Condensed tannins are high molecular weight polymers, comprised of flavan-3-ol monomers stabilized by C-C interflavan bonds. Condensed tannins are found in most higher plants, and affect food properties such as astringency and protein availability, N- and C-cycles in soil systems, and agronomic properties such as insect and pathogen resistance. Tannins are highly sorptive molecules, making them difficult to purify or analyze using existing methods.
MALDI-TOF MS has been used in combination with chemical degradation methods to establish subunit composition and degree of polymerization of condensed tannins. Recently we developed methods with high resolution FT-ICR MALDI-MS to obtain exact masses of components of polydisperse condensed tannins. Using highly purified procyanidin from Sorghum grain, we obtained spectra that exhibited the expected peaks at intervals of 289 m/z, consistent with the (epi)catechin composition previously proposed for this material. However, the spectra also exhibited a strong secondary pattern with peaks at intervals of 152 m/z. This interval suggested that the tannin contained gallate ester groups, in contradiction to earlier reports. Using tools such as thiolytic degradation, methanolysis, HSQC NMR and IR and UV-visible spectroscopy, we have confirmed that the tannin does not contain ester groups. Instead, we propose that the MALDI matrix 2,5-dihydroxy benzoic acid forms tight adducts with the procyanidin that survive ionization and yield misleading peaks in the spectra.
To further characterize the Sorghum tannin, we are developing new RP-HPLC methods that separate polymers with degrees of polymerization up to about 25. This is a substantial improvement over previous methods that give only a large “hump” for most tannin extracts. By combining the high resolution MALDI analysis with better HPLC methods, we are improving the potential to establish the role of condensed tannins in foods, the environment and human health.
Author: Savanah Reeves
Faculty Advisor: Dr. Ann Hagerman, Department of Chemistry and Biochemistry
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