molecules Editorial Special Issue: NMR-Based Metabolomics Miriam Pérez-Trujillo 1, * and Toby J. Athersuch 2, * 1 Nuclear Magnetic Resonance Facility, Universitat Autònoma de Barcelona, 08193 Barcelona, Spain 2 Department of Metabolism, Digestion and Reproduction, Faculty of Medicine, Imperial College London, London SW7 2AZ, UK * Correspondence: miriam.perez@uab.cat (M.P.-T.); toby.athersuch@imperial.ac.uk (T.J.A.) Nuclear magnetic resonance (NMR) spectroscopy remains one of the core analytical platforms for metabolomics, providing complementary chemical information to others, such as mass spectrometry, and offering particular advantages in some areas of research on account of its inherent robustness, reproducibility, and phenomenal dynamic range. While routine experimental protocols for profiling and related statistical analysis pipelines have been established, they often present considerable challenges to the analyst, including spectral overlap, accurate and precise quantification, and chemical shift variation. Conse- quently, there is still much activity across all areas of NMR spectroscopic analysis in relation to metabolomics. Furthermore, there remain many biological systems and sample types that have not been extensively explored using NMR spectroscopy-based metabolomics. In this Special Issue, several advances in methodology, and new applications in the field of NMR-based metabolomics, have been presented. In addition, the SI includes au- thoritative review articles focused on the state-of-the-art of quantitative NMR spectroscopy in biomedical metabolomics applications, and novel applications in the agri-food sector. The use of new NMR spectroscopic pure shift methods in NMR spectroscopy-based  metabolomics is evaluated by Stark et al. [1] in Hypericum plant extracts; pure shift methods  improve spectral resolution in 1 H-NMR by suppressing homonuclear coupling, turning Citation: Pérez-Trujillo, M.; multiplets into singlets. They test the PSYCHE (pure shift yielded by chirp excitation) and Athersuch, T.J. Special Issue: the Zangger–Sterk pulse sequences with conventional 1 H-NMR spectroscopic sequences NMR-Based Metabolomics. Molecules used routinely in profiling and suggest the implementation of such methods in the follow- 2021, 26, 3283. https://doi.org/ up experiments of the metabolomics workflow, as a complementary data source that can 10.3390/molecules26113283 aid data interpretation. In their SI paper, Mehendale et al. [2] describe the development of a novel NMR-compatible platform for the automated real-time monitoring of biochemical Received: 20 May 2021 Accepted: 24 May 2021 reactions using a flow shuttling configuration. This system offers high-resolution analysis, Published: 29 May 2021 automation, minimal user intervention, and, therefore, time-efficient measurement per sample. The platform described suits bioprocessing applications, in which conversion to a Publisher’s Note: MDPI stays neutral value-added product is the objective, since the intermediate and final product yields can be with regard to jurisdictional claims in monitored in real time. published maps and institutional affil- Gargallo-Garriga et al. [3] conducted metabolic profiling using 31 P NMR spectroscopic iations. methods in a study of phosphorus utilization in the trees of a French Guiana rainforest; the productivity and diversity of tropical lowland moist forests are often limited by the avail- ability and functional allocation of phosphorus, and thus 31 P NMR offers a direct window on the relevant aspects of plant biochemistry. The authors conclude that phosphorus use Copyright: © 2021 by the authors. is species-specific and that species diversity relates to both species phosphorus use and Licensee MDPI, Basel, Switzerland. concentrations of phosphorus-containing compounds. This article is an open access article In their contribution, Kim et al. [4] investigate meat-aging processes, specifically the distributed under the terms and characteristic metabolic changes of the crust from dry-aged beef using 2D quantitative conditions of the Creative Commons NMR-based metabolomics. Multivariate analysis of NMR spectra from various examples of Attribution (CC BY) license (https:// aged meats allows the method, degree, and doneness of the aging process to be effectively creativecommons.org/licenses/by/ differentiated and the key discriminatory metabolites to be identified. Special attention 4.0/). is given to the investigation of methylmalonic aciduria (MMA) by Nicolescu et al. [5]. Molecules 2021, 26, 3283. https://doi.org/10.3390/molecules26113283 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 3283 2 of 2 They report on monitoring MMA-specific and non-specific metabolites via NMR-based metabolomics and present an extended dataset of targeted metabolites. The current status and applications of quantitative NMR-based biomedical metabolomics is reviewed by Crook et al. [6]. The authors highlight the significant improvements in metabo- lite quantification that have taken place in recent years. They cover advances in the use of reference standards, pulse sequence development, physical and chemical deconvolution methods, hyperpolarization techniques, physical and chemical deconvolution methods, as well as the integration of multiple analytical platforms. Finally, particular attention is given to applications in the agri-food sector. The discrimination of the variables influ- encing tomato (Solanum lycopersicum) using NMR-based metabolomics is reviewed by Abreu et al. [7]. The knowledge derived from the reported studies is especially helpful in improving fruit quality through cultural practices and agronomical conditions. Funding: T.J.A. is supported by the NIHR Imperial Biomedical Research Centre (BRC). Acknowledgments: The Guest Editors wish to thank all the authors for their contributions to this Special Issue. Reviewers of the manuscripts are thanked for their hard work and thoughtful comments while evaluating the submitted articles. We also wish to thank the editorial staff of MDPI Molecules for their kind assistance and patience in the compilation of this SI during a particularly testing time. Conflicts of Interest: The authors declare no conflict of interest. References 1. Stark, P.; Zab, C.; Porzel, A.; Franke, K.; Rizzo, P.; Wessjohann, L.A. PSYCHE—A Valuable Experiment in Plant NMR- Metabolomics. Molecules 2020, 25, 5125. [CrossRef] [PubMed] 2. Mehendale, N.; Jenne, F.; Joshi, C.; Sharma, S.; Masakapalli, S.K.; MacKinnon, N. A Nuclear Magnetic Resonance (NMR) Platform for Real-Time Metabolic Monitoring of Bioprocesses. Molecules 2020, 25, 4675. [CrossRef] [PubMed] 3. Gargallo-Garriga, A.; Sardans, J.; Llusià, J.; Peguero, G.; Asensio, D.; Ogaya, R.; Urbina, I.; Langenhove, L.V.; Verryckt, L.T.; Courtois, E.A.; et al. 31 P-NMR Metabolomics Revealed Species-Specific Use of Phosphorous in Trees of a French Guiana Rainforest. Molecules 2020, 25, 3960. [CrossRef] [PubMed] 4. Kim, H.C.; Baek, K.H.; Ko, Y.-J.; Lee, H.J.; Yim, D.-G.; Jo, C. Characteristic Metabolic Changes of the Crust from Dry-Aged Beef Using 2D NMR Spectroscopy. Molecules 2020, 25, 3087. [CrossRef] [PubMed] 5. Nicolescu, A.; Blanita, D.; Boiciuc, C.; Hlistun, V.; Cristea, M.; Rotaru, D.; Pinzari, L.; Oglinda, A.; Stamati, A.; Tarcomnicu, I.; et al. Monitoring Methylmalonic Aciduria by NMR Urinomics. Molecules 2020, 25, 5312. [CrossRef] [PubMed] 6. Crook, A.A.; Powers, R. Quantitative NMR-Based Biomedical Metabolomics: Current Status and Applications. Molecules 2020, 25, 5128. [CrossRef] [PubMed] 7. Abreu, A.C.; Fernández, I. NMR Metabolomics Applied on the Discrimination of Variables Influencing Tomato (Solanum lycopersicum). Molecules 2020, 25, 3738. [CrossRef] [PubMed]