Location: Commodity Utilization ResearchTitle: Roles of conjugated double bonds in electron-donating capacity of sorghum grains
Submitted to: African Journal of Agricultural Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2016
Publication Date: 6/16/2016
Citation: Uchimiya, M., Wang, M.L. 2016. Roles of conjugated double bonds in electron-donating capacity of sorghum grains. African Journal of Agricultural Research. 11(24):2146-2156.
Interpretive Summary: Tannin coating is present in varying amounts in sorghum seeds. Tannins are expected to protect seeds from molding and bird damage on farm. This unique property of tannins results from the ability to donate electrons (antioxidant) and complex metals. This study developed cheap, rapid, and easy methods to quantify tannins in sorghum seeds. The methods are robust enough for uses by the producers on-farm and at processing factories having minimal instruments. The methods utilized the electron transfer from tannins to iron under specific conditions; light absorbance and emission unique to different tannin structures and amounts; and interaction of tannins with protein in sorghum seeds.
Technical Abstract: Electron-donating and metal ion complexation ability of tannins play key roles as antioxidants and in mold/bird resistance. In this study, rapid, sensitive, and nondestructive fluorescence excitation-emission (EEM) spectrophotometry was utilized to correlate structural attributes of sorghum tannins with its redox and coordination chemistry. Four sorghum bicolor accessions were extracted using (i) acetone-water (70:30 vol%) to preserve original tannin structures and (ii) 0.1 M NaOH to examine the oxidative degradation at elevated pH. UV (360 nm) absorbance, FeIII reduction by ferrozine method, and acid-butanol assay were followed as a function of pH and time. Greater aromaticity, condensed tannin content, and FeIII reduction were observed for brown seeds than white seeds. In contrast, acetone-water extracts of all seeds showed EEM peak (380/480) attributable to high MW, hydrophobic, polyphenolic (macro)molecules. Seed extracts in NaOH showed distinctively different peaks (230/320 and 280/320) attributable to protein-like structures and quinone oxidation products. Addition of FeIII to NaOH seed extracts of brown seeds formed a new aromatic EEM peak (320/440) that could be used as the characteristic wavelengths to understand the redox and coordination chemistry of sorghum tannins.