perfectBASH: Band-selective homonuclear decoupling in peptides and peptidomimetics

Implementation of pure shift 1 H NMR in metabolic phenotyping for structural information recovery of biofluid metabolites with complex spin systems

NMR spectroscopy is a mainstay of metabolic profiling approaches to investigation of physiological and pathological processes. The one-dimensional proton pulse sequences typically used in phenotyping large numbers of samples generate spectra that are rich in information but where metabolite identification is often compromised by peak overlap. Recently developed pure shift (PS) NMR spectroscopy, where all J-coupling multiplicities are removed from the spectra, has the potential to simplify the complex proton NMR spectra that arise from biosamples and hence to aid metabolite identification. Here we have evaluated two complementary approaches to spectral simplification: the HOBS (band-selective with real-time acquisition) and the PSYCHE (broadband with pseudo-2D interferogram acquisition) pulse sequences. We compare their relative sensitivities and robustness for deconvolving both urine and serum matrices. Both methods improve resolution of resonances ranging from doublets, triplets and quartets to more complex signals such as doublets of doublets and multiplets in highly overcrowded spectral regions. HOBS is the more sensitive method and takes less time to acquire in comparison with PSYCHE, but can introduce unavoidable artefacts from metabolites with strong couplings, whereas PSYCHE is more adaptable to these types of spin system, although at the expense of sensitivity. Both methods are robust and easy to implement. We also demonstrate that strong coupling artefacts contain latent connectivity information that can be used to enhance metabolite identification. Metabolite identification is a bottleneck in metabolic profiling studies. In the case of NMR, PS experiments can be included in metabolite identification workflows, providing additional capability for biomarker discovery.

Gradient selected pure shift EASY-ROESY techniques facilitate the quantitative measurement of 1H,1H-distance restraints in congested spectral regions

For elucidating molecular structure and dynamics in solution, NMR experiments such as NOESY, ROESY and EXSY have been used excessively over the past decades, to provide interatomic distance restraints or rates for chemical exchange. The extraction of such information, however, is often prohibited by signal overlap in these spectra. To reduce this problem, pure shift methods for improving the spectral resolution have become popular. We report on pure shift EASY-ROESY experiments and their application to extract cross-relaxation rates, proton-proton distances and exchange rates. Homonuclear decoupling (pure shift) is applied in the indirect dimension using the PSYCHE or the perfectBASH technique, to enhance the spectral resolution of severely overcrowded spectral regions. The spectral quality is further improved by using a gradient selected F1-PSYCHE-EASY-ROESY, which produces significantly less t₁-noise than the experiment used previously, as also demonstrated by employing the recently introduced SAN (signal-artefact-noise) plots. Applications include the quantification of distance restraints in a peptide organocatalyst and the extraction of a number of distance restraints in cyclosporine A, which were previously not available for analysis, because they were either located in overcrowded spectral regions or hidden under t₁-noise. Distances extracted and exchange rates obtained are accurate. Also, the 2D gradient-selected F1-perfectBASH-EASY-ROESY with the additional gradient selection proposed herein, which is superior in terms of sensitivity, can be used to accurately quantify cross-relaxation.

PE-SERF: A sensitivity-improved experiment to measure JHH in crowded spectra

Aiming at facilitating the analysis of molecular structure, the gradient-encoded selective refocusing methods (G-SERF) and a great number of its variants for measuring proton-proton coupling constants have been proposed. However, the sensitivity is an issue in the 2D gradient-encoded experiments, because the signal intensity is determined by the slice thickness of the sample that depends on encoding gradient and the bandwidth of selective pulses which is limited by the smallest chemical shift difference of any two coupled protons. Here, we present a method dubbed PE-SERF (perfect echo selective refocusing) which can determine all J_HH values involving a selected proton with improved sensitivity compared to original G-SERF experiment. The modules of perfect echo involving selective pulses and gradient-encoded selective refocusing are combined in the method, so that the unwanted J couplings arising from coupled spin pairs in the same sample slice would be nullified. In this way, instead of single proton, a pair of coupled protons is allowed to share a sample slice, and thus the slice thickness can be increased and the spectral sensitivity can be improved. The performance of the method is demonstrated by experiments on quinine and strychnine.