The contribution of polyhydroxyl aromatic compounds to tetramethylammonium hydroxide lignin-based proxies

A problem inherent in analytical procedures where thermochemolyis or pyrolysis is coupled with in situ methylation by tetramethylammonium hydroxide is the inability to determine if an observed aromatic methoxyl group was originally present as a hydroxyl or a methoxyl functionality. This analytical insensitivity makes it impossible to ascertain relative inputs of fresh lignin, demethylated lignin and polyphenols (hydrolysable) into natural organic matter pools. ¹³C-TMAH thermochemolysis, however, methylates using a ¹³C-labeled methyl group permitting the differentiation of original and analytically-added methyl groups and therefore the potential source of many "lignin" phenols. Common oak root, senescent leaf, wood, and bark as well as brown rot decayed oak wood are compared herein to test if the ¹³C-TMAH modification can be used to differentiate sources of phenols. Oak was chosen as it exhibits extremely high contents of hydrolysable tannins which interfere with the study of wood decomposition but also confound any comparative studies of the fate of lignin and tannin in soil profiles, dissolved organic matter, and sediments when using the unlabeled TMAH thermochemolysis technique. The average % monohydroxyl (i.e. 3,5-dimethoxyl, 4-hydroxyl in source) content of syringyl compounds for each sample varies as leaves (70%) <root (75%) <bark (77%) <brown rot wood (85.9%) <wood (89%). Some compounds such as permethylated syringic acid were almost 90% 3,4,5-trihydroxyl benzoic acid in origin as was determined for oak leaf, or, put another way, S6 was derived almost exclusively from gallic acid. Similarly, what was assumed to be permethylated ferulic acid in roots turned out to be predominantly caffeic acid with a 3,4-dihydroxyl substitution rather than the 3-methoxy, 4-hydroxy substitution. When individual compound concentrations are corrected for the polyhydroxyl content to better reflect "true" lignin, or at least lignin predominantly bound via a β-O-4 configuration as is released by this chemolysis procedure, the values for lignin proxies changed substantially. As such, it must be considered that many unlabeled TMAH methylated compounds released from natural systems and ascribed to a "lignin" source may in fact be from tannins or other fresh or degraded phenolics.