Simultaneous Amino Acid Analysis Based on 19F NMR Using a Modified OPA-Derivatization Method

Novel Pseudo-Two-Dimensional 19F NMR Spectroscopy for Rapid Simultaneous Detection of Amines in Complex Mixture

Rapid detection of amines in complex mixtures presents a significant challenge. Here, we introduce a novel nuclear magnetic resonance (NMR) method for amine detection utilizing a probe with two fluorine atoms in distinct chemical environments. Upon interaction with an amine, the probe generates two atomic resonance peaks, which are used to create coordinates, revealing fluorine chemical shifts on the 19F NMR spectroscopy. This innovative approach allows for the clear distinction of amine signals in a two-dimensional plane. This method has been effectively employed in analyzing amines in pharmaceuticals and amino acids in Ophiopogon japonicus and dry white wine, providing a robust and general approach for amine analysis.

Bioinspired Fluorine Labeling for 19F NMR-Based Plasma Amine Profiling

Metabolite profiling serves as a powerful tool that advances our understanding of biological systems, disease mechanisms, and environmental interactions. In this study, we present an approach employing 19F-nuclear magnetic resonance (19F NMR) spectroscopy for plasma amine profiling. This method utilizes a highly efficient and reliable fluorine-labeling reagent, 3,5-difluorosalicylaldehyde, which effectively emulates pyridoxal phosphate, facilitating the formation of Schiff base compounds with primary amines. The fluorine labeling allows for distinct resolution of 19F NMR signals from amine mixtures, leading to precise identification and quantification of amine metabolites in human plasma. This advancement offers valuable tools for furthering metabolomics research.

Detecting Submicromolar Analytes in Mixtures with a 5 min Acquisition on 600 MHz NMR Spectrometers

Amino compounds are widely present in complex mixtures in chemistry, biology, medicine, food, and environmental sciences involving drug impurities and metabolisms of proteins, biogenic amines, neurotransmitters, and pyrimidine in biological systems. Nuclear magnetic resonance (NMR) spectroscopy is an excellent tool for simultaneously identifying and quantifying these in-mixture compounds but has a limit-of-detection (LOD) over several micromolarities (>5 μM). To break such a sensitivity barrier, we developed a sensitive and rapid method by combining the probe-induced sensitivity enhancement and nonuniform-sampling-based 1H-13C HSQC 2D-NMR (PRISE-NUS-HSQC). We introduced two 13CH3 tags for each analyte to respectively increase the 1H and 13C abundances for up to 6 and 200 fold. This enabled high-resolution detection of 0.4-0.8 μM analytes in mixtures in 5 mm tubes with a 5 min acquisition on 600 MHz spectrometers. The method is much more sensitive and faster than traditional 1H-13C HSQC methods (∼50 μM, >10 h). Using sulfanilic acid as a single reference, furthermore, we established a database covering chemical shifts and relative-response factors for >100 compounds, enabling reliable identification and quantification. The method showed good quantitation linearity, accuracy, precision, and applicability in multiple biological matrices, offering a rapid and sensitive approach for quantitative analysis of large cohorts of chemical, medicinal, metabolomic, food, and other mixtures.

Solvent Effects Used for Optimal Simultaneous Analysis of Amino Acids via 19F NMR Spectroscopy

¹⁹F NMR has been extensively used in simultaneous analysis of multicomponent due to its 100% natural isotope abundance, high NMR-sensitivity, and wide-range chemical shifts. The solvent effects are usually observed in NMR spectroscopy and cause large changes in ¹⁹F chemical shifts. Herein, we propose that the simultaneous analysis of a complex mixture can be achieved using solvent effects via ¹⁹F NMR spectroscopy, such as a mixture solution of amino acids (AAs). AAs are not only cell-signaling molecules, but are also considered as biomarkers of some diseases. Hence, the analysis of AAs is important for human health and the diagnosis of diseases. In this work, the key to the success of sensing 19 biogenic AAs is the use of 2-fluorobenzaldehyde (2FBA) as a highly sensitive derivatizing agent and solvent effects to produce distinguishable ¹⁹F NMR signals. As a result, the resolution of ¹⁹F NMR spectroscopy of multiple 2FBA-labeled AAs is obviously higher than other methods based on ¹⁹F NMR. Moreover, 14 and 18 AAs can be satisfactorily differentiated and unambiguously identified in different complicated media supporting the growth of mammalian cells. Furthermore, quantification of the concentration of AAs can be made, and the limit of detection reaches 10 μM. Our work provides new insights into the simultaneous analysis of a multicomponent mixture based on solvent effects by ¹⁹F NMR spectroscopy.

Analysis of Small-Molecule Mixtures by Super-Resolved 1H NMR Spectroscopy

Analysis of small molecules is essential to metabolomics, natural products, drug discovery, food technology, and many other areas of interest. Current barriers preclude from identifying the constituent molecules in a mixture as overlapping clusters of NMR lines pose a major challenge in resolving signature frequencies for individual molecules. While homonuclear decoupling techniques produce much simplified pure shift spectra, they often affect sensitivity. Conversion of typical NMR spectra to pure shift spectra by signal processing without a priori knowledge about the coupling patterns is essential for accurate analysis. We developed a super-resolved wavelet packet transform based 1H NMR spectroscopy that can be used in high-throughput studies to reliably decouple individual constituents of small molecule mixtures. We demonstrate the efficacy of the method on the model mixtures of saccharides and amino acids in the presence of significant noise.

NMR-based isotope editing, chemoselection and isotopomer distribution analysis in stable isotope resolved metabolomics

NMR is a very powerful tool for identifying and quantifying compounds within complex mixtures without the need for individual standards or chromatographic separation. Stable Isotope Resolved Metabolomics (or SIRM) is an approach to following the fate of individual atoms from precursors through metabolic transformation, producing an atom-resolved metabolic fate map. However, extracts of cells or tissue give rise to very complex NMR spectra. While multidimensional NMR experiments may partially overcome the spectral overlap problem, additional tools may be needed to determine site-specific isotopomer distributions. NMR is especially powerful by virtue of its isotope editing capabilities using NMR active nuclei such as 13C, 15N, 19F and 31P to select molecules containing just these atoms in a complex mixture, and provide direct information about which atoms are present in identified compounds and their relative abundances. The isotope-editing capability of NMR can also be employed to select for those compounds that have been selectively derivatized with an NMR-active stable isotope at particular functional groups, leading to considerable spectral simplification. Here we review isotope analysis by NMR, and methods of chemoselection both for spectral simplification, and for enhanced isotopomer analysis.

Analytical Validation of an Assay for Concurrent Measurement of Amino Acids in Dog Serum and Comparison of Amino Acid Concentrations between Whole Blood, Plasma, and Serum from Dogs

Amino acids play an important role in metabolism. Comprehensive analytical validation of an assay for the concurrent measurement of a large number of amino acids in dogs is lacking, which precludes its usefulness in a clinical setting. Amino acids are often measured in plasma or whole blood. However, serum is commonly used for gastrointestinal diagnostic testing in dogs and is therefore convenient to use. This study aimed to analytically validate an assay for the concurrent measurement of amino acids in dog serum and to evaluate differences in amino acid concentrations in whole blood, plasma, and serum in dogs. Analytical validation of the assay (Biochrom 30+ Amino Acid Analyzer) was performed on fresh or banked serum samples from dogs. Whole blood, plasma, and serum from 36 healthy dogs were analyzed, and concentrations of the three sample types were compared. The assay was demonstrated to be precise, reproducible, accurate, linear, and stable for the measurement of the majority of compounds detected in dog serum. Cystine, glutamic acid, and ethanolamine were shown to be unstable at conditions commonly encountered in clinical settings. Significant differences in concentrations were identified between whole blood, plasma, and serum for 33 of 42 compounds. Amino acid profiles in serum and plasma were more similar to each other than to those in whole blood. While some amino acids are present in similar concentrations in whole blood, plasma, and serum, others are highly dependent on the type of biofluid, and measurements warrant strict adherence to sample type-based reference intervals.

Ratiometric Strategy Based on Intramolecular Internal Standard for Reproducible and Simultaneous Fingerprint Recognition of Diols via 19F NMR Spectroscopy

¹⁹F NMR spectroscopy has been widely used as a convenient and noninvasive analytical technique for understanding complex natural phenomena at the atomic level. However, current NMR referencing techniques are most optimized for ¹H NMR, which causes some limitations while referencing heteronuclear NMR. Despite its promising advantages, ¹⁹F NMR spectroscopy often exhibits large variations in experimental results and lacks consistency compared with ¹H NMR. Herein, we propose a new strategy to improve the consistency of ¹⁹F NMR referencing using an internal standard method. As a proof-of-concept, BA-Py-TFP was applied as a sensor for diols via ¹⁹F NMR spectroscopy. This strategy proved to be a robust and reproducible referencing method with acceptable deviation (Δδ_F = 43-58 ppb) across diverse NMR spectrometers at different institutions. In particular, this new strategy allows reliable fingerprint recognition for analytes and enables qualitative and quantitative analyses of mixtures of multiple analytes simultaneously. The high recovery rates for d-glucose in the human serum matrix suggest its potential suitability for a diverse range of applications, such as in diabetes-related diagnostics.

Rapid and Selective 19F NMR-Based Sensors for Fingerprint Identification of Ribose

Ribose plays an important role in the process of life. Excessive ribose in the human cerebrospinal fluid or urine can be used as an early diagnostic marker of leukoencephalopathy. Fluorinated phenylboronic acid combined with ¹⁹F NMR spectroscopy was a powerful method for molecular recognition. However, phenylboronic acid-based sensors for selective detection of ribose are rarely reported in the literature. In this study, the rapid and highly selective recognition of ribose was studied by ¹⁹F NMR and 2-fluorophenylboric acid. It was found that 2-fluoro-phenylboric acid was an appropriate ¹⁹F NMR-based sensor molecule for the determination of ribose under physiological conditions with high selectivity and robust anti-interference ability. When 2-fluorophenylboric acid was used for the detection of ribose in human urine without any sample pretreatment, a limit of detection of 78 μM was obtained at room temperature under given ¹⁹F NMR experimental conditions (400 MHz, 512 scans, ca. 12 min), which can well meet the needs of practical application.

Simultaneous analysis of amino acids based on discriminative 19F NMR spectroscopy

The simultaneous analysis of amino acids (AAs) is crucial for human health, diagnosis and treatment of disease, and nutritional quality evaluation in foodstuffs. Here, we establish an easy and rapid method for the simultaneous analysis of AAs using a single reagent 2-(trifluoromethyl)benzaldehyde (oTFMBA) based on spectral-separation-enabled ¹⁹F NMR spectroscopy. oTFMBA, a highly sensitive chemosensor, is capable of analyzing 19 proteinogenic AAs or non-amino acid amines (non-AAs) in a complex mixture by adjusting the pH in a toilless way. The ¹⁹F signals of oTFMBA-labeled AAs are distributed over a wide range of ∼ 0.7 ppm, demonstrating oTFMBA with higher resolution for simultaneous analysis of AAs compared to the o-phthaldialdehyde (OPA) method (<0.6 ppm). Additionally, 12 AAs were unambiguously identified in human urine, including Asp, Ser, Gly, Thr, Glu, Arg, Ala, Val, Ile, Tyr, His, and Phe. Furthermore, our method's detection limit for AAs is 5.83 μM, illustrating sensitivity with an ∼100-fold improvement over the OPA method. This work represents an approach to the analysis of AAs or non-AAs in a complicated mixture (even biofluid) using a ¹⁹F NMR probe with high sensitivity, which is of great significance for the simultaneous analysis of multiple analytes.

Simultaneous Quantification of Biothiols and Deciphering Diverse GSH Stability in Different Live Cells by 19F-Tag

GSH, Cys, Hcy, and H₂S are important biothiols and play important roles in the living systems. Quantitative and simultaneous determination of these biothiols under physiological conditions is still a challenge. Herein, we developed an effective ¹⁹F-reactive tag that readily interacts with these four biothiols for the generation of stable thioether products that have distinguishable ¹⁹F-chemical shifts. These thioester compounds encode the characteristic fingerprint profiles of each biothiols, allowing one to simultaneously quantify and determine these biothiols by 1D ¹⁹F NMR spectroscopy. The intra-/extracellular GSH in live cells was assessed by the established strategy, and remarkable variations in the GSH stability were determined between the normal mammalian cells and cancer cells. It is notable that GSH hydrolyzes efficiently in the out-membrane of the cancer cells and the lysates. In contrast, GSH remains stable in the tested normal cells.

Simultaneous identification and quantification of amino acids in biofluids by reactive 19F-tags

A robust method to identify and quantify amino acids close to physiological conditions by 1D ¹⁹F NMR was established. Each ¹⁹F-derivatized amino acid has its characteristic chemical-shift profile that is readily identified in the mixture of amino acids or in biofluids including fetal bovine serum and cell lysates. The method shows great potential in metabolomics and biochemical analysis.

A Bisboronic Acid Sensor for Ultra-High Selective Glucose Assay by 19F NMR Spectroscopy

Glucose is a significant analyte both in biology and biomedical science, it is of great importance to selectively detect glucose both in body fluids and complex mixture. In this study, a simple ¹⁹F NMR based sensor was synthesized easily, which exhibited a high selectivity and robust anti-interference ability toward glucose detection both in a mixture containing up to 10 saccharides and human urine samples without any pretreatment. Combined with this sensor system, glucose could be well detected in human urine samples and the limit of detection was 0.41 mM by using a 400 MHz NMR spectrometer with 128 scans (ca. 4 min). This method had a potential for specific detection of glucose in complex mixture and diagnosis of diabetes mellitus related diseases in body fluid.

Attenuation of doxorubicin-induced oxidative damage in rat brain by regulating amino acid homeostasis with Astragali Radix

The nervous system disorders caused by doxorubicin (DOX) are among the severe adverse effects that dramatically reduce the quality of life of cancer survivors. Astragali Radix (AR), a popular herbal drug and dietary supplement, is believed to help treat brain diseases by reducing oxidative stress and maintaining metabolic homeostasis. Here we show the protective effects of AR against DOX-induced oxidative damage in rat brain via regulating amino acid homeostasis. By constructing a clinically relevant low-dose DOX-induced toxicity rat model, we first performed an untargeted metabolomics analysis to discover specific metabolic features in the brain after DOX treatment and AR co-treatment. It was found that the amino acid (AA) metabolism pathways altered most significantly. To accurately characterize the brain AA profile, we established a sensitive, fast, and reproducible hydrophilic interaction chromatography-tandem mass spectrometry method for the simultaneous quantification of 22 AAs. The targeted analysis further confirmed the changes of AAs between different groups of rat brain. Specifically, the levels of six AAs, including glutamate, glycine, serine, alanine, citrulline, and ornithine, correlated (Pearson |r| >  0.47, p < 0.05) with the brain oxidative damage that was caused by DOX and rescued by AR. These findings present that AAs are among the regulatory targets of DOX-induced brain toxicity, and AR is a promising therapeutic agent for it.

Tailoring Sensors and Solvents for Optimal Analysis of Complex Mixtures Via Discriminative 19F NMR Chemosensing

Separation-free analytic techniques capable of providing precise and real-time component information are in high demand. ¹⁹F NMR-based chemosensing, where the reversible binding between analytes and a ¹⁹F-labeled sensor produces chromatogram-like output, has emerged as a valuable tool for the rapid analysis of complex mixtures. However, the potential overlap of the ¹⁹F NMR signals still limits the number of analytes that can be effectively differentiated. In this study, we systematically investigated the influence of the sensor structure and NMR solvents on the resolution of structurally similar analytes. The substituents adjacent and distal to the ¹⁹F labels are both important to the resolving ability of the ¹⁹F-labeled sensors. More pronounced separation between ¹⁹F NMR peaks was observed in nonpolar and aromatic solvents. By using a proper sensor and solvent combination, more than 20 biologically relevant analytes can be simultaneously identified.