At the center of a hydrolyzable tannin molecule, there is a carbohydrate (usually D-glucose but also cyclitols like quinic or shikimic acids). The hydroxyl groups of the carbohydrate are partially or totally esterified with phenolic groups such as gallic acid in gallotannins or ellagic acid in ellagitannins. Hydrolysable tannins are mixtures of polygalloyl glucoses and/or poly-galloyl quinic acid derivatives containing in between 3 up to 12 gallic acid residues per molecule.
Hydrolyzable tannins are hydrolyzed by weak acids or weak bases to produce carbohydrate and phenolic acids.
Hydrolysable tannins can be extracted from different vegetable plants, such as chestnut wood (Castanea sativa), oak wood (Quercus robur, Quercus petraea and Quercus alba), tara pods (Caesalpinia spinosa), gallnuts (Quercus infectoria and Rhus semialata), myrobalan (Terminalia chebula), sumac (Rhus coriaria) and Aleppo gallnuts (Andricus kollari).
- Gallic acid determination
50 mg of sample tannin in 5 ml 2N H2SO4 are put into constricted test tubes and frozen. The tubes are vacuum-sealed and heated for 24 hours at 100 °C. The tubes are cooled, opened and the contents made up to 50 ml with water. Then 1.5 ml of freshly prepared 0.667% w/v rhodanine in methanol and 1 ml of sample are mixed. After exactly 5 min 1 ml of 0.5 N KOH solution is added. After 2.5 min the mixture is diluted to 25 ml with distilled water and 5–10 min later the absorbance at 520 nm is measured. The measured absorbance obeys the relationship:
A520= [0.l3 × (mg of gallic acid) ] +0.03
Gallic acid is used as a standard and the data are based on experiments carried out in triplicate.
- Ellagic acid determination
10 mg of samples tannin in 1 ml 2N H2SO4 are put into constricted test tubes and frozen. The tubes are vacuum-waled and heated for 24 hours at 100 °C. Tubes were cooled, opened and the filtered content made up to 10 ml with pyridine. Then 1.1 ml of pyridine and l ml of sample are mixed in a dry test tube. After adding 0.10 ml of concentrated HCl and mixing, the sample is brought to 30 °C. The sample is quickly mixed after 0.10 ml of 1% (w/v) NaNO2 in water and the absorbance 538 nm is immediately recorded. After a 36 min incubating period at 30 °C, the absorbance is again recorded. The difference between the initial absorbance and the absorbance at 36 min (D A538) is proportional to the ellagic acid concentration. The measured absorbance obeys the relationship:
A538 = [0.03 × (mg of ellagic acid)] – 0.04
Ellagic acid is used as a standard and the data were based on experiments carried out in triplicate.
- Alkaline hydrolysis
4.8 g sample tannin in 9 ml water are refluxed in 4.2 ml 40% NaOH for 6h at pH 12 - 13. Neutralization to pH = 6.8 - 7 is performed with 62% H2SO4.
- HPLC determination
Tannins, including gallo and ellagic acid (epigallitannins), are inhibitors of HIV replication. 1,3,4-Tri-O-galloylquinic acid, 3,5-di-O-galloyl-shikimic acid, 3,4,5-tri-O-galloylshikimic acid, punicalin, punicalagin inhibited HIV replication in infected H9 lymphocytes with little cytotoxicity. Two compounds, punicalin and punicacortein C, inhibited purified HIV reverse transcriptase.
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