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

Improving quality of analysis by suppression of unwanted signals through band-selective excitation in NMR spectroscopy for metabolomics studies

INTRODUCTION

Nuclear Magnetic Resonance (NMR) spectroscopy stands as a preeminent analytical tool in the field of metabolomics. Nevertheless, when it comes to identifying metabolites present in scant amounts within various types of complex mixtures such as plants, honey, milk, and biological fluids and tissues, NMR-based metabolomics presents a formidable challenge. This predicament arises primarily from the fact that the signals emanating from metabolites existing in low concentrations tend to be overshadowed by the signals of highly concentrated metabolites within NMR spectra.

OBJECTIVES

The aim of this study is to tackle the issue of intense sugar signals overshadowing the desired metabolite signals, an optimal pulse sequence with band-selective excitation has been proposed for the suppression of sugar's moiety signals (SSMS). This sequence serves the crucial purpose of suppressing unwanted signals, with a particular emphasis on mitigating the interference caused by sugar moieties' signals.

METHODS

We have implemented this comprehensive approach to various NMR techniques, including 1D 1H presaturation (presat), 2D J-resolved (RES), 2D 1H-1H Total Correlation Spectroscopy (TOCSY), and 2D 1H-13C Heteronuclear Single Quantum Coherence (HSQC) for the samples of dates-flesh, honey, a standard stock solution of glucose, and nine amino acids, and commercial fetal bovine serum (FBS).

RESULTS

The outcomes of this approach were significant. The suppression of the high-intensity sugar signals has considerably enhanced the visibility and sensitivity of the signals emanating from the desired metabolites.

CONCLUSION

This, in turn, enables the identification of a greater number of metabolites. Additionally, it streamlines the experimental process, reducing the time required for the comparative quantification of metabolites in statistical studies in the field of metabolomics.

1H NMR with Partial Transition Selectivity

¹H NMR has unique strengths, owing, for one, to ¹H being the most sensitive NMR nucleus. However, the limited frequency range of ¹H chemical shifts implies spectral crowding, leading to difficulties in assignment and interpretation of the spectra. Homonuclear broadband decoupling has been developed as a means of simplifying ¹H NMR spectra but clearly leads to the inevitable and complete loss of precious information on homonuclear scalar couplings in solution state. A novel experiment is introduced in this work, which leads to partial ¹H multiplet selectivity, thereby reducing spectral crowding, while at the same time permitting couplings to be inferred. The present one-dimensional (1D) experiment relies on two-way coherence transfer starting from ¹H to coupled ¹³C carbons at natural abundance and ending finally with ¹H detection. The experiment may be termed CArbon Single transition EDited (CASED) ¹H NMR. The unusual spectral patterns that result are summarized, demonstrated, and rationalized for various molecular fragments. Artifacts in the present version of the CASED experiment are also described, and an application to the ¹H NMR of a disaccharide is demonstrated as a first practical example.

Metabolic mechanisms of a drug revealed by distortion-free 13C tracer analysis

Metabolomic isotopic tracing can provide flux information useful for understanding drug mechanisms. For that, NMR has the unique advantage of giving positional isotope enrichment information, but the current ¹³C 1D NMR approach suffers from low sensitivity and high overlaps. We developed a new 2D heteronuclear NMR experiment incorporating J-scaling and distortion-free elements that allows for quantitative analysis of multiplets with high sensitivity and resolution. When applied to an old chemotherapeutic drug, the approach provided a quantitative estimation of TCA-cycle turns, confirming the conventional mechanism of its mitochondrial metabolic enhancement. Additionally, the approach identified a new mechanism of the higher contribution of the pentose phosphate pathway to serine synthesis in the cytosolic compartment, possibly explaining the broad pharmacological activities of the drug. Our approach may prove beneficial in helping to find new usages or metabolic mechanisms of other drugs.

Our approach provides high-resolution and distortion-free NMR for metabolic tracer analysis. It confirmed the conventional mechanism of dichloroacetate and suggested a new one involving an enhanced contribution of PPP to cytosolic serine synthesis.