Gallic acid

Gallic acid
IUPAC name 3,4,5-trihydroxybenzoic acid
Other names Gallic acid Gallate 3,4,5-trihydroxybenzoate
Identifiers
CAS number	149-91-7 Y, [5995-86-8] (Monohydrate)
PubChem	370
ChemSpider	361 Y
UNII	632XD903SP Y
KEGG	C01424 Y
ChEBI	CHEBI:30778 Y
ChEMBL	CHEMBL288114 Y
Jmol-3D images	Image 1
SMILES O=C(O)c1cc(O)c(O)c(O)c1
InChI InChI=1S/C7H6O5/c8-4-1-3(7(11)12)2-5(9)6(4)10/h1-2,8-10H,(H,11,12) Y Key: LNTHITQWFMADLM-UHFFFAOYSA-N Y InChI=1/C7H6O5/c8-4-1-3(7(11)12)2-5(9)6(4)10/h1-2,8-10H,(H,11,12) Key: LNTHITQWFMADLM-UHFFFAOYAN
Properties
Molecular formula	C7H6O5
Molar mass	170.12 g/mol
Appearance	White, yellowish-white, or pale fawn-colored crystals.
Density	1.7 g/cm3 (anhydrous)
Melting point	250 °C, 523 K, 482 °F
Solubility in water	1.1 g/100 ml water @ 20°C (anhydrous) 1.5 g/100 ml water @ 20 °C (monohydrate)
Acidity (pKa)	COOH: 4.5, OH: 10.
Hazards
MSDS	External MSDS
Main hazards	Irritant
Related compounds
Related compounds	Benzoic acid, Phenol, Pyrogallol
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Gallic acid is a trihydroxybenzoic acid, a type of phenolic acid, a type of organic acid, also known as 3,4,5-trihydroxybenzoic acid, found in gallnuts, sumac, witch hazel, tea leaves, oak bark, and other plants.^[1] The chemical formula is C₆H₂(OH)₃COOH. Gallic acid is found both free and as part of hydrolyzable tannins.

Salts and esters of gallic acid are termed 'gallates'. Despite its name, it does not contain gallium.

Gallic acid is commonly used in the pharmaceutical industry.^[2] It is used as a standard for determining the phenol content of various analytes by the Folin-Ciocalteau assay; results are reported in gallic acid equivalents.^[3] Gallic acid can also be used as a starting material in the synthesis of the psychedelic alkaloid mescaline.^[4]

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Gallic acid seems to have anti-fungal and anti-viral properties. Gallic acid acts as an antioxidant and helps to protect human cells against oxidative damage. Gallic acid was found to show cytotoxicity against cancer cells, without harming healthy cells. Gallic acid is used as a remote astringent in cases of internal haemorrhage. Gallic acid is also used to treat albuminuria and diabetes. Some ointments to treat psoriasis and external haemorrhoids contain gallic acid.^[5]

Historical context and uses [edit]

Gallic acid is an important component of iron gall ink, the standard European writing and drawing ink from the 12th to 19th century with a history extending to the Roman empire and the Dead Sea Scrolls. Pliny the Elder (23-79 AD) describes his experiments with it and writes that it was used to produce dyes. Galls (also known as oak apples) from oak trees were crushed and mixed with water, producing tannic acid (a macromolecular complex containing gallic acid). It could then be mixed with green vitriol (ferrous sulfate) — 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.^[6]

Gallic acid was one of the substances used by Angelo Mai (1782–1854), 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.^{[citation needed]}

Gallic acid was first studied by the Swedish chemist Carl Wilhelm Scheele in 1786.^[7] In 1818 the French chemist and pharmacist Henri Braconnot (1780–1855) devised a simpler method of purifying gallic acid from galls;^[8] gallic acid was also studied by the French chemist Théophile-Jules Pelouze (1807–1867),^[9] among others.

Early photographers, including Joseph Bancroft Reade (1801–1870) and William Fox Talbot (1800–1877), used gallic acid for developing latent images in calotypes. It has also been used as a coating agent in zincography.

George Washington used gallic acid to communicate with spies^{[clarification needed]} during the American Revolutionary War, according to the miniseries America: The Story of Us.^{[citation needed]}

Gallic acid is a component of some pyrotechnic whistle mixtures.

Metabolism [edit]

Biosynthesis [edit]

Chemical structure of 3,5-didehydroshikimate

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 coincidently occurring aromatization.^[10][11]

Degradation [edit]

Gallate dioxygenase is an enzyme found in Pseudomonas putida that catalyses the reaction gallate + O₂ → (1E)-4-oxobut-1-ene-1,2,4-tricarboxylate.

Gallate decarboxylase is another enzyme in the degradation of gallic acid.

Conjugation [edit]

Gallate 1-beta-glucosyltransferase is an enzyme that uses UDP-glucose and gallate, whereas its two products are UDP and 1-galloyl-beta-D-glucose.

Natural occurrences [edit]

Gallic acid is found in a number of land plants. It is also found in the aquatic plant Myriophyllum spicatum and shows an allelopathic effect on the growth of the blue-green alga Microcystis aeruginosa.^[12]

List of plants that contain the chemical [edit]

• Gallic acid is found in oaks species like the North American white oak (Quercus alba) and European red oak (Quercus robur).^[13]
• Caesalpinia mimosoides^[14]
• stem bark of Boswellia dalzielii^[15]
• Drosera (sundew)
• Rhodiola rosea (Golden root)
• Triphala (Ayurvedic herbal rasayana formula)
• Toona sinensis

In food [edit]

• Areca nut
• Bearberry (Arctostaphylos sp)
• Bergenia sp
• Blackberry
• Hot chocolate
• Juglans regia (Common walnut)
• Mango in peels and leaves
• Phyllanthus emblica (Indian gooseberry) in fruits
• Raspberry
• Syzygium aromaticum (clove)^[16]
• Vinegar^[17]
• wine
• Witch hazel (Hamamelis virginiana)
• White tea

Spectral data [edit]

UV-Vis
Lambda-max:	220, 271 nm (ethanol) Spectrum of gallic acid
Extinction coefficient (log ε)
IR
Major absorption bands	ν : 3491, 3377, 1703, 1617, 1539, 1453, 1254 cm−1 (KBr)
NMR
Proton NMR (acetone-d6): d : doublet, dd : doublet of doublets, m : multiplet, s : singlet	δ : 7.15 (2H, s, H-3 and H-7)
Carbon-13 NMR (acetone-d6):	δ : 167.39 (C-1), 144.94 (C-4 and C-6), 137.77 (C-5), 120.81 (C-2), 109.14 (C-3 and C-7)
Other NMR data
MS
Masses of main fragments	ESI-MS [M-H]- m/z : 169.0137

Reference^[14]

Esters [edit]

Also known as galloylated esters:

• Methyl gallate
• Ethyl gallate, a food additive with E number E313
• Propyl gallate, or propyl 3,4,5-trihydroxybenzoate, an ester formed by the condensation of gallic acid and propanol
• Octyl gallate, the ester of octanol and gallic acid
• Dodecyl gallate, or lauryl gallate, the ester of dodecanol and gallic acid
• Epicatechin gallate, a flavan-3-ol, a type of flavonoid, present in green tea
• Epigallocatechin gallate (EGCG), also known as epigallocatechin 3-gallate, the ester of epigallocatechin and gallic acid, and a type of catechin
• Gallocatechin gallate (GCG), the ester of gallocatechin and gallic acid and a type of flavan-3ol
• Theaflavin-3-gallate, a theaflavin derivative

Health effects [edit]

It is a weak carbonic anhydrase inhibitor.^[18]

Potential uses [edit]

It can be used to produce polyesters based on phloretic acid and gallic acid.^[19]

See also [edit]

• Hydrolyzable tannin
• Pyrogallol
• Syringol
• Syringaldehyde
• Shikimic acid

References [edit]

External links [edit]

• King's American Dispensatory (1898) entry on Gallic acid