Gallic acid is an important component of iron gall ink, the standard European writing and drawing ink from the 12th to 19th centuries, with a history extending to the Roman empire and the Dead Sea Scrolls. Pliny the Elder describes the use of gallic acid as a means of detecting an adulteration of verdigris and writes that it was used to produce dyes. Galls from oak trees were crushed and mixed with water, producing tannic acid. It could then be mixed with green vitriol — obtained by allowing sulfate-saturated water from a spring or mine drainage to evaporate — and gum arabic from acacia trees; this combination of ingredients produced the ink. Gallic acid was one of the substances used by Angelo Mai, among other early investigators of palimpsests, 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. Gallic acid was first studied by the Swedish chemist Carl Wilhelm Scheele in 1786. In 1818, French chemist and pharmacist Henri Braconnot devised a simpler method of purifying gallic acid from galls; gallic acid was also studied by the French chemist Théophile-Jules Pelouze, among others. Gallic acid is a component of some pyrotechnic whistle mixtures.
Metabolism
Biosynthesis
Gallic acid is formed from 3-dehydroshikimate by the action of the enzyme shikimate dehydrogenase to produce 3,5-didehydroshikimate. This latter compound tautomerizes to form the redox equivalent gallic acid, where the equilibrium lies essentially entirely toward gallic acid because of the coincidentally occurring aromatization.
Degradation
is an enzyme found in Pseudomonas putida that catalyses the reaction Gallate decarboxylase is another enzyme in the degradation of gallic acid.
Conjugation
is an enzyme that uses UDP-glucose and gallate, whereas its two products are UDP and 1-galloyl-beta-D-glucose.
Gallic acid is found in a number of land plants, such as the parasitic plantCynomorium coccineum, the aquatic plantMyriophyllum spicatum, and the blue-green algaMicrocystis aeruginosa. Gallic acid is also found in various oak species, Caesalpinia mimosoides, and in the stem bark of Boswellia dalzielii, among others. Many foodstuffs contain various amounts of gallic acid, especially fruits, as well as teas, cloves, and vinegars. Carob fruit is a rich source of gallic acid.
Production
Gallic acid is easily freed from gallotannins by acidic or alkaline hydrolysis. When gallic acid is heated with concentrated sulfuric acid, rufigallol is produced by condensation. Oxidation with arsenic acid, permanganate, persulfate, or iodine yields ellagic acid, as does reaction of methyl gallate with iron chloride.
