Gallic acid reactions

Noyes for bromate-malonic acid reaction and involves formation of BrOg radicals which oxidize the reduced metal complex. Silver(i) inhibition in the gallic acid reaction is observed, presumably by reduction of the Br present... 

Gallic acid is present in tea leaf and is a known reactant during the complex enzymatic and organochemical reactions that occur when tea components are oxidized.51 The gallic and quinic acids originate via the shikimate/arogenate pathway. The key enzymes in shikimic acid biosyn-... 

Performing the assay is reduced to putting an alcoholic solution of the analysed sample, Folin-Ciocalteu reagent and solution of sodium carbonate into a reaction tube, which brings the pH of the reaction environment to approx. 10. According to various literature reports, the reaction runs in the darkness for 10 to 120 minutes. After that time, the blue colour of the solution is observed colorimetrically at 725 nm - 760 nm [34, 35, 36, 37, 38]. The results are expressed based on calibration curves prepared for catechol and gallic acid. 

Most pyrogallol derivatives did not precipitate either (Table III). A small precipitation occurred with pyrogallol (which may be considered as a hydroxyresorcinol) and it reacts with vanillin/HCl also (7). However, any additional substitution eliminates this reactivity, e.g., gallic acid (Table IV). Purpurogallin did precipitate with formaldehyde. It is not very soluble in the reaction mixture, and it is a tropolone. Structures of this type might interfere, but in tea and probably in wine the known tropolone derivatives are flavonoids (I, 25). 

Gallic acid is quite stable in dry form, but will oxidize once it is in solution. This reaction is enhanced at higher temperatures. The author has found that the standard solutions should be stored in full or nearly full bottles that are kept tightly sealed between uses and are stored under refrigeration. The author has observed that >5% potency is lost after -1 week at room temperature, but this same loss takes -2 weeks with refrigeration. The authors would expect that greater air exposure will also accelerate decomposition. 

Esters (RCOOR) are formed by reaction of a carboxylic acid with the hydroxyl group of an alcohol. The hydroxyl group of phenolic compounds can participate in ester formation. Esters of two phenols are not particularly common in nature. The most familiar is the diester of gallic acid (2.22), which is ellagic acid (2.23), along with other gallotannin compounds. 

Gallotannins contain an additional 10-12 gallic acid moieties per molecule. This is effectively a continuation of the esterification reactions that resulted in the formation of pentagalloylglucose (3.99). The major... 

The contents of the ampule are diluted in water to a final volume of 50 mL. A 1-mL sample is then taken for the assay. To this sample 1.5 mL 0.667% (w/v) rhodanine in methanol is added. After exactly 5 min. 1 mL 0.5N KOH is added. After 2.5 min. water is added to a final volume of 25 mL. The absorbance is read at 520 nm after a 5-10 min. incubation. A standard curve is made by reaction of gallic acid in 0.2N sulfuric acid with the rhodanine solution. Hagerman and Butler (1989) argued that this assay is more suitable than the potassium iodate assay for the determination of hydrolysable tannins, although it has to be kept in mind that the rhodanine assay is sensitive to any gallic acid ester, including those in non-tannin compounds. 

Wilson and Hagerman (1990) described an alternative spectrophoto metric procedure to quantify ellagic acid. This method has three advantages over the procedure developed by Bate-Smith (1977) 1) it is not sensitive to gallic acid, which can result in an overestimate of the ellagitannin content, 2) it is more sensitive, and 3) it is more convenient because oxygen does not interfere with the reaction. 

This procedure is based on the formation of the electophile NO+, which can react with an ellagic acid residue (4.2) at two sites, either via a substitution reaction which results in 4.3, or an addition reaction that results in the nitrosyl dienone 4.4. These compounds can decay to form the quinine oxime 4.5, which under alkaline conditions forms the red product 4.6. When related compounds, such as gallic acid, phloroglucinol, hydroxycinnamic acids, and phenol are subjected to this assay, a yellow-brown product is formed, which does not interfere with the spectrophotometric detection of ellagic acid. 

Except for ET reactions with strong oxidants, 02 is not very reactive (for a compilation of rate constants, see Bielski et al. 1985). For example, practically no reaction has been detected with amino acids (Bielski and Shiue 1979), and there is no reaction to speak of with the DNA constituents, that is, it is also practically unreactive towards DNA. However, where substantial reactivity has been recognized, its main route of reaction seems to be by addition. This has not only been proposed for its reaction with pyrogallol and the propyl ester of gallic acid [k = 3.4 x 10s and 2.6 x 10s dm3 mol1 s 1, respectively cf. reactions (67)-(71) Deeble et al. 1987, 1988], but it seems that an addition reaction triggers a number of chain reactions (von Sonntag et al. 1993, see below). 

Oxygen radical absorbency capacity (ORAC) method presumes registration of substrate (B-phycoerythrin or fluorescein) fluorescence [43,44] after reaction with AOC in the sample to be compared with reference template. Trolox is used as a standard to determine per-oxyl radicals and gallic acid to determine hydroxyl radicals. 

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