A General Method for Extracting Individual Coupling Constants from Crowded 1H NMR Spectra

Couplings between protons, whether scalar or dipolar, provide a wealth of structural information. Unfortunately, the high number of ¹H‐¹H couplings gives rise to complex multiplets and severe overlap in crowded spectra, greatly complicating their measurement. Many different methods exist for disentangling couplings, but none approaches optimum resolution. Here, we present a general new 2D J‐resolved method, PSYCHEDELIC, in which all homonuclear couplings are suppressed in F₂, and only the couplings to chosen spins appear, as simple doublets, in F₁. This approaches the theoretical limit for resolving ¹H‐¹H couplings, with close to natural linewidths and with only chemical shifts in F₂. With the same high sensitivity and spectral purity as the parent PSYCHE pure shift experiment, PSYCHEDELIC offers a robust method for chemists seeking to exploit couplings for structural, conformational, or stereochemical analyses.     

HyperBIRD: A Sensitivity‐Enhanced Approach to Collecting Homonuclear‐Decoupled Proton NMR Spectra

Samples prepared following dissolution dynamic nuclear polarization (DNP) enable the detection of NMR spectra from low‐γ nuclei with outstanding sensitivity, yet have limited use for the enhancement of abundant species like ¹H nuclei. Small‐ and intermediate‐sized molecules, however, show strong heteronuclear cross‐relaxation effects: spontaneous processes with an inherent isotopic selectivity, whereby only the ¹³C‐bonded protons receive a polarization enhancement. These effects are here combined with a recently developed method that delivers homonuclear‐decoupled ¹H spectra in natural abundance samples based on heteronuclear couplings to these same, ¹³C‐bonded nuclei. This results in the HyperBIRD methodology; a single‐shot combination of these two effects that can simultaneously simplify and resolve complex, congested ¹H NMR spectra with many overlapping spin multiplets, while achieving 50–100 times sensitivity enhancements over conventional thermal counterparts.     

Uniform Reduction of Scalar Coupling by Real‐Time Homonuclear J‐Downscaled NMR

Scalar coupling in proton NMR spectra provides important information for the structural analysis. However, the low resolution due to the resulting signal splitting, together with the rather narrow spectral range of hydrogen, often prevents the extraction of J‐coupling information. Here we present a method to achieve real‐time homonuclear J‐downscaling. Thereby, all J‐values are uniformly reduced by an arbitrary scaling factor. In the resulting one‐dimensional spectra, signal overlap is reduced, while scalar coupling information is still available.     

Precise Measurement of Long‐Range Heteronuclear Coupling Constants by a Novel Broadband Proton–Proton‐Decoupled CPMG‐HSQMBC Method

A broadband proton–proton‐decoupled CPMG‐HSQMBC method for the precise and direct measurement of long‐range heteronuclear coupling constants is presented. The Zangger–Sterk‐based homodecoupling scheme reported herein efficiently removes unwanted proton–proton splittings from the heteronuclear multiplets, so that the desired heteronuclear couplings can be determined simply by measuring frequency differences between singlet maxima in the resulting spectra. The proposed pseudo‐1D/2D pulse sequences were tested on nucleotides, a metal complex incorporating P heterocycles, and diglycosyl (di)selenides, as well as on other carbohydrate derivatives, for the extraction of ⁿJ(¹H,³¹P), ⁿJ(¹H,⁷⁷Se), and ⁿJ(¹H,¹³C) values, respectively.     

Boosting the Resolution of 1H NMR Spectra by Homonuclear Broadband Decoupling

Broadband homonuclear decoupling of proton spectra, that is, the collapse of all multiplets into singlets, has the potential of boosting the resolution of ¹H NMR spectra. Several methods have been described in the last 40 years to achieve this goal. Most of them can only be applied in the indirect dimension of multi‐dimensional NMR spectra or special data processing is necessary to yield decoupled 1D proton spectra. Recently, complete decoupling of proton spectra during acquisition has been introduced; this not only significantly reduced the experimental time to record these spectra, but also removed the need for any sophisticated processing schemes. Here we present an introduction and overview of the techniques and applications of broadband proton‐decoupled proton experiments.     

PSYCHE CPMG–HSQMBC: An NMR Spectroscopic Method for Precise and Simple Measurement of Long‐Range Heteronuclear Coupling Constants

Among the NMR spectroscopic parameters, long‐range heteronuclear coupling constants convey invaluable information on torsion angles relevant to glycosidic linkages of carbohydrates. A broadband homonuclear decoupled PSYCHE CPMG–HSQMBC method for the precise and direct measurement of multiple‐bond heteronuclear couplings is presented. The PSYCHE scheme built into the pulse sequence efficiently eliminates unwanted proton–proton splittings from the heteronuclear multiplets so that the desired heteronuclear couplings can be determined simply by measuring frequency differences between peak maxima of pure antiphase doublets. Moreover, PSYCHE CPMG–HSQMBC can provide significant improvement in sensitivity as compared to an earlier Zangger–Sterk‐based method. Applications of the proposed pulse sequence are demonstrated for the extraction of ⁿJ(¹H,⁷⁷Se) and ⁿJ(¹H,¹³C) values, respectively, in carbohydrates; further extensions can be envisioned in any J‐based structural and conformational studies.     

Scalar Coupling Constants—Their Analysis and Their Application for the Elucidation of Structures

Since their discovery in the early fifties, scalar/coupling constants have been of great interest to the NMR spectroscopist. Their impact on structure determination by NMR spectroscopy is founded on the fact that the size of the coupling constant is directly related to molecular conformation. Today, for most chemical substances the parameters for the Karplus relationship, which relates the vicinial (3-bond) coupling constant to the dihedral angle, have been determined. In addition to proton–proton distances, the application of coupling constants in modern conformational analysis is indispensable. In the study of larger molecules which are of current interest, more and more involved experiments are necessary in order to overcome signal overlap and increasing line widths. A large number of experimental techniques for the determination of coupling constants has been developed; however, for this reason the choice of the most appropriate experiment to use has become more difficult. This decision must be made carefully to maximize instrument usage and obtain the largest number of couplings with the greatest accuracy possible. Many of the computer programs used in structure calculations can directly apply coupling constant restraints, similar to proton–proton distances developed from NOEs. Therefore, not only is the quality of the structure improved, but the molecular motions (internal dynamics) are better described. In this article, we review the techniques that exist today with particular attention paid to helping the non-expert to choose the appropriate experiment for the problem at hand. In addition, the use of coupling constants in computer simulations are discussed.     

Exclusively Heteronuclear NMR Experiments to Obtain Structural and Dynamic Information on Proteins

Provided that ¹³C‐detected NMR experiments are either preferable or complementary to ¹H detection, we report here tools to determine C^α? C′, C′? N, and C^α? H^α residual dipolar couplings on the basis of the CON experiment. The coupling constants determined on ubiquitin are consistent with the subset measured with the ¹H‐detected HNCO sequences. Since the utilization of residual dipolar couplings may depend on the mobility of the involved nuclei, we also provide tools to measure longitudinal and transverse relaxation rates of N and C′. This new set of experiments is a further development of a whole strategy based on ¹³C direct‐detection NMR spectroscopy for the study of biological macromolecules.     

NMR Spin–Spin Coupling Constants in Polymethine Dyes as Polarity Indicators

Herein, we explore the use of spin–spin coupling constants (SSCCs) in merocyanine (MCYNE) dyes as indicators of polarity. For this purpose, we use Car–Parrinello hybrid quantum mechanics/molecular mechanics (QM/MM) to determine the structures of MCYNE in solvents of different polarity, followed by computations of the SSCCs by using QM/MM linear‐response theory. The molecular geometry of MCYNE switches between neutral, cyanine‐like, and zwitterionic depending on the polarity of the solvent. This structural variation is clearly reflected in the proton SSCCs in the polymethine backbone, which are highly sensitive to the dielectric nature of the environment; this mechanism can be used as a “polarity indicator” for different microenvironments. This result is highlighted by computing the SSCCs of the MCYNE probe in the cavity of the beta‐lactoglobulin protein. The computed SSCCs clearly indicate a non‐polar hydrophobic dielectric nature of this cavity.     

1H NMR‐Guided Isolation of Formyl‐Phloroglucinol Meroterpenoids from the Leaves of Eucalyptus robusta

Nine formyl‐phloroglucinolmeroterpenoids (FPMs), namely, eucalrobusones A–I ( 1 – 9 ), were isolated from the leaves of Eucalyptus robusta by tracking the phenolic hydroxyl ¹H NMR peaks. The Snatzke helicity rules for the Cotton effects of twisted benzene rings were applied to elucidate the absolute configurations of the FPMs. These findings, along with NMR spectroscopy, the circular dichroism (CD) exciton chirality method, and CD calculations, allowed complete structures for the FPMs to be assigned. Eucalrobusones A–F ( 1 – 6 ) are novel adducts formed between a formyl‐derived carbon atom on the phloroglucinol ring and monoterpene and sesquiterpene components. Eucalrobusones G–I ( 7 – 9 ) are the first examples of FPMs with cubebane part structures connected by an unusual 1‐oxaspiro[5.5]undecane subunit. Among these isolates, eucalrobusone C ( 3 ) showed significant cytotoxicity against HepG2, MCF‐7, and U2OS cancer cell lines, with IC₅₀ values less than 10 μm . Compound 3 significantly blocks cell proliferation in MCF‐7 cells and induces MCF‐7 cell death through apoptosis.     

J‐Edited Pure Shift NMR for the Facile Measurement of nJHH for Specific Protons

We report a novel 1D J‐edited pure shift NMR experiment (J‐PSHIFT) that was constructed from a pseudo 2D experiment for the direct measurement of proton–proton scalar couplings. The experiment gives homonuclear broad‐band ¹H‐decoupled ¹H NMR spectra, which provide a single peak for chemically distinct protons, and only retain the homonuclear‐scalar‐coupled doublet pattern at the chemical‐shift positions of the protons in the coupled network of a specific proton. This permits the direct and unambiguous measurement of the magnitudes of the couplings. The incorporation of a 1D selective correlation spectroscopy (COSY)/ total correlation spectroscopy (TOCSY) block in lieu of the initial selective pulse, results in the exclusive detection of the correlated spectrum of a specific proton.     

Characterization of Doubly Ionic Hydrogen Bonds in Protic Ionic Liquids by NMR Deuteron Quadrupole Coupling Constants: Differences to H‐bonds in Amides,Peptides, and Proteins

We present the first deuteron quadrupole coupling constants (DQCCs) for selected protic ionic liquids (PILs) measured by solid‐state NMR spectroscopy. The experimental data are supported by dispersion‐corrected density functional theory (DFT‐D3) calculations and molecular dynamics (MD) simulations. The DQCCs of the N−D bond in the triethylammonium cations are the lowest reported for deuterons in PILs, indicating strong hydrogen bonds between ions. The NMR coupling parameters are compared to those in amides, peptides, and proteins. The DQCCs show characteristic behavior with increasing interaction strength of the counterion and variation of the H‐bond motifs. We report the similar presence of the quadrupolar splitting pattern and the narrow liquid line in the NMR spectra over large temperature ranges, indicating the heterogeneous nature of PILs.