Quantitative analysis of 14N quadrupolar coupling using 1H detected 14N solid-state NMR

NMR of a single nuclear spin detected by a scanning tunnelling microscope

We detect a single spin nuclear magnetic resonance (NMR) by monitoring the intensity modulations of a selected hyperfine line in the electron spin resonance (ESR) spectrum. We analyse the power spectrum of the corresponding hyperfine intensity and obtain the nuclear magnetic resonance (NMR) spectrum. Our process also demonstrates ionization of a molecule with the bias voltage of a Scanning Tunnelling Microscope (STM), allowing detection of NMR even in molecules that are non-radical in their neutral state. We have observed this phenomenon in four types of molecules: toluene, triphenylphosphine, TEMPO and adenosine triphosphate (ATP) showing NMR of 1H, 13C, 31P and 14N nuclei. The spectra are detailed and show signatures of the chemical environment, i.e. chemical shifts. A theoretical model to account for these data is outlined.

Probing the Effect of Fluorine on Hydrogen Bonding Interactions in a Pharmaceutical Hydrate Using Advanced Solid-State NMR

Structural studies of pharmaceutical hydrates are essential to understanding stability-related issues, especially during the heating process of formulation. A thorough understanding of the hydration and dehydration behavior of active pharmaceutical ingredient (API) hydrate is also important since phase transitions can occur during the formulation process. This is because dehydration could result in a considerable rearrangement in the structure if water-API hydrogen bonding is present. We perform advanced solid-state NMR experiments on regorafenib monohydrate to investigate the role of fluorine in hydrogen bonding interaction, and the results are compared to its anhydrous form and its structural analogue, namely, sorafenib. Our results show that significant structural changes could not be observed on dehydration. Based on our study, it can be concluded that the introduction of fluorine restricts the intramolecular hydrogen bonding and the asymmetry in the structure of regorafenib monohydrate is absent, in comparison to sorafenib.

Spin-dynamics and efficiency of single 14N-1H cross-polarization at fast magic angle spinning in solids

The naturally abundant 14N isotope (>99 %) is sparingly employed for characterization in solid-state nuclear magnetic resonance (NMR) despite the importance of nitrogen atoms in shaping molecular structures and properties. This inhibition can be attributed to large quadrupolar couplings (∼several MHz), resulting in more involved spin methodologies for 14N nuclei. Experimentally, spin-½ nuclei are utilized for excitation and detection through two-way (1H→14N→1H) polarization transfer between spin-½ nuclei and 14N. Herein, we show direct 14N spin excitation followed by 14N→1H cross-polarization (CP) is an efficient method for polarization transfer even for 14N spins with a large quadrupolar coupling constant (3-4 MHz). This contrasts previous studies, which indicate that 1H-14N spectra can only be observed with a pair of at least a rotor period-long symmetric 14N pulses (J. Chem. Phys. 151 (2019) 154202). The 14N→1H CP spin dynamics have been experimentally established and can be explained in analogy to spin-½ Hartmann-Hahn CP if visualized in the quadrupolar jolting frame. The 14N→1H CP is ∼1.9-2.7 times more efficient in polarization transfer than other 14N edited experiments. Considering shorter 14N T1 relaxation times compared to protons, 14N edited spectra were recorded using 14N→1H CP, resulting in enhanced sensitivity per unit of time.

Electron diffraction and solid-state NMR reveal the structure and exciton coupling in a eumelanin precursor

Eumelanin, a versatile biomaterial found throughout the animal kingdom, performs essential functions like photoprotection and radical scavenging. The diverse properties of eumelanin are attributed to its elusive and heterogenous structure with DHI (5,6-dihydroxyindole) and DHICA (5,6-dihydroxyindole-2-carboxylic acid) precursors as the main constituents. Despite DHICA being recognized as the key eumelanin precursor, its crystal structure and functional role in the assembled state remain unknown. Herein, we employ a synthesis-driven, bottom-up approach to elucidate the structure and assembly-specifics of DHICA, a critical building block of eumelanin. We introduce an interdisciplinary methodology to analyse the nanocrystalline assembly of DHICA, employing three-dimensional electron diffraction (3D ED), solid-state NMR and density functional theory (DFT), while correlating the structural aspects with the electronic spectroscopic features. The results underscore charge-transfer exciton delocalization as the predominant energy transfer mechanism within the π-π stacked and hydrogen-bonded crystal network of DHICA. Additionally, extending the investigation to the 13C-labelled DHICA-based polymer improves our understanding of the chemical heterogeneity across the eumelanin pigment, providing crucial insights into the structure of eumelanin.

Comparison of methods for 14N-1H recoupling in 14N-1H HMQC MAS NMR

¹H-detected ¹⁴N heteronuclear multiple-quantum coherence (HMQC) magic-angle-spinning (MAS) NMR experiments performed at fast magic-angle spinning (≥50 kHz) are finding increasing application, e.g., to pharmaceuticals. Of importance to the efficacy of these techniques is the recoupling technique applied to reintroduce the ¹H-¹⁴N dipolar coupling. In this paper, we compare, by experiment and 2-spin density matrix simulations, two classes of recoupling scheme: first, those based on n = 2 rotary resonance, namely R³ and spin-polarisation inversion SPI-R³, and the symmetry based SR4₁² method and, second, the TRAPDOR method. Both classes require optimisation depending on the magnitude of the quadrupolar interaction, and thus there is a compromise choice for samples with more than one nitrogen site, as is the case for the studied dipeptide β-AspAla that contains two nitrogen sites with a small and large quadrupolar coupling constant. Considering this, we observe better sensitivity for the TRAPDOR method, though noting the marked sensitivity of TRAPDOR to the ¹⁴N transmitter offset, with both SPI-R³ and SR4₁² giving similar recoupling performance.

Ultrafast Magic Angle Spinning Solid-State NMR Spectroscopy: Advances in Methodology and Applications

Solid-state NMR spectroscopy is one of the most commonly used techniques to study the atomic-resolution structure and dynamics of various chemical, biological, material, and pharmaceutical systems spanning multiple forms, including crystalline, liquid crystalline, fibrous, and amorphous states. Despite the unique advantages of solid-state NMR spectroscopy, its poor spectral resolution and sensitivity have severely limited the scope of this technique. Fortunately, the recent developments in probe technology that mechanically rotate the sample fast (100 kHz and above) to obtain "solution-like" NMR spectra of solids with higher resolution and sensitivity have opened numerous avenues for the development of novel NMR techniques and their applications to study a plethora of solids including globular and membrane-associated proteins, self-assembled protein aggregates such as amyloid fibers, RNA, viral assemblies, polymorphic pharmaceuticals, metal-organic framework, bone materials, and inorganic materials. While the ultrafast-MAS continues to be developed, the minute sample quantity and radio frequency requirements, shorter recycle delays enabling fast data acquisition, the feasibility of employing proton detection, enhancement in proton spectral resolution and polarization transfer efficiency, and high sensitivity per unit sample are some of the remarkable benefits of the ultrafast-MAS technology as demonstrated by the reported studies in the literature. Although the very low sample volume and very high RF power could be limitations for some of the systems, the advantages have spurred solid-state NMR investigation into increasingly complex biological and material systems. As ultrafast-MAS NMR techniques are increasingly used in multidisciplinary research areas, further development of instrumentation, probes, and advanced methods are pursued in parallel to overcome the limitations and challenges for widespread applications. This review article is focused on providing timely comprehensive coverage of the major developments on instrumentation, theory, techniques, applications, limitations, and future scope of ultrafast-MAS technology.

Improved resolution for spin-3/2 isotopes in solids via the indirect NMR detection of triple-quantum coherences using the T-HMQC sequence

The indirect NMR detection of quadrupolar nuclei in solids under magic-angle spinning (MAS) is possible with the through-space HMQC (heteronuclear multiple-quantum coherence) scheme incorporating the TRAPDOR (transfer of population in double-resonance) dipolar recoupling. This sequence, called T-HMQC, exhibits limited t₁-noise. In this contribution, with the help of numerical simulations of spin dynamics, we show that most of the time, the fastest coherence transfer in the T-HMQC scheme is achieved when TRAPDOR recoupling employs the highest radiofrequency (rf) field compatible with the probe specifications. We also demonstrate how the indirect detection of the triple-quantum (3Q) coherences of spin-3/2 quadrupolar nuclei in solids improves the spectral resolution for these isotopes. The sequence is then called T-HMQC₃. We demonstrate the gain in resolution provided by this sequence for the indirect proton detection of ³⁵Cl nuclei in l-histidine∙HCl and l-cysteine∙HCl, as well as that of ²³Na isotope in NaH₂PO₄. These experiments indicate that the gain in resolution depends on the relative values of the chemical and quadrupolar-induced shifts (QIS) for the different spin-3/2 species. In the case of NaH₂PO₄, we show that the transfer efficiency of the T-HMQC₃ sequence employing an rf-field of 80 kHz with a MAS frequency of 62.5 kHz reaches 75% of that of the t₁-noise eliminated (TONE) dipolar-mediated HMQC (D-HMQC) scheme.

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.

Optimisation of 1H PMLG homonuclear decoupling at 60 kHz MAS to enable 15N-1H through-bond heteronuclear correlation solid-state NMR spectroscopy

The Lee-Goldburg condition for homonuclear decoupling in 1H magic-angle spinning (MAS) solid-state NMR sets the angle θ, corresponding to arctan of the ratio of the rf nutation frequency, ν1, to the rf offset, to be the magic angle, θm, equal to tan-1(√2) = 54.7°. At 60 kHz MAS, we report enhanced decoupling compared to MAS alone in a 1H spectrum of 15N-glycine with at θ = 30° for a ν1 of ∼100 kHz at a 1H Larmor frequency, ν0, of 500 MHz and 1 GHz, corresponding to a high chemical shift scaling factor (λCS) of 0.82. At 1 GHz, we also demonstrate enhanced decoupling compared to 60 kHz MAS alone for a lower ν1 of 51 kHz, i.e., a case where the nutation frequency is less than the MAS frequency, with θ = 18°, λCS = 0.92. The ratio of the rotor period to the decoupling cycle time, Ψ = τr/τc, is in the range 0.53 to 0.61. Windowed decoupling using the optimised parameters for a ν1 of ∼100 kHz also gives good performance in a 1H spin-echo experiment, enabling implementation in a 1H-detected 15N-1H cross polarisation (CP)-refocused INEPT heteronuclear correlation NMR experiment. Specifically, initial 15N transverse magnetisation as generated by 1H-15N CP is transferred back to 1H using a refocused INEPT pulse sequence employing windowed 1H decoupling. Such an approach ensures the observation of through-bond N-H connectivities. For 15N-glycine, while the CP-refocused INEPT experiment has a lower sensitivity (∼50%) as compared to a double CP experiment (with a 200 μs 15N to 1H CP contact time), there is selectivity for the directly bonded NH3+ moiety, while intensity is observed for the CH21H resonances in the double CP experiment. Two-dimensional 15N-1H correlation MAS NMR spectra are presented for the dipeptide β-AspAla and the pharmaceutical cimetidine at 60 kHz MAS, both at natural isotopic abundance. For the dipeptide β-AspAla, different build-up dependence on the first spin-echo duration is observed for the NH and NH3+ moieties demonstrating that the experiment could be used to distinguish resonances for different NHx groups.

Combining heteronuclear correlation NMR with spin-diffusion to detect relayed Cl-H-H and N-H-H proximities in molecular solids

Analysis of short-to-intermediate range intermolecular interactions offers a great way of characterizing the solid-state organization of small molecules and materials. This can be achieved by two-dimensional (2D) homo- and heteronuclear correlation NMR spectroscopy, for example, by carrying out experiments at high magnetic fields in conjunction with fast magic-angle spinning (MAS) techniques. But, detecting 2D peaks for heteronuclear dipolar coupled spin pairs separated by greater than 3 Å is not always straightforward, particularly when low-gamma quadrupolar nuclei are involved. Here, we present a 2D correlation NMR experiment that combines the advantages of heteronuclear-multiple quantum coherence (HMQC) and proton-based spin-diffusion (SD) pulse sequences using radio-frequency-driven-recouping (RFDR) to probe inter and intramolecular ¹H-X (X = ¹⁴N, ³⁵Cl) interactions. This experiment can be used to acquire 2D ¹H{X}-HMQC filtered ¹H-¹H correlation as well as 2D ¹H-X HMQC spectra. Powder forms of dopamine·HCl and l-histidine·HCl·H₂O are characterized at high fields (21.1 T and 18.8 T) with fast MAS (60 kHz) using the 2D HMQC-SD-RFDR approach. Solid-state NMR results are complemented with NMR crystallography analyses using the gauge-including projector augmented wave (GIPAW) approach. For histidine·HCl·H₂O, 2D peaks associated with ¹⁴N-¹H-¹H and ³⁵Cl-¹H-¹H distances of up to 4.4 and 3.9 Å have been detected. This is further corroborated by the observation of 2D peaks corresponding to ¹⁴N-¹H-¹H and ³⁵Cl-¹H-¹H distances of up to 4.2 and 3.7 Å in dopamine·HCl, indicating the suitability of the HMQC-SD-RFDR experiments for detecting medium-range proximities in molecular solids.

Detection of remote proton-nitrogen correlations by 1H-detected 14N overtone solid-state NMR at fast MAS

Detecting proton and nitrogen correlations in solid-state nuclear magnetic resonance (NMR) is important for the structural determination of biological and chemical systems. Recent advances in proton detection-based approaches under fast magic-angle spinning have facilitated the detection of ¹H-¹⁴N correlations by solid-state NMR. However, observing remote ¹H-¹⁴N correlations by these approaches is still a challenge, especially for ¹⁴N sites having large quadrupolar couplings. To address this issue, we introduce the ¹H-¹⁴N overtone continuous wave rotational-echo saturation-pulse double-resonance (¹H-¹⁴N OT CW-RESPDOR) sequence. Unlike regular 2D correlation experiments where the indirect dimension is recorded in the time domain, the ¹H-¹⁴N OT CW-RESPDOR experiment is directly observed in the frequency domain. A set of ¹H-¹⁴N OT CW-RESPDOR filtered ¹H spectra is recorded at varying ¹⁴N OT frequencies. Thanks to the selective nature of the ¹⁴N OT pulse, the filtered ¹H spectra appear only if the ¹⁴N OT frequency hits the positions of the ¹⁴N OT central band or one of the spinning sidebands. This set of filtered ¹H spectra represents a 2D ¹H-¹⁴N OT correlation map. We have also investigated the optimizable parameters for CW-RESPDOR and figured out that these parameters are not strictly needed for our working magnetic field of 14.1 T. Hence, the experiment is easy to set up and requires almost no optimization. We have demonstrated the experimental feasibility of ¹H-¹⁴N OT CW-RESPDOR on monoclinic L-histidine and L-alanyl L-alanine. The remote ¹H-¹⁴N correlations have been efficiently detected, no matter how large the ¹⁴N quadrupolar interaction is, and agree with the crystal structures. In addition, based on the remote ¹H-¹⁴N correlations from the non-protonated ¹⁴N site of L-histidine, we can unambiguously distinguish the orthorhombic and monoclinic forms.

Separating an overlapped 1H peak and identifying its 1H-1H correlations with the use of single-channel 1H solid-state NMR at fast MAS

Fast magic-angle spinning (≥60 ​kHz) technique has enabled the acquisition of high-resolution ¹H NMR spectra of solid materials. However, the spectral interpretation is still difficult because the ¹H peaks are overlapped due to the narrow chemical shift range and broad linewidths. An additional ¹³C or ¹⁴N or ¹H dimension possibly addresses the issues of overlapped proton resonances, but it leads to the elongated experimental time. Herein, we introduce a single-channel ¹H experiment to separate the overlapped ¹H peak and identify its spatially proximal ¹H-¹H correlations. This sequence combines selective excitation, selective ¹H-¹H polarization transfer by selective recoupling of protons (SERP), and broadband ¹H recoupling by back-to-back (BABA) recoupling sequences. The concept for ¹H separation is based on (i) the selective excitation of a well-resolved ¹H peak and (ii) the selective dipolar polarization transfer from this isolated ¹H peak to one of the ¹H peaks in the overlapped/poor resolution region by SERP and (iii) the detection of ¹H-¹H correlations from these two ¹H peaks to other neighboring ¹Hs by BABA. We demonstrated the applicability of this approach to identify overlapped peaks on two molecules, β-L-aspartyl-l-alanine and Pioglitazone.HCl. The sequence allows the clear observation of ¹H-¹H correlations from an overlapped ¹H peak without an additional heteronuclear dimension and ensures efficient polarization transfers that leads to twelve fold reduction in experimental time compared to ¹⁴N edited experiments. The limitation and the conditions of applicability for this approach are discussed in detail.

A dimeric chlorophyll electron acceptor differentiates type I from type II photosynthetic reaction centers

This research addresses one of the most compelling issues in the field of photosynthesis, namely, the role of the accessory chlorophyll molecules in primary charge separation. Using a combination of empirical and computational methods, we demonstrate that the primary acceptor of photosystem (PS) I is a dimer of accessory and secondary chlorophyll molecules, Chl_2A and Chl_3A, with an asymmetric electron charge density distribution. The incorporation of highly coupled donors and acceptors in PS I allows for extensive delocalization that prolongs the lifetime of the charge-separated state, providing for high quantum efficiency. The discovery of this motif has widespread implications ranging from the evolution of naturally occurring reaction centers to the development of a new generation of highly efficient artificial photosynthetic systems.

Video abstract

Selective 1H-14N Distance Measurements by 14N Overtone Solid-State NMR Spectroscopy at Fast MAS

Accurate distance measurements between proton and nitrogen can provide detailed information on the structures and dynamics of various molecules. The combination of broadband phase-modulated (PM) pulse and rotational-echo saturation-pulse double-resonance (RESPDOR) sequence at fast magic-angle spinning (MAS) has enabled the measurement of multiple 1H-14N distances with high accuracy. However, complications may arise when applying this sequence to systems with multiple inequivalent 14N nuclei, especially a single 1H sitting close to multiple 14N atoms. Due to its broadband characteristics, the PM pulse saturates all 14N atoms; hence, the single 1H simultaneously experiences the RESPDOR effect from multiple 1H-14N couplings. Consequently, no reliable H-N distances are obtained. To overcome the problem, selective 14N saturation is desired, but it is difficult because 14N is an integer quadrupolar nucleus. Alternatively, 14N overtone (OT) NMR spectroscopy can be employed owing to its narrow bandwidth for selectivity. Moreover, owing to the sole presence of two energy levels (m = ± 1), the 14N OT spin dynamics behaves similarly to that of spin-1/2. This allows the interchangeability between RESPDOR and rotational-echo double-resonance (REDOR) since their principles are the same except the degree of 14N OT population transfer; saturation for the former whereas inversion for the latter. As the ideal saturation/inversion is impractical due to the slow and orientation-dependent effective nutation of 14N OT, the working condition is usually an intermediate between REDOR and RESPDOR. The degree of 14N OT population transfer can be determined from the results of protons with short distances to 14N and then can be used to obtain long-distance determination of other protons to the same 14N site. Herein, we combine the 14N OT and REDOR/RESPDOR to explore the feasibility of selective 1H-14N distance measurements. Experimental demonstrations on simple biological compounds of L-tyrosine.HCl, N-acetyl-L-alanine, and L-alanyl-L-alanine were performed at 14.1 T and MAS frequency of 62.5 kHz. The former two consist of a single 14N site, whereas the latter consists of two 14N sites. The experimental optimizations and reliable fittings by the universal curves are described. The extracted 1H-14N distances by OT-REDOR are in good agreement with those determined by PM-RESPDOR and diffraction techniques.

Low-power synchronous helical pulse sequences for large anisotropic interactions in MAS NMR: Double-quantum excitation of 14N

We develop a theoretical framework for a class of pulse sequences in the nuclear magnetic resonance (NMR) of rotating solids, which are applicable to nuclear spins with anisotropic interactions substantially larger than the spinning frequency, under conditions where the radiofrequency amplitude is smaller than or comparable to the spinning frequency. The treatment is based on average Hamiltonian theory and allows us to derive pulse sequences with well-defined relationships between the pulse parameters and spinning frequency for exciting specific coherences without the need for any detailed calculations. This framework is applied to the excitation of double-quantum spectra of ¹⁴N and is used both to evaluate the existing low-power pulse schemes and to predict the new ones, which we present here. It is shown that these sequences can be designed to be γ-encoded and therefore allow the acquisition of sideband-free spectra. It is also shown how these new double-quantum excitation sequences are incorporated into heteronuclear correlation NMR, such as ¹H-¹⁴N dipolar double-quantum heteronuclear multiple-quantum correlation spectroscopy. The new experiments are evaluated both with numerical simulations and experiments on glycine and N-acetylvaline, which represent cases with "moderate" and "large" quadrupolar interactions, respectively. The analyzed pulse sequences perform well for the case of a "moderate" quadrupolar interaction, however poorly with a "large" quadrupolar interaction, for which future work on pulse sequence development is necessary.

Fast Acquisition of Proton-Detected HETCOR Solid-State NMR Spectra of Quadrupolar Nuclei and Rapid Measurement of NH Bond Lengths by Frequency Selective HMQC and RESPDOR Pulse Sequences

Fast magic-angle spinning (MAS), frequency selective (FS) heteronuclear multiple quantum coherence (HMQC) experiments which function in an analogous manner to solution SOFAST HMQC NMR experiments, are demonstrated. Fast MAS enables efficient FS excitation of ¹ H solid-state NMR signals. Selective excitation and observation preserves ¹ H magnetization, leading to a significant shortening of the optimal inter-scan delay. Dipolar and scalar ¹ H{¹⁴ N} FS HMQC solid-state NMR experiments routinely provide 4- to 9-fold reductions in experiment times as compared to conventional ¹ H{¹⁴ N} HMQC solid-state NMR experiments. ¹ H{¹⁴ N} FS resonance-echo saturation-pulse double-resonance (RESPDOR) allowed dipolar dephasing curves to be obtained in minutes, enabling the rapid determination of NH dipolar coupling constants and internuclear distances. ¹ H{¹⁴ N} FS RESPDOR was used to assign multicomponent active pharmaceutical ingredients (APIs) as salts or cocrystals. FS HMQC also provided enhanced sensitivity for ¹ H{¹⁷ O} and ¹ H{³⁵ Cl} HMQC experiments on ¹⁷ O-labeled Fmoc-alanine and histidine hydrochloride monohydrate, respectively. FS HMQC and FS RESPDOR experiments will provide access to valuable structural constraints from materials that are challenging to study due to unfavorable relaxation times or dilution of the nuclei of interest.

Selective detection of active pharmaceutical ingredients in tablet formulations using solid-state NMR spectroscopy

Atomic-level characterization of active pharmaceutical ingredients (API) is crucial in pharmaceutical industry because APIs play an important role in physicochemical properties of drug formulations. However, the analysis of targeted APIs in intact tablet formulations is less straightforward due to the coexistence of excipients as major components and different APIs at dilute concentrations (often below 10 ​wt% loading). Although solid-state (ss) NMR spectroscopy is widely used to investigate short-range order, polymorphism, and pseudo-polymorphism in neat pharmaceutical compounds, the analysis of complex drug formulations is often limited by overlapped signals that originate from structurally different APIs and excipients. In particular, such examples are frequently encountered in the analysis of ¹H ssNMR spectra of pharmaceutical formulations. While the high-resolution in ¹H ssNMR spectra can be attained by, for example, high magnetic fields accompanied by fast magic-angle spinning (MAS) approaches, the spectral complexity associated with the mixtures of compounds hinders the accurate determination of chemical shifts and through-space proximities. Here we propose a fast MAS (70 ​kHz) NMR experiment for the selective detection of ¹H signals associated with an API from a severely overlapped NMR spectrum of a tablet formulation. Spectral simplification is achieved by combining (i) symmetry-based dipolar recoupling (SR4₁²) rotational-echo saturation-pulse double-resonance (RESPDOR) with phase-modulate (PM) saturation pulses, (ii) radio frequency-driven recoupling (RFDR), and (iii) double-quantum excitation using Back-to-Back (BaBa) pulse sequence elements. First, ¹H sites in close proximities to ¹⁴N nuclei of an API are excited using a PM-S-RESPDOR sequence, and simultaneously, the other unwanted ¹H signals of excipients are suppressed. Then, ¹H magnetization transfer to adjacent ¹H sites in the API is achieved by spin diffusion process using a RFDR sequence, which polarizes to ¹H sites within the crystalline API regions of the drug formulation. Next, a PM-S-RESPDOR-RFDR sequence is combined with a Back-to-Back (BaBa) sequence to elucidate local-structures and ¹H-¹H proximities of the API in a dosage form. The PM-S-RESPDOR-RFDR-BaBa experiment is employed in one- (1D) and two-dimensional (2D) versions to selectively detect the ¹H ssNMR spectrum of l-cysteine (10.6 ​wt% or 0.11 ​mg) in a commercial formulation, and compared with the spectra of neat l-cysteine recorded using a standard BaBa experiment. The 2D ¹H double-quantum-single-quantum (DQ-SQ) spectrum of the API (l-cysteine)-detected pharmaceutical tablet is in good agreement with the 2D ¹H DQ-SQ spectrum obtained from the pure API molecule. Furthermore, the sensitivity and robustness of the experiment is examined by selectively detecting ¹H{¹⁴N} signals in an amino acid salt, l-histidine^.H₂O^.HCl.

Evaluation of excitation schemes for indirect detection of 14N via solid-state HMQC NMR experiments

It has previously been shown that ¹⁴N NMR spectra can be reliably obtained through indirect detection via HMQC experiments. This method exploits the transfer of coherence between single-(SQ) or double-quantum (DQ) ¹⁴N coherences, and SQ coherences of a suitable spin-1/2 'spy' nucleus, e.g., ¹H. It must be noted that SQ-SQ methods require a carefully optimized setup to minimize the broadening related to the first-order quadrupole interaction (i.e., an extremely well-adjusted magic angle and a highly stable spinning speed), whereas DQ-SQ ones do not. In this work, the efficiencies of four ¹⁴N excitation schemes (DANTE, XiX, Hard Pulse (HP), and Selective Long Pulse (SLP)) are compared using J-HMQC based numerical simulations and either SQ-SQ or DQ-SQ ¹H-{¹⁴N} D-HMQC experiments on l-histidine HCl and N-acetyl-l-valine at 18.8 T and 62.5 kHz MAS. The results demonstrate that both DANTE and SLP provide a more efficient ¹⁴N excitation profile than XiX and HP. Furthermore, it is shown that the SLP scheme: (i) is efficient over a large range of quadrupole interaction, (ii) is highly robust to offset and rf-pulse length and amplitude, and (iii) is very simple to set up. These factors make SLP ideally suited to widespread, non-specialist use in solid-state NMR analyses of nitrogen-containing materials.