Improved detection of magnetic interactions in proteins based on long-lived coherences.

Living systems rely on molecular building blocks with low structural symmetry. Therefore, constituent amino acids and nucleotides yield short-lived nuclear magnetic responses to electromagnetic radiation. Magnetic signals are at the basis of molecular imaging, structure determination and interaction studies. In solution state, as the molecular weight of analytes increases, coherences with long lifetimes are needed to yield advantageous through-space magnetisation transfers. Interactions between magnetic nuclei can only be detected provided the lifetimes of spin order are sufficient. In J-coupled pairs of nuclei, long-lived coherences (LLC's) connect states with different spin-permutation symmetry. Here in, we show sustained LLC's in protein Lysozyme, weighing 14.3 kDa, with lifetimes twice as long as those of classical magnetisation for the aliphatic protons of glycine residues. We found for the first time that, in a protein of significant molecular weight, LLC's yield substantial through-space magnetisation transfers: spin-order transfer stemming from LLC's overcame transfers from classical coherences by factors > 2. Furthermore, in agreement with theory, the permutation symmetry of LLC-based transfers allows mapping interacting atoms in the protein structure with respect to the molecular plane of glycine residues in a stereospecific manner. These findings can extend the scope of liquid-state high-resolution biomolecular spectroscopy.

Cross polarization from dipolar-order under magic angle spinning: The ADRF-CPMAS NMR experiment.

Techniques for enhancing the signals arising from low-γ, insensitive (I) nuclei are central to solid-state nuclear magnetic resonance. One of the leading and best-established methods to sensitize these unreceptive species is Hartmann-Hahn cross polarization (HH-CP), a polarization transfer mechanism often executed under MAS. Herein, we explore the possibility of utilizing the 1H dipolar order created via adiabatic demagnetization in the rotating frame (ADRF), to enhance the unreceptive spins under MAS. It is found that an efficient polarization transfer via ADRF-CPMAS is not only possible but can exceed, at least in some instances involving plastic crystals, the efficiency of an optimized HH-CPMAS transfer. The experiment requires low radiofrequency nutation fields on both the 1H- and the I-spin channels, and displays unusual matching conditions that are reminiscent of the zero- and double-quantum matching conditions arising under CPMAS, albeit centered at zero frequency and demanding the simultaneous involvement of several spins. The origin of these multi-spin transfer processes is analytically derived and numerically simulated in predictions that compare well with experimental 13C and 15N results collected on model compounds at different spinning speeds. These derivations start from descriptions that depart from traditional thermodynamic arguments, and treat instead the ADRF processes in static and spinning solids on the basis of coherent evolutions. The predictions of these analytical derivations are corroborated by numerical simulations. The effects of additional factors, including chemical shift anisotropies, J-couplings, and radiofrequency inhomogeneities, are also theoretically and experimentally explored.

Identifying and Overcoming Artifacts in 1 H-Based Saturation Transfer NOE NMR Experiments.

Magnetization transfer experiments are versatile nuclear magnetic resonance (NMR) tools providing site-specific information. We have recently discussed how saturation magnetization transfer (SMT) experiments could leverage repeated repolarizations arising from exchanges between labile and water protons to enhance connectivities revealed via the nuclear Overhauser effect (NOE). Repeated experience with SMT has shown that a number of artifacts may arise in these experiments, which may confound the information being sought - particularly when seeking small NOEs among closely spaced resonances. One of these pertains to what we refer to as "spill-over" effects, originating from the use of long saturation pulses leading to changes in the signals of proximate peaks. A second, related but in fact different effect, derives from what we describe as NOE "oversaturation", a phenomenon whereby the use of overtly intense RF fields overwhelms the cross-relaxation signature. The origin and ways to avoid these two effects are described. A final source of potential artifact arises in applications where the labile 1Hs of interest are bound to 15N-labeled heteronuclei. SMT's long 1H saturation times will then be usually implemented while under 15N decoupling based on cyclic schemes leading to decoupling sidebands. Although these sidebands usually remain invisible in NMR, they may lead to a very efficient saturation of the main resonance when touched by SMT frequencies. All of these phenomena are herein experimentally demonstrated, and solutions to overcome them are proposed.

Effective Hamiltonian and spin dynamics in fast MAS TRAPDOR-HMQC experiments involving spin-3/2 quadrupolar nuclei.

We present a theoretical and numerical description of the spin dynamics associated with TRAPDOR-HMQC (T-HMQC) experiment for a 1H (I) - 35Cl (S) spin system under fast magic angle spinning (MAS). Towards this an exact effective Hamiltonian describing the system is numerically evaluated with matrix logarithm approach. The different magnitudes of the heteronuclear and pure S terms in the effective Hamiltonian allow us to suggest a truncation approximation, which is shown to be in excellent agreement with the exact time evolution. Limitations of this approximation, especially at the rotary resonance condition, are discussed. The truncated effective Hamiltonian is further employed to monitor the buildup of various coherences during TRAPDOR irradiation. We observe and explain a functional resemblance between the magnitude of different terms in the truncated effective Hamiltonian and the amplitudes of various coherences during TRAPDOR irradiation, as function of crystallite orientation. Subsequently, the dependence of the sign (phase) of the T-HMQC signal on the coherence type generated is investigated numerically and analytically. We examine the continuous creation and evolution of various coherences at arbitrary times, i.e., at and between avoided level crossings. Behavior between consecutive crossings is described analytically and reveals 'quadrature' evolution of pairs of coherences and coherence interconversions. The adiabatic, sudden, and intermediate regimes for T-HMQC experiments are discussed within the approach established by A. J. Vega. Equations as well as numerical simulations suggest the existence of a driving coherence which builds up between consecutive crossings and then gets distributed at crossings among other coherences. In the intermediate regime, redistribution of the driving coherence to other coherences is almost uniform such that coherences involving S-spin double-quantum terms may be efficiently produced.

Selective Excitation of Long-Lived Nuclear Spin States.

Nuclear magnetization storage, once limited by longitudinal and transverse relaxation lifetimes, T1 and T2, can be prolonged by symmetry-adapted nuclear spin order, i.e. long-lived states (LLS) and long-lived coherences (LLC), which have significantly extended relaxation time constants compared to T1 and T2, respectively. Excitation and/or detection of LLS currently involves pulses covering wide frequency ranges in high-magnetic-field spectrometers. This leads to excitation of unwanted signals that may overlap and interfere with the resonances of interest. Herein, we present a new pulse sequence that converts longitudinal magnetization to LLS and further to detectable magnetization using only frequency-selective pulses. We demonstrate the suitability of this sequence for different J-coupled spin pairs in dipeptide AlaGly and protein Ubiquitin. The newly developed method is adapted for investigations of LLS in complex systems such as proteins and mixtures of metabolites where selected molecular groups are to be investigated separately.

The Extended Hadamard Transform: Sensitivity-Enhanced NMR Experiments Among Labile and Non-Labile 1 Hs of SARS-CoV-2-derived RNAs.

Hadamard encoded saturation transfer can significantly improve the efficiency of NOE-based NMR correlations from labile protons in proteins, glycans and RNAs, increasing the sensitivity of cross-peaks by an order of magnitude and shortening experimental times by ≥100-fold. These schemes, however, fail when tackling correlations within a pool of labile protons - for instance imino-imino correlations in RNAs or amide-amide correlations in proteins. Here we analyze the origin of the artifacts appearing in these experiments and propose a way to obtain artifact-free correlations both within the labile pool as well as between labile and non-labile 1 Hs, while still enjoying the gains arising from Hadamard encoding and solvent repolarizations. The principles required for implementing what we define as the extended Hadamard scheme are derived, and its clean, artifact-free, sensitivity-enhancing performance is demonstrated on RNA fragments derived from the SARS-CoV-2 genome. Sensitivity gains per unit time approaching an order of magnitude are then achieved in both imino-imino and imino-amino/aromatic protons 2D correlations; similar artifact-free sensitivity gains can be observed when carrying out extended Hadamard encodings of 3D NOESY/HSQC-type experiments. The resulting spectra reveal significantly more correlations than their conventionally acquired counterparts, which can support the spectral assignment and secondary structure determination of structured RNA elements.

Heteronuclear transfers from labile protons in biomolecular NMR: Cross polarization, revisited.

INEPT- and HMQC-based pulse sequences are widely used to transfer polarization between heteronuclei, particularly in biomolecular spectroscopy: they are easy to setup and involve low power deposition. Still, these short-pulse polarization transfers schemes are challenged by fast solvent chemical exchange. An alternative to improve these heteronuclear transfers is J-driven cross polarization (J-CP), which transfers polarization by spin-locking the coupled spins under Hartmann-Hahn conditions. J-CP provides certain immunity against chemical exchange and other T2-like relaxation effects, a behavior that is here examined in depth by both Liouville-space numerical and analytical derivations describing the transfer efficiency. While superior to INEPT-based transfers, fast exchange may also slow down these J-CP transfers, hurting their efficiency. This study therefore explores the potential of repeated projective operations to improve 1H→15N and 1H→15N→13C J-CP transfers in the presence of fast solvent chemical exchanges. It is found that while repeating J-CP provides little 1H→15N transfer advantages over a prolonged CP, multiple contacts that keep both the water and the labile protons effectively spin-locked can improve 1H→15N→13C transfers in the presence of chemical exchange. The ensuing Looped, Concatenated Cross Polarization (L-CCP) compensates for single J-CP losses by relying on the 13C's longer lifetimes, leading to a kind of "algorithmic cooling" that can provide high polarization for the 15N as well as carbonyl and alpha 13Cs. This can facilitate certain experiments, as demonstrated with triple resonance experiments on intrinsically disordered proteins involving labile, chemically exchanging protons.

3D Heteronuclear Magnetization Transfers for the Establishment of Secondary Structures in SARS-CoV-2-Derived RNAs.

Multidimensional NOESY experiments targeting correlations between exchangeable imino and amino protons provide valuable information about base pairing in nucleic acids. It has been recently shown that the sensitivity of homonuclear correlations involving RNA's labile imino protons can be significantly enhanced, by exploiting the repolarization brought about by solvent exchanges. Homonuclear correlations, however, are of limited spectral resolution, and usually incapable of tackling relatively large homopolymers with repeating structures like RNAs. This study presents a heteronuclear-resolved version of those NOESY experiments, in which magnetization transfers between the aqueous solvent and the nucleic acid protons are controlled by selecting specific chemical shift combinations of a coupled 1H-15N spin pair. This selective control effectively leads to a pseudo-3D version of HSQC-NOESY, but with cross-peaks enhanced by ∼2-5× as compared with conventional 2D NOESY counterparts. The enhanced signal sensitivity as well as access to both 15N-1H and 1H-1H NOESY dimensions can greatly facilitate RNA assignments and secondary structure determinations, as demonstrated here with the analysis of genome fragments derived from the SARS-CoV-2 virus.

Effective Hamiltonian and 1H-14N cross polarization/double cross polarization at fast MAS.

In this work we investigate in detail the underlying spin-dynamics associated with 1H-14N double CP experiments under fast MAS, recently demonstrated by Carnevale et al. We employ matrix logarithm and Floquet theory to compute numerically the effective Hamiltonian associated to the time-dependent problem. Certain common features related to construction of effective Hamiltonians by both approaches are discussed. The main observations related to 1H-14N CPMAS/double CP transfer are: (a) various spin terms of the effective Hamiltonian strongly depend on the crystallite orientation; (b) significant CP transfer occurs only when the magnitudes of the effective1H and 14N RF strengths are comparable, and simultaneously all pure 14N terms in the effective Hamiltonian are small, except for the longitudinal and the RF terms; (c) the sign of 14N CPMAS signal follows the sign of 14N effective RF strength; (d) sign of the double CP signal is largely independent of crystallite orientation. We predict and verify matching conditions employing multiples of the spinning frequency or involving different 14N RF strengths. We provide an analytical proof for (d). The proof also provides an estimate for the ratio of 1H-14N and 14N-1H transfer amplitudes which is further substantiated through simulations. In addition, we find that double CP signals include contributions from several single-quantum coherences present after the first CP process. The uneven contribution from different coherences leads to a reversal of signal at very short contact times, a feature noted experimentally by Carnevale et al. The connection between CPMAS transfer and efficient spin-lock is discussed and illustrated. The factors affecting second-order quadrupolar lineshapes in double CP experiment are examined. With a linear ramp of 1H RF amplitude we have observed that significant CP transfer occurs for more crystallite orientations resulting in improved sensitivity.

Sensitivity enhancement in 2D Double Cross Polarization experiments under fast MAS by recycling unused protons.

We demonstrate sensitivity enhancement via recycling of proton magnetization in 2D Double Cross Polarization (Double CP) experiments performed on fully protonated and uniformly labeled (13C, 15N) samples at a magic angle spinning rate of 60 kHz. Unused proton magnetization is preserved during t1 evolution either by locking it with CW irradiation or by employing rotor-synchronized pi pulses. A flip-back pulse together with a modified second CP block preserves unused proton magnetization resulting in enhanced sensitivity. We have achieved sensitivity enhancements of 15-20% and 25-28% in 1H-13C and 1H-15N 2D Double CP experiments respectively. At shorter recycle delays (∼0.25T1), relative sensitivity enhancements of 40-45% and 55% were obtained in 1H-13C and 1H-15N 2D Double CP experiments respectively. An analysis of the sensitivity enhancements and theoretical estimation of lineshapes in indirect dimension in the presence of proton recycling is provided.

Multidimensional J-driven NMR correlations by single-scan offset-encoded recoupling.

Two-dimensional (2D) correlations between bonded heteroatoms, lie at the cornerstone of many uses given to contemporary nuclear magnetic resonance (NMR). Improving the efficiency with which these correlations are established is an important topic in modern NMR, with potential applications in rapid chemical analysis and dynamic biophysical studies. Alternatives have been developed over the last decade to speed up these experiments, based among others on reducing the number of data points that need to be sampled, and/or shortening the inter-scan delays. Approaches have also been proposed to forfeit multi-scan schemes altogether, and complete full 2D correlations in a single shot. Here we explore and discuss a new alternative enabling the collection of such very fast - in principle, single-scan - acquisitions of 2D heteronuclear correlations among bonded species, which operates on the basis of a partial reintroduction of J couplings. Similar approaches had been proposed in the past based on collecting coupled spectra for arrays of off-resonance decoupling values; the proposal that is here introduced operates on the basis of suitably incorporating frequency-swept pulses, into spin-echo sequences. Thanks to the offset-dependent amplitude modulations of the in- and anti-phase components that such sequences impart, chemical shifts of coupled but otherwise unobserved nuclear species, can be extracted from the relative intensities and phases of J-coupled multiplets observed in one-dimensional acquisitions. A description of the steps needed to implement this rapid acquisition approach in a quantitative fashion, as well as applications of the ensuing sequences, are presented.

Heteronuclear cross-relaxation effects in the NMR spectroscopy of hyperpolarized targets.

Dissolution dynamic nuclear polarization (DNP) enables high-sensitivity solution-phase NMR experiments on long-lived nuclear spin species such as (15)N and (13)C. This report explores certain features arising in solution-state (1)H NMR upon polarizing low-γ nuclear species. Following solid-state hyperpolarization of both (13)C and (1)H, solution-phase (1)H NMR experiments on dissolved samples revealed transient effects, whereby peaks arising from protons bonded to the naturally occurring (13)C nuclei appeared larger than the typically dominant (12)C-bonded (1)H resonances. This enhancement of the satellite peaks was examined in detail with respect to a variety of mechanisms that could potentially explain this observation. Both two- and three-spin phenomena active in the solid state could lead to this kind of effect; still, experimental observations revealed that the enhancement originates from (13)C→(1)H polarization-transfer processes active in the liquid state. Kinetic equations based on modified heteronuclear cross-relaxation models were examined, and found to well describe the distinct patterns of growth and decay shown by the (13)C-bound (1)H NMR satellite resonances. The dynamics of these novel cross-relaxation phenomena were determined, and their potential usefulness as tools for investigating hyperpolarized ensembles and for obtaining enhanced-sensitivity (1)H NMR traces was explored.

Rapid acquisition of 14N solid-state NMR spectra with broadband cross polarization.

Nitrogen is an element of utmost importance in chemistry, biology and materials science. Of its two NMR-active isotopes, (14)N and (15)N, solid-state NMR (SSNMR) experiments are rarely conducted upon the former, due to its low gyromagnetic ratio (γ) and broad powder patterns arising from first-order quadrupolar interactions. In this work, we propose a methodology for the rapid acquisition of high quality (14)N SSNMR spectra that is easy to implement, and can be used for a variety of nitrogen-containing systems. We demonstrate that it is possible to dramatically enhance (14)N NMR signals in spectra of stationary, polycrystalline samples (i.e., amino acids and active pharmaceutical ingredients) by means of broadband cross polarization (CP) from abundant nuclei (e.g., (1)H). The BRoadband Adiabatic INversion Cross-Polarization (BRAIN-CP) pulse sequence is combined with other elements for efficient acquisition of ultra-wideline SSNMR spectra, including Wideband Uniform-Rate Smooth-Truncation (WURST) pulses for broadband refocusing, Carr-Purcell Meiboom-Gill (CPMG) echo trains for T2-driven S/N enhancement, and frequency-stepped acquisitions. The feasibility of utilizing the BRAIN-CP/WURST-CPMG sequence is tested for (14)N, with special consideration given to (i) spin-locking integer spin nuclei and maintaining adiabatic polarization transfer, and (ii) the effects of broadband polarization transfer on the overlapping satellite transition patterns. The BRAIN-CP experiments are shown to provide increases in signal-to-noise ranging from four to ten times and reductions of experimental times from one to two orders of magnitude compared to analogous experiments where (14)N nuclei are directly excited. Furthermore, patterns acquired with this method are generally more uniform than those acquired with direct excitation methods. We also discuss the proposed method and its potential for probing a variety of chemically distinct nitrogen environments.

Two-dimensional RF pulses: a new approach to selectively exciting J-coupled spins in nuclear magnetic resonance.

A new scheme for the excitation of spins according to the joint values of their heteronuclear or homonuclear J couplings and of their chemical shifts, is proposed and demonstrated. The principles of the new pulses involved derive from those employed in NMR imaging for exciting arbitrary 2D spatial shapes, using so-called "multidimensional" RF pulses. It is shown that if recast in a suitable spectroscopic framework, the distinction that π-pulses enable to establish between linear and bilinear interactions, support the selective excitation of coherences possessing arbitrary combinations of chemical shift and J-coupling values--a flexibility akin to that provided by a 2D J-resolved NMR spectrum. Details on the execution of the resulting 2D "J-shift" RF pulses are given, and examples where excitation only addresses spins with particular chemical shift offsets fulfilling specific J-coupling displacements, are demonstrated. Additional instances where such pulses could be applied, as well as main limitations of this new approach, are briefly discussed.

Broadband adiabatic inversion pulses for cross polarization in wideline solid-state NMR spectroscopy.

Efficient acquisition of ultra-wideline solid-state NMR powder patterns is a continuing challenge. In particular, when the breadth of the powder pattern is much larger than the cross-polarization (CP) excitation bandwidth, transfer efficiencies suffer and experimental times are greatly increased. Presented herein is a CP pulse sequence with an excitation bandwidth that is up to ten times greater than that available from a conventional spin-locked CP pulse sequence. The pulse sequence, broadband adiabatic inversion CP (BRAIN-CP), makes use of the broad, uniformly large frequency profiles of chirped inversion pulses, to provide these same characteristics to the polarization transfer process. A detailed theoretical analysis is given, providing insight into the polarization transfer process involved in BRAIN-CP. Experiments on spin-1/2 nuclei including (119)Sn, (199)Hg and (195)Pt nuclei are presented, and the large bandwidth improvements possible with BRAIN-CP are demonstrated. Furthermore, it is shown that BRAIN-CP can be combined with broadband frequency-swept versions of the Carr-Purcell-Meiboom-Gill experiment (for instance with WURST-CPMG, or WCPMG for brevity); the combined BRAIN-CP/WCPMG experiment then provides multiplicative signal enhancements of both CP and multiple-echo acquisition over a broad frequency region.

Toward single-shot pure-shift solution 1H NMR by trains of BIRD-based homonuclear decoupling.

Achieving homonuclear 1H decoupling remains one of the key challenges in liquid-state NMR. Such spectra would endow a variety of organic and analytical applications with an increased resolution, and would ideally do so even in a one-dimensional format. A number of parallel efforts aimed at achieving this goal using two-dimensional acquisitions have been proposed; approaches demonstrated over recent years include, among others, new modes for achieving purely-absorptive J spectroscopy, the use of spatially-selective manipulations, and exploiting the natural spin dilution afforded by heteronuclei. The present study relies on the latter approach, and explores the use of BIRD pulses distinguishing between protons bonded to (13)C from those bonded to (12)C, to achieve homonuclear decoupling in a continuous 1D scan. Studies on several representative compounds demonstrate that this goal can be implemented in a robust format, provided that suitable care is also taken to suppress unwanted coherences, of making all manipulations sufficiently broad-banded, and to provide adequate heteronuclear decoupling of the targeted protons. Dependable homonuclear decoupling performance can then be achieved, with minimal line width, fine-tuning, and sensitivity penalties.

Sensitizing solid state nuclear magnetic resonance of dilute nuclei by spin-diffusion assisted polarization transfers.

Recent years have witnessed efforts geared at increasing the sensitivity of NMR experiments, by relying on the suitable tailoring and exploitation of relaxation phenomena. These efforts have included the use of paramagnetic agents, enhanced (1)H-(1)H incoherent and coherent transfers processes in 2D liquid state spectroscopy, and homonuclear (13)C-(13)C spin diffusion effects in labeled solids. The present study examines some of the opportunities that could open when exploiting spontaneous (1)H-(1)H spin-diffusion processes, to enhance relaxation and to improve the sensitivity of dilute nuclei in solid state NMR measurements. It is shown that polarization transfer experiments executed under sufficiently fast magic-angle-spinning conditions, enable a selective polarization of the dilute low-γ spins by their immediate neighboring protons. Repolarization of the latter can then occur during the time involved in monitoring the signal emitted by the low-γ nuclei. The basic features involved in the resulting approach, and its potential to improve the effective sensitivity of solid state NMR measurements on dilute nuclei, are analyzed. Experimental tests witness the advantages that could reside from utilizing this kind of approach over conventional cross-polarization processes. These measurements also highlight a number of limitations that will have to be overcome for transforming selective polarization transfers of this kind into analytical methods of choice.

Indirect detection of nitrogen-14 in solid-state NMR spectroscopy.

NMR spectra of (14)N (spin I=1) are obtained by indirect detection in powders spinning at the magic angle. The method relies on the transfer of coherence from a neighboring "spy" nucleus with S=1/2, such as (13)C or (1)H, to single- or double-quantum transitions of (14)N nuclei. The transfer of coherence can occur through a combination of scalar and residual dipolar splittings (RDS); the latter are also known as second-order quadrupole-dipole cross terms. The two-dimensional NMR spectra reveal powder patterns determined by second- and third-order quadrupolar couplings. These spectra depend on the quadrupolar coupling constant C(Q) (typically a few megahertz), on the asymmetry parameter eta(Q) of the (14)N nucleus, and on the orientation of the internuclear vector r(IS) between the I ((14)N) and S (spy) nuclei with respect to the quadrupolar tensor. These parameters, which can be subject to motional averaging, can reveal valuable information about the structure and dynamics of nitrogen-containing solids. Application of this technique to various amino acids, either enriched in (13)C or with natural carbon isotope abundance, with spectra recorded at various magnetic fields, illustrates the scope of the method.

Indirect detection of nitrogen-14 in solids via protons by nuclear magnetic resonance spectroscopy.

This Communication describes the indirect detection of 14N nuclei (spin I=1) in solids by nuclear magnetic resonance (NMR) spectroscopy. The two-dimensional correlation method used here is closely related to the heteronuclear multiple quantum correlation (HMQC) experiment introduced in 1979 to study molecules in liquids, which has recently been used to study solids spinning at the magic angle. The difference is that the coherence transfer from neighboring 1H nuclei to 14N is achieved via a combination of J couplings and residual dipolar splittings (RDS). Projections of the two-dimensional correlation spectra onto the 14N dimension yield powder patterns which reflect the 14N quadrupolar interaction. In contrast to the indirect detection of 14N via 13C nuclei that was recently demonstrated [Gan, J. Am. Chem. Soc. 128 (2006) 6040; Cavadini et. al., J. Am. Chem. Soc., 128 (2006) 7706], this approach may benefit from enhanced sensitivity, and does not require isotopic enrichment in 13C, although the 1H line-widths may have to be reduced upon selective deuteration.

Nitrogen-14 NMR spectroscopy using residual dipolar splittings in solids.

It is shown that nuclear magnetic resonance (NMR) spectra of nitrogen-14 (spin I = 1) can be obtained by indirect detection in powders spinning at the magic angle (MAS). The method relies on the transfer of coherence from a neighboring nucleus with S = 1/2, such as carbon-13, to single- or double-quantum transitions of nitrogen-14 nuclei. The transfer of coherence occurs through second-order quadrupole-dipole cross terms, also known as residual dipolar splittings. The two-dimensional NMR spectra reveal powder patterns determined by the second-order quadrupolar interactions of nitrogen-14. Analysis of the spectra yields the quadrupolar coupling constant, CQ, and asymmetry parameter, etaQ, of nitrogen-14. These parameters can be related to the structure of nitrogen-containing solids.