The application of multidimensional NMR to the study of soil humic substances

Humic substances are the most abundant organic macromolecules in soils, and comprehension of their chemical structure is essential to understanding th eir role in terrestrial ecosystems. The one-dimensional nuclear magnetic re sonance (NMR) spectroscopy techniques now used widely to study humic substa nces have provided important insight into humic structures, but the complex ity of these macromolecules gives rise to resonance signals that are broad and have spectral overlap. This has prevented the definitive functional gro up assignments necessary for structural determination, Hence, interest has focused on more powerful two-dimensional NMR experiments, such as the homon uclear TOtal Correlation SpectroscopY (TOCSY) and Heteronuclear Multiple Qu antum Coherence (HMQC), which were employed in the study of a soil humic ac id standard. The purpose of this paper is to outline the potential of these techniques to the study of soil humic structures. The 2-D spectra produced were extremely encouraging, with multitudes of cross-peaks produced from b oth TOCSY and HMQC experiments. The identification of fatty ester/acid chai ns and amino acid couplings are given as examples. Results obtained with th ese NMR experiments indicate substantial improvements in functional group a ssignment capabilities and the potential for marked progress in the determi nation of the chemical structure of soil humic substances.