Application of Nuclear Magnetic Resonance Spectroscopy in Molecular Structure Identification: A Microbial Biogeochemistry Perspective

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The issues of climate change, land degradation, and sustainable agriculture are arguably three of the most significant scientific, political, and environmental challenges of the 21st century. However, one common denominator that is frequently overlooked when trying to understand and respond to key processes in the environment is the role of soil microbial biomass. As an example, soil microbial biomass controls the production of soil organic matter—the largest pool of terrestrial carbon—which is an important variable in studies of climate change and is also central to sustainable agriculture. Traditionally, the complex nature of this biomaterial renders it difficult to study at sufficiently fine resolution. At the molecular level, NMR spectroscopy is the single most powerful analytical technique available for the determination of structural components and their interactions in various environmental matrices, and can be performed on components of different physical phases [solids, semi-solids (gels), liquids, and gases]. NMR spectroscopy provides information about the chemical environment of nuclei within a molecule, and the three most frequently studied nuclei in microbial biomass are protons (1H), carbons (13C) and (15N). However, carbon and nitrogen NMR are generally considered relatively insensitive analytical techniques, because at natural abundance only 1% of the total carbon (13C) and 0.37% of the total nitrogen (15N) is NMR observable, while 99.9% of all protons are detectable. The complexity and multitude of components within a natural organic matter sample lead to extensive spectral overlap, especially in solid-state NMR. Therefore, the transformations of minor components are difficult to monitor unless we enhance NMR sensitivity by increasing the relative abundance of 13C and 15N through isotopic enrichment. This chapter provides an overview of the application of various NMR techniques used to provide comprehensive molecular and structural information, as well as some of the key considerations in the field of microbial biogeochemistry.