Characterization of a nitrogen-rich fulvic acid and its precursor algae from solid state NMR

A nitrogen-rich fulvic acid (FA) from Pony Lake, a coastal pond in Antarctica, was investigated using advanced solid state nuclear magnetic resonance (NMR) techniques and compared with the precursor algal material. A significant fraction of non-protonated alkyl carbons, some bonded to nitrogen, was detected; most must be humification products, since they were undetectable in the lake algae. The corresponding branched alkyl structures account for 25-50% of carbon in the FA. Spectral editing detected methylenes (20% of all C) and ethyl groups attached to a branch point. COOH groups are also enriched (similar to 10% of all C); N-15 NMR and spectral editing of CH and CN groups indicates peptides (similar to 1/4 of all C, similar to 2/5 of all N), but the N-15 NMR spectrum also exhibits bands up- and downfield from the peptide signal; these must be associated with a 157 ppm sp(2)-hybridized carbon bonded to three heteroatoms, of which at least two are N, according to C-13{N-14} SPIDER (saturation pulse induced dipolar exchange with recoupling) NMR. This component, possibly a degradation product of penguin guano, accounts for nearly half the N. OCH, O-CH-O and OCH2 moieties, typically found in sugar rings, were identified using spectral editing (similar to 12% of all C). The quantitative C-13 spectrum shows an aromaticity of 8%; many of the aromatic carbons are bonded to N. H-1-C-13 NMR with H-1 spin diffusion shows that most components are within 1 nm from non-polar alkyl segments, which excludes polysaccharide or aromatic domains. Overall, the structural differences between the algal precursor and the FA are striking. (C) 2007 Elsevier Ltd. All rights reserved.