Electrostatic potential map of surface of gallic acid moleculeEllagic acid molecule structure resembles that of two gallic acid molecules assembled in head to tail position and linked together by a C–C bond (as inbiphenyl) and two cyclicester links (lactones) forming two additional 6-piece cycles.
Alkaline solutions of gallic acid are readily oxidized by air. The oxidation is catalyzed by the enzymegallate dioxygenase, an enzyme found inPseudomonas putida.
Oxidative coupling of gallic acid with arsenic acid, permanganate, persulfate, or iodine yieldsellagic acid, as does reaction of methyl gallate withiron(III) chloride.[5] Gallic acid forms intermolecular esters (depsides) such asdigallic and cyclic ether-esters (depsidones).[5]
Gallic acid is an important component ofiron gall ink, the standard European writing and drawing ink from the 12th to 19th centuries, with a history extending to the Roman empire and theDead Sea Scrolls.Pliny the Elder (23–79 AD) describes the use of gallic acid as a means of detecting an adulteration ofverdigris[7] and writes that it was used to produce dyes. Galls (also known as oak apples) from oak trees were crushed and mixed with water, producingtannic acid. It could then be mixed withgreen vitriol (ferrous sulfate)—obtained by allowing sulfate-saturated water from a spring or mine drainage to evaporate[citation needed]—andgum arabic from acacia trees; this combination of ingredients produced the ink.[8]
Gallic acid was one of the substances used byAngelo Mai (1782–1854), among other early investigators ofpalimpsests, to clear the top layer of text off and reveal hidden manuscripts underneath. Mai was the first to employ it, but did so "with a heavy hand", often rendering manuscripts too damaged for subsequent study by other researchers.[9]
Gallic acid was first studied by the Swedish chemistCarl Wilhelm Scheele in 1786.[10] In 1818, French chemist and pharmacistHenri Braconnot (1780–1855) devised a simpler method of purifying gallic acid from galls;[11] gallic acid was also studied by the French chemistThéophile-Jules Pelouze (1807–1867),[12] among others.
When mixed withacetic acid, gallic acid had uses in early types of photography, like thecalotype to make the silver more sensitive to light; it was also used in developing photographs.[13]
Gallate esters areantioxidants useful in food preservation, with propyl gallate being the most commonly used. Their use in human health is scantly supported by evidence.
^Albert W. Burgstahler and Zoe J. Bithos (1962). "Hexahydrogallic Acid and Hexahydrogallic Acid Triacetate".Organic Syntheses.42: 62.doi:10.15227/orgsyn.042.0062.
^Pliny the Elder with John Bostock and H.T. Riley, trans.,The Natural History of Pliny (London, England: Henry G. Bohn, 1857), vol. 6,p. 196. In Book 34, Chapter 26 of hisNatural History, Pliny states that verdigris (a form of copper acetate (Cu(CH3COO)2·2Cu(OH)2), which was used to process leather, was sometimes adulterated with copperas (a form of iron(II) sulfate (FeSO4·7H2O)). He presented a simple test for determining the purity of verdigris. From p. 196: "The adulteration [of verdigris], however, which is most difficult to detect, is made with copperas; ... The fraud may also be detected by using a leaf of papyrus, which has been steeped in an infusion of nut-galls; for it becomes black immediately upon the genuine verdigris being applied."
^L.D. Reynolds and N.G. Wilson, "Scribes and Scholars" 3rd Ed. Oxford: 1991, pp 193–4.
^Carl Wilhelm Scheele (1786)"Om Sal essentiale Gallarum eller Gallåple-salt" (On the essential salt of galls or gall-salt),Kongliga Vetenskaps Academiens nya Handlingar (Proceedings of the Royal [Swedish] Academy of Science),7: 30–34.
^Mämmelä, Pirjo; Savolainen, Heikki; Lindroos, Lasse; Kangas, Juhani; Vartiainen, Terttu (2000). "Analysis of oak tannins by liquid chromatography-electrospray ionisation mass spectrometry".Journal of Chromatography A.891 (1):75–83.doi:10.1016/S0021-9673(00)00624-5.PMID10999626.
^abChanwitheesuk, Anchana; Teerawutgulrag, Aphiwat; Kilburn, Jeremy D.; Rakariyatham, Nuansri (2007). "Antimicrobial gallic acid from Caesalpinia mimosoides Lamk".Food Chemistry.100 (3):1044–1048.doi:10.1016/j.foodchem.2005.11.008.
^Hodgson JM, Morton LW, Puddey IB, Beilin LJ, Croft KD (2000). "Gallic acid metabolites are markers of black tea intake in humans".Journal of Agricultural and Food Chemistry.48 (6):2276–80.Bibcode:2000JAFC...48.2276H.doi:10.1021/jf000089s.PMID10888536.
^Pathak, S. B.; Niranjan, K.; Padh, H.; Rajani, M.; et al. (2004). "TLC Densitometric Method for the Quantification of Eugenol and Gallic Acid in Clove".Chromatographia.60 (3–4):241–244.doi:10.1365/s10337-004-0373-y.S2CID95396304.
^Gálvez, Miguel Carrero; Barroso, Carmelo García; Pérez-Bustamante, Juan Antonio (1994). "Analysis of polyphenolic compounds of different vinegar samples".Zeitschrift für Lebensmittel-Untersuchung und -Forschung.199:29–31.doi:10.1007/BF01192948.S2CID91784893.
