Fast and simultaneous determination of 1 H-1 H and 1 H-19 F scalar couplings in complex spin systems: Application of PSYCHE homonuclear broadband decoupling

Rapid Measurement of Heteronuclear Coupling Constants in Complex NMR Spectra

The NMR analysis of fluorine-containing molecules, increasingly widespread due to their importance in pharmaceuticals and biochemistry, poses significant challenges. Severe peak overlap in the proton spectrum often hinders the extraction of critical structural information in the form of heteronuclear scalar coupling constants, which are crucial for determining pharmaceutical properties and biological activity. Here, a new method, IPAP-FESTA, is reported that drastically simplifies measurements of the signs and magnitudes of proton-fluorine couplings. Its usefulness is demonstrated for the structural study of the steroidal drug fluticasone propionate extracted from a commercial formulation and for assessing solvent effects on the conformational equilibrium in a physically inseparable fluorohydrin mixture.

Pure shift edited NMR methodologies for the extraction of Homo- and heteronuclear couplings with ultra-high resolution

The scalar couplings that result in the splitting of the signals in the NMR spectrum arise due to the interaction of the nuclear spins, whereby the spin polarization is transmitted through chemical bonds. The interaction strengths depend inter alia on the number of consecutive chemical bonds intervening between the two interacting spins and on the molecular conformation. The pairwise interaction of many spins in a molecule resulting in a complex spectrum poses a severe challenge to analyse the spectrum and hence the determination of magnitudes and signs of homo- and heteronuclear couplings. The problem is more severe in the analysis of 1H spectra than the spectra of most of the other nuclei due to the often very small chemical shift dispersion. As a consequence, the straightforward analysis and the accurate extraction of the coupling constants from the 1H spectrum of a complex spin system continues to remain a challenge, and often may be a formidable task. Over the years, the several pure shift-based one-dimensional and two-dimensional methodologies have been developed by workers in the field, which provide broadband homonuclear decoupling of proton spectra, removing the complexity but at the cost of the very informative scalar couplings. To circumvent this problem, several one-dimensional and two-dimensional NMR experiments have been developed for the determination of homonuclear and heteronuclear couplings (nJHX, where n = 1,2,3) while retaining the high resolution obtained by implementing pure shift strategies. This review attempts to summarize the extensive work reported by a large number of researchers over the years for the accurate determination of homo- and heteronuclear scalar couplings.

Simultaneous Broadband Suppression of Homonuclear and Heteronuclear Couplings in 1 H NMR Spectroscopy

The ¹ H NMR analysis of species containing NMR-active heteronuclei can be difficult due to signal overlap caused by the combined effects of homonuclear and heteronuclear scalar (J) couplings. Here, a general pure shift method is presented for obtaining ultra-high resolution ¹ H NMR spectra where spectral overlap is drastically reduced by suppressing both homonuclear and heteronuclear J-couplings, giving one single signal per ¹ H chemical environment. Its usefulness is demonstrated in the analysis of fluorine- and phosphorus-containing compounds of pharmaceutical and biochemical interest.

Slice selective absorption-mode J-resolved NMR spectroscopy

Limited chemical shift dispersion and broad multiplet patterns limit resolution in ¹H NMR spectra. J-Resolved spectroscopy overcomes this problem to a great extent. However, the phase-twist line shape in J-Resolved spectroscopy allows only the magnitude mode of the experiment to be practical, which degrades resolution. Recently, various pure shift or broadband homonuclear decoupling approaches have been integrated with J-Resolved spectroscopy to eliminate the broad dispersive contribution. In the present work, we demonstrate a broadband ¹H-¹H J-Resolved spectrum with a greatly reduced dispersive contribution using the concept of slice selection. We show that slice selective excitation, t₁ encoding, storage, and detection of the in-phase absorptive signals can be executed, while a gradient-based suppression of the dispersive antiphase signals can be performed during the storage period. In more than two spin systems, a small part of the doubly antiphase absorptive signal may also contribute to the spectrum in addition to the inphase absorptive signals. The overall effect is a reduced multiplet pattern similar to a regular J-Resolved case as the passive spins remain unflipped due to slice selective pulses. However, the effect is broadband for a fraction of the spins when all slices are considered analogous to Zangger-Sterk (ZS) broadband homo-decoupling. Further, the fresh magnetization from neighboring slices can be accessed in different scans by frequency shifting of the slice selective pulses without a recycle delay-an elegant aspect of the ZS pulse element. This allows faster signal averaging, improving sensitivity which depends on the T₁ relaxation time of the signals. This method displays sensitivity up to 4-20 percent of the regular J-RES ¹H signals.

Pure shift HMQC: Resolution and sensitivity enhancement by bilinear rotation decoupling in the indirect and direct dimensions

The heteronuclear multiple-quantum coherence in the indirect dimension of the two-dimensional HMQC experiment evolves under the passive ¹H-¹H J-couplings leading to multiplet structures in the F₁ dimension. Besides, ¹H-¹H J-multiplets appear in the direct dimension as well. Thus, multiplets along both dimensions lower the resolution and sensitivity of this technique, when high resolution is required along both dimensions. An efficient broadband homodecoupling scheme along the F₁ dimension of the HMQC experiment has not been realized to date. We have implemented broadband homonuclear decoupling using bilinear rotation decoupling (BIRD) by adding a ¹H SQ evolution period followed by BIRD before the ¹H-¹³C multiple-quantum evolution period in the HMQC. In the direct time domain, BIRD is implemented using a real-time or single-scan scheme, which further improves resolution and sensitivity of this technique. The resulting pure shift HMQC provides singlet peak per chemical site along F₁ as well as F₂ axes and, hence, better resolution and sensitivity than conventional HMQC spectrum for all peaks except diastereotopic methylene protons. Due to the incorporation of the BIRD, the indirect time domain becomes double in length compared to the conventional HMQC. However, slow relaxation of small molecules favors better sensitivity for ps-HMQC relative to conventional HMQC under all conditions. We also found that the sensitivity of ps-HMQC is only slightly less than ps-HSQC for small molecules.

Perfecting band selective homo-decoupling for decoupling two signals coupled within the same band

Recently, pure shift NMR methods have delivered ultrahigh resolution broadband proton NMR spectra that display singlet peak per chemical site. BASH/HOBS (band selective homo-decoupling/homonuclear band selective) decoupling is the only method that provides singlet only spectrum for a selected signal or a group of signals with a sensitivity higher than regular proton NMR, and an order of magnitude higher than broadband pure shift techniques. It is this aspect that makes this technique important. In the present work, we show that perfect echo (PE) when combined with band selective homo-decoupling (BASHD) permits increasing the bandwidth of the BASH/HOBS decoupling resulting in reduced experimental time for this class of experiments. Using new PE-BASHD technique two closely resonating coupled signals could be decoupled in a single experiment which otherwise demands two separate BASHD experiments. In polypeptides, it also allows decoupling of Hα and HN signals simultaneously from each other as well as from the side chain protons reducing experimental time. Further, pseudo 2D based PE-BASHD scheme provides spectrum superior to the real-time BASHD spectrum when applied to closely resonating coupled signals. Numerical simulation as well as experimental results indicate that the PE-BASHD sequence display good quality singlet only spectrum even in the presence of moderate strong coupling.

Analyses of Complex Mixtures by F1 Homo-Decoupled Diagonal Suppressed Total Correlation Spectroscopy

A diagonal suppressed F₁ decoupled total correlation spectroscopy(TOCSY) experiment is developed for analyses of complex mixtures. In 2D homonuclear correlation, assignment of the cross peaks is crucial for structure elucidation. However, when cross peaks are close to the diagonal peaks in overcrowded spectral regions, their assignment becomes tedious. In complex mixtures, the presence of multiple spectra along with broad and complex proton multiplets owing to homonuclear scalar couplings degrade the resolution to the extent that assignment of various cross peaks becomes tedious or impossible. Herein, a diagonal suppressed total correlation technique with F₁ decoupling is presented to improve the resolution of the cross peaks. The resolution of the cross peaks is improved by both diagonal suppression as well as the collapse of the multiplets to singlets. Application of the method to a few mixtures of organic compounds reveals better identification of the cross peaks relative to other TOCSY variants.