Author
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BUNZEL, MIRKO - UNIV. OF HAMBURG, GERMANY |
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Ralph, John |
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Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 8/10/2006 Publication Date: 9/14/2006 Citation: Bunzel, M., Ralph, J. 2006. NMR characterization of lignins isolated from fruit and vegetable insoluble dietary fiber. Journal of Agricultural and Food Chemistry. 54(21):8352-8361. Interpretive Summary: Cereal grains, fruits and vegetables are important source of dietary fiber. Dietary fiber components are "polysaccharides, oligosaccharides, lignin, and associated plant substances". It has marked effects on gut function, and may afford protection against colon cancer. The aim of this study was to authenticate the presence of lignin in cereal grain, fruit, and vegetable dietary fiber and to delineate its structural characteristics, as a prelude to understanding the role of fiber in the health effects noted above. The results of this study demonstrate that wheat bran, which are often noted as being high in lignin, has only very low amounts of the authentic polymer. It is likely that conventional methods of measurement have severely overestimated this component in wheat and other cereal brans, but that other polymer components may play a crucial role. Kiwifruit and pear, however, have very nice isolable lignins amenable to detailed structural investigation which showed them to be similar to lignins in trees. Rhubarb lignins were difficult to extract and were not obtained in pure form, but were striking in that they were among the highest ever noted in the ratio of one lignin precursor to the other (over 86% "syringyl"). The structural analyses here provide a basis to begin to explain the possible health effects of lignins in food, especially the binding of carcinogens in the human gastrointestinal tract. Technical Abstract: 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, including as antioxidants and carcinogen adsorbants, and may be partially converted to mammalian lignans, the confusing current information derived from non-specific lignin determination methods needs to be augmented by diagnostic structural studies. Here we isolated lignin fractions from kiwi, pear, rhubarb and, for comparison, wheat bran insoluble dietary fiber - wheat bran is regarded as being a source rich in lignin. 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, highlighting the need for diagnostic structural studies. Syringyl/guaiacyl measures from volume-integration of NMR contours in 2D 13C-1H correlation spectra show that pear and wheat lignins have comparable guaiacyl and syringyl contributions (from the two lignin monomers, coniferyl and sinapyl alcohols), that kiwi lignins are particularly guaiacyl-rich, and suggest that rhubarb lignins, which could not be isolated from contaminating materials, are as syringyl-rich as lignins from any known natural or transgenic fiber source. Typical lignin structures, including the newly discovered ones (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 indeed contain authentic lignins, some with intriguing features, and that the detailed structural analysis exposes limitations with data from currently accepted analytical methods. |
