Interpretation of heteronuclear and multidimensional NMR spectroscopy of humic substances

Mapping the chemical structures and organization of humic substances is vital for a fundamental understanding of their roles and interactions in the soil. One-dimensional nuclear magnetic resonance (NMR) techniques have advanced our awareness of the composition of humic materials, but modem developments in two-dimensional NMR are soon likely to make obsolete reliance on one-dimensional spectra alone. The advantages of using heteronuclear two-dimensional NMR spectroscopy are illustrated in this paper in studies of the structural units in a fulvic acid fraction isolated from the Bh horizon of a Podzol. The structures identified from the NMR (at 500 MHz) experiments can be summarized as: mono- and dicarboxylic acids (in about equal amounts), with an average chain length of about 10 carbon atoms (these are easily the major components); smaller amounts (about 10-20% of the acids) of esters and alcohols or ethers; some carbohydrate and amino acid residues (evidence from chemical shift data would suggest that these were likely to be in the form of chains); and very small amounts of 1,2-, 1,4-, and 1,3,4-substituted benzenes and of cinnamic acids. The results suggest that applications of heteronuclear and multidimensional NMR spectroscopy will allow considerable progress to be made in understanding the nature of the structural units and their connectivities in humic molecules provided that the heterogeneity of the humic mixtures can first be decreased significantly.