NMR Characterization of Lignins Isolated from Fruit and Vegetable Insoluble Dietary Fiber

Compositional information for lignins in food is rare and concentrated on cereal grains and brans. As
lignins are suspected to have important health roles in the dietary fiber complex, the confusing current
information derived from nonspecific lignin determination methods needs to be augmented by
diagnostic structural studies. For this study, lignin fractions were isolated from kiwi, pear, rhubarb,
and, for comparison, wheat bran insoluble dietary fiber. Clean pear and kiwi lignin isolates allowed
for substantive structural profiling, but it is suggested that the significance of lignin in wheat has
been overestimated by reliance on nonspecific analytical methods. Volume integration of NMR contours
in two-dimensional 13C−1H correlation spectra shows that pear and wheat lignins have comparable
guaiacyl and syringyl contributions and that kiwi lignins are particularly guaiacyl-rich (∼94% guaiacyl)
and suggest that rhubarb lignins, which could not be isolated from contaminating materials, are as
syringyl-rich (∼96% syringyl) as lignins from any known natural or transgenic fiber source. Typical
lignin structures, including those newly NMR-validated (glycerols, spirodienones, and dibenzodioxocins), and resinols implicated as possible mammalian lignan precursors in the gut are
demonstrated via their NMR correlation spectra in the fruit and vegetable samples. A novel putative
benzodioxane structure appears to be associated with the kiwi lignin. It is concluded that the fruits
and vegetables examined contain authentic lignins and that the detailed structural analysis exposes
limitations of currently accepted analytical methods.
Keywords: Dietary fiber; plant cell wall; dioxane−water milled lignin; ball-milling; acidolysis lignin;
pear; kiwi; rhubarb; wheat; NMR; HSQC; HMBC; gut fermentation; lignans