective recoupling of proton (SERP) in terms of the selectivity and efficiency of 1H-1H polarization transfers in a dense network of proton spins and explore the possibility of measuring 1H-1H distances. We use theoretical considerations, numerical simulations, and experiments to support the distinct advantages and disadvantages of each recoupling sequence. Experiments were performed on L-histidine.HCl.H2O at a MAS frequency of 71.43 kHz. This study rationalizes the proper selection of 1H-1H recoupling sequences when working with fully protonated solids.

Combining Molecular and Spin Dynamics Simulations with Solid-State NMR: A Case Study of Amphiphilic Lysine-Leucine Repeat Peptide Aggregates.

Interpreting dynamics in solid-state molecular systems requires characterization of the potentially heterogeneous environmental contexts of molecules. In particular, the analysis of solid-state nuclear magnetic resonance (ssNMR) data to elucidate molecular dynamics (MD) involves modeling the restriction to overall tumbling by neighbors, as well as the concentrations of water and buffer. In this exploration of the factors that influence motion, we utilize atomistic MD trajectories of peptide aggregates with varying hydration to mimic an amorphous solid-state environment and predict ssNMR relaxation rates. We also account for spin diffusion in multiply spin-labeled (up to 19 nuclei) residues, with several models of dipolar-coupling networks. The framework serves as a general approach to determine essential spin couplings affecting relaxation, benchmark MD force fields, and reveal the hydration dependence of dynamics in a crowded environment. We demonstrate the methodology on a previously characterized amphiphilic 14-residue lysine-leucine repeat peptide, LKα14 (Ac-LKKLLKLLKKLLKL-c), which has an α-helical secondary structure and putatively forms leucine-burying tetramers in the solid state. We measure the R1 relaxation rates of uniformly 13C-labeled and site-specific 2H-labeled leucines in the hydrophobic core of LKα14 at multiple hydration levels. Studies of 9 and 18 tetramer bundles reveal the following: (a) for the incoherent component of 13C relaxation, the nearest-neighbor spin interactions dominate, while the 1H-1H interactions have minimal impact; (b) the AMBER ff14SB dihedral barriers for the leucine Cγ-Cδ bond ("methyl rotation barriers") must be lowered by a factor of 0.7 to better match the 2H data; (c) proton-driven spin diffusion explains some of the discrepancy between experimental and simulated rates for the Cβ and Cα nuclei; and (d) 13C relaxation rates are mostly underestimated in the MD simulations at all hydrations, and the discrepancies identify likely motions missing in the 50 ns MD trajectories.

Quantitative 1H-1H Distances in Protonated Solids by Frequency-Selective Recoupling at Fast Magic Angle Spinning NMR.

Nuclear magnetic resonance (NMR) spectroscopy of protons in protonated solids is challenging. Fast magic angle spinning (MAS) and homonuclear decoupling schemes, in conjunction, with high magnetic fields have improved the proton resolution. However, experiments to quantitatively measure 1H-1H distances still remain elusive due to the dense proton-proton dipolar coupling network. A novel MAS solid-state NMR pulse sequence is proposed to selectively recouple and measure interproton distances in protonated samples. The phase-modulated sequence combined with a judicious choice of transmitter frequency is used to measure quantitative 1H-1H distances on the order of 3 Å in l-histidine·HCl·H2O, despite the presence of other strongly coupled protons. This method provides a major boost to NMR crystallography approaches for structural determination of pharmaceutical molecules by directly measuring 1H-1H distances. The band-selective nature of the sequence also enables observation of selective 1H-1H correlations (e.g., HN-HN/HN-Hα/ΗΝ-ΗMethyl) in peptides and proteins, which should serve as useful restraints in structure determination.

Asynchronous through-bond homonuclear isotropic mixing: application to carbon-carbon transfer in perdeuterated proteins under MAS.

Multiple-bond carbon-carbon homonuclear mixing is a hurdle in extensively deuterated proteins and under fast MAS due to the absence of an effective proton dipolar-coupling network. Such conditions are now commonly employed in solid-state NMR spectroscopy. Here, we introduce an isotropic homonuclear (13)C-(13)C through-bond mixing sequence, MOCCA, for the solid state. Even though applied under MAS, this scheme performs without rotor synchronization and thus does not pose the usual hurdles in terms of power dissipation for fast spinning. We compare its performance with existing homonuclear (13)C-(13)C mixing schemes using a perdeuterated and partially proton-backexchanged protein. Based on the analysis of side chain carbon-carbon correlations, we show that particularly MOCCA with standard 180-degree pulses and delays leading to non-rotor-synchronized spacing performs exceptionally well. This method provides high magnetization transfer efficiency for multiple-bond transfer in the aliphatic region compared with other tested mixing sequences. In addition, we show that this sequence can also be tailor-made for recoupling within a selected spectral region using band-selective pulses.

Proton-Detected Solid-State NMR Spectroscopy of Bone with Ultrafast Magic Angle Spinning.

While obtaining high-resolution structural details from bone is highly important to better understand its mechanical strength and the effects of aging and disease on bone ultrastructure, it has been a major challenge to do so with existing biophysical techniques. Though solid-state NMR spectroscopy has the potential to reveal the structural details of bone, it suffers from poor spectral resolution and sensitivity. Nonetheless, recent developments in magic angle spinning (MAS) NMR technology have made it possible to spin solid samples up to 110 kHz frequency. With such remarkable capabilities, 1H-detected NMR experiments that have traditionally been challenging on rigid solids can now be implemented. Here, we report the first application of multidimensional 1H-detected NMR measurements on bone under ultrafast MAS conditions to provide atomistic-level elucidation of the complex heterogeneous structure of bone. Our investigations demonstrate that two-dimensional 1H/1H chemical shift correlation spectra for bone are obtainable using fp-RFDR (finite-pulse radio-frequency-driven dipolar recoupling) pulse sequence under ultrafast MAS. Our results infer that water exhibits distinct 1H−1H dipolar coupling networks with the backbone and side-chain regions in collagen. These results show the promising potential of proton-detected ultrafast MAS NMR for monitoring structural and dynamic changes caused by mechanical loading and disease in bone.

Unraveling long range residual dipolar coupling networks in strongly aligned proteins

Long-range residual dipolar couplings (lrRDCs) have the potential to serve as powerful structural restraints in protein NMR spectroscopy as they can provide both distance and orientation information about nuclei separate in sequence but close in space. Current nonselective methods for their measurement are limited to moderate alignment strengths due to the sheer abundance of active couplings at stronger alignment. This limits the overall magnitude and therefore distance across which couplings can be measured. We have developed a double resonance technique for the inversion of individual coupled spin pairs, called Selective Inversion by Single Transition Cross Polarization (SIST-CP). This technique enables the selective recoupling of lrRDCs, thus allowing the complex multiplets occurring in strongly aligned systems to be disentangled. This technique is demonstrated in the context of an application to the measurement of 13C′–1HN lrRDCs in strongly aligned proteins.

Long lived NMR signal in bone

Solids and rigid tissues, such as bone, ligaments, and tendons, typically appear dark in MRI, which is due to the extremely short-lived proton nuclear magnetic resonance signals. This short lifetime is due to strong dipolar interactions between immobilized proton spins, which render it challenging to detect these signals with sufficient resolution and sensitivity. Here we show the possibility of exciting long-lived signals in cortical bone tissue with a signature consistent with that of bound water signals. It is further shown that dipolar coupling networks are an integral requirement for the excitation of these long-lived signals. The use of these signals could enhance the ability to visualize rigid tissues and solid samples with high resolution and sensitivity via MRI.

Second-order dipolar order in magic-angle spinning nuclear magnetic resonance

Generating dipolar order under magic-angle spinning (MAS) conditions is explored for different pulse sequences and dipolar-coupling networks. It is shown that under MAS second-order dipolar order can be generated reliably with 10% to 30% efficiency using the Jeener-Broekaert sequence in systems where the second-order average Hamiltonian is a (near) constant of the motion. When using adiabatic demagnetization and remagnetization, second-order dipolar order can be generated and reverted back to Zeeman order with up to 60% efficiency. This requires a maximum field strength with a nutation frequency that is less than one-quarter of the rotor frequency, and that the spin system can be properly spinlocked under such conditions. A simple coherent description accounts for the principal features of the spin dynamics, even using the smallest possible system of three coupled spins. For the systems investigated, the lifetime of second-order dipolar order under MAS was found to be on the order of T(1).

Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate

Magic-angle spinning (MAS) NMR spin-echo dephasing is systematically investigated for the spin I = 3/2 (11)B nucleus in lithium diborate, Li(2)O·2B(2)O(3). A clear dependence on the quadrupolar frequency (ω(Q)(PAS)/2π = 3C(Q)/[4I(2I- 1)]) is observed: the B3 (larger C(Q)) site dephases more slowly than the B4 site at all investigated MAS frequencies (5 to 20 kHz) at 14.1 T. Increasing the MAS frequency leads to markedly slower dephasing for the B3 site, while there is a much less evident effect for the B4 site. Considering samples at 5, 25, 80 (natural abundance) and 100% (11)B isotopic abundance, dephasing becomes faster for both sites as the (11)B isotopic abundance increases. The experimental behaviour is rationalised using density matrix simulations for two and three dipolar-coupled (11)B nuclei. The experimentally observed slower dephasing for the larger C(Q) (B3) site is reproduced in all simulations and is explained by the reintroduction of the dipolar coupling by the so-called "spontaneous quadrupolar-driven recoupling mechanism" having a different dependence on the MAS frequency for different quadrupolar frequencies. Specifically, isolated spin-pair simulations show that the spontaneous quadrupolar-driven recoupling mechanism is most efficient when the quadrupolar frequency is equal to twice the MAS frequency. While for isolated spin-pair simulations, increasing the MAS frequency leads to faster dephasing, agreement with experiment is observed for three-spin simulations which additionally include the homogeneous nature of the homonuclear dipolar coupling network. First-principles calculations, using the GIPAW approach, of the (2)J(11B-11B) couplings in lithium diborate, metaborate and triborate are presented: a clear trend is revealed whereby the (2)J(11B-11B) couplings increase with increasing B-O-B bond angle and B-B distance. However, the calculated (2)J(11B-11B) couplings are small (0.95, 1.20 and 2.65 Hz in lithium diborate), thus explaining why no zero crossing due to J modulation is observed experimentally, even for the sample at 25% (11)B where significant spin-echo intensity remains out to durations of ∼200 ms.

Double-Nucleus Enhanced Recoupling for Efficient 13C MAS NMR Correlation Spectroscopy of Perdeuterated Proteins

The use of both proton and deuterium dipolar coupling networks to obtain efficient (13)C magic-angle-spinning NMR correlation spectroscopy is introduced. This new strategy aims to improve the efficiency of (13)C spin diffusion in perdeuterated proteins. The method is called double-nucleus enhanced recoupling (DONER), and it provides significantly improved transfer efficiency for carbon spin diffusion at low proton density. The C(alpha) cross-peak intensity in the DONER experiment is approximately 3 and approximately 5 times stronger than those in conventional radio-frequency-assisted diffusion (RAD) and proton-driven spin diffusion (PDSD) experiments, respectively. Remarkably, the full cross-peak pattern for the aliphatic region of an extensively perdeuterated SH3 protein sample can be obtained using the DONER approach with direct (13)C excitation.

Quantification of homonuclear dipolar coupling networks from magic-angle spinning 1H NMR

Numerical simulations of magic-angle spinning (MAS) spectra of dipolar-coupled nuclear spins have been used to assess different approaches to the quantification of dipolar couplings from 1H solid-state NMR. Exploiting the translational symmetry of periodic spin systems allows extended networks with ‘realistic’ numbers of spins to be considered. The experimentally accessible parameter is shown to be the root-sum-square of the dipolar couplings to a given spin. The effectiveness of either fitting the resulting spinning sideband spectra to small spin system models, or using analyses based on moment expansions, has been examined. Fitting of the spinning sideband pattern is found to be considerably more robust with respect to experimental noise than frequency domain moment analysis. The influence of the MAS rate and system geometry on robustness of the quantification is analysed and discussed.

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.

Proton residual dipolar couplings by NMR magnetization exchange in cross-linked elastomers: determination and imaging

Proton nuclear magnetic resonance (NMR) magnetization exchange is used to investigate residual dipolar couplings in a series of cross-linked poly(styrene–cobutadiene) elastomers. A new model for the dipolar unit is used for the evaluation of the signal decay in magnetization exchange experiments. It takes into account an extended residual dipolar coupling network along the polymer chain. It is shown that in the regime of short mixing times, information about the residual dipolar coupling between methine and methylene protons can be obtained which is not affected by other inter- and intramolecular dipolar couplings. The dynamic order parameter of methine–methylene protons is measured and correlated with cross-link density. This study certifies the quality of a filter for magnetization from residual dipolar couplings which exploit magnetization exchange. The filter can be used to generate contrast in NMR images of heterogeneous elastomers. The first proton NMR parameter image of a dynamic order parameter is presented for a phantom made from poly(styrene–cobutadiene) samples with different cross-link densities.

High-Speed Magic-Angle Spinning13C MAS NMR Spectra of Adamantane: Self-Decoupling of the Heteronuclear Scalar Interaction and Proton Spin Diffusion

We have investigated the carbon line shape of solid adamantane under high-speed magic-angle sample spinning (MAS) acquired without proton decoupling. The CH-group shows a spinning-speed-dependent line broadening while the CH2-group consists of a spinning-speed-independent sharp component and a spinning-speed-dependent broader part. These phenomena can be explained by self-decoupling of theJ-interaction due to proton spin diffusion. Such a self-decoupling process can be described by a magnetization exchange process between the multiplet lines. Changing the spin-diffusion rate constant by off-resonance irradiation of the protons allows us to observe the full range from slow exchange to coalescence to fast exchange of the carbon spectra. One of the multiplet components in the CH2-group corresponds to a group spin of the protons of zero and therefore does not couple to the other protons. This gives rise to the sharp central line. The magnetization exchange rate constant between the different multiplet lines can be determined from the spectra and is a measure for the spinning-speed-dependent proton spin-diffusion rate constant. Even at an MAS speed of 30 kHz, proton spin diffusion is still observable despite the relatively weak intermolecular proton dipolar-coupling network in adamantane which results in a static proton line width of only 14 kHz (full width at half height).