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Malate

Not to be confused with maleic acid or malonic acid.
"Malate" redirects here. For the district in Manila, see Malate, Manila.
Malic acid

DL-Malic acid
Identifiers
CAS number 6915-15-7 YesY
PubChem 525
ChemSpider 193317 L-(–)-malic acid YesY
UNII 817L1N4CKP YesY
EC number 230-022-8
KEGG C00149 YesY
ChEBI CHEBI:6650 YesY
Jmol-3D images Image 1
Properties
Molecular formula C4H6O5
Molar mass 134.09 g mol−1
Density 1.609 g cm−3
Melting point

130 °C, 403 K, 266 °F

Solubility in water 558 g/L (at 20 °C)[1]
Acidity (pKa) pKa1 = 3.40, pKa2 = 5.20 [2]
Related compounds
Other anions malate
Related carboxylic acids succinic acid
tartaric acid
fumaric acid
Related compounds butanol
butyraldehyde
crotonaldehyde
sodium malate
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Malic acid is an organic compound with the formula HO2CCH2CHOHCO2H. It is a dicarboxylic acid that is made by all living organisms, contributes to the pleasantly sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms (L- and D-enantiomers), though only the L-isomer exists naturally. The salts and esters of malic acid are known as malates. The malate anion is an intermediate in the citric acid cycle.

Biochemistry

L-Malic acid is the naturally occurring form, whereas a mixture of L- and D-malic acid is produced synthetically.

Malate plays an important role in biochemistry. In the C4 carbon fixation process, malate is a source of CO2 in the Calvin cycle. In the citric acid cycle, (S)-malate is an intermediate, formed by the addition of an -OH group on the si face of fumarate. It can also be formed from pyruvate via anaplerotic reactions.

Malate is also synthesized by the carboxylation of phosphoenolpyruvate in the guard cells of plant leaves. Malate, as a double anion, often accompanies potassium cations during the uptake of solutes into the guard cells in order to maintain electrical balance in the cell. The accumulation of these solutes within the guard cell decreases the solute potential, allowing water to enter the cell and promote aperture of the stomata.

Malic acid in food

Malic acid was first isolated from apple juice by Carl Wilhelm Scheele in 1785. Antoine Lavoisier in 1787 proposed the name acide malique which is derived from the Latin word for apple, mālum.[3] Malic acid contributes to the sourness of green apples. It is present in grapes and in most wines with concentrations sometimes as high as 5 g/l.[4] It confers a tart taste to wine, although the amount decreases with increasing fruit ripeness. The taste of malic acid is very clear and pure in rhubarb, a plant for which it is the primary flavor.

The process of malolactic fermentation converts malic acid to much milder lactic acid. Malic acid occurs naturally in all fruits and many vegetables, and is generated in fruit metabolism.[5]

Malic acid, when added to food products, is denoted by E number E296. Malic acid is the source of extreme tartness in USA-produced confectionery, the so-called extreme candy. It is also used with or in place of the less sour citric acid in sour sweets. These sweets are sometimes labeled with a warning stating that excessive consumption can cause irritation of the mouth. It is approved for use as a food additive in the EU,[6] USA[7] and Australia and New Zealand[8] (where it is listed by its INS number 296).

Production and main reactions

Racemic malic acid is produced industrially by the double hydration of maleic anhydride. In 2000, American production capacity was 5000 tonnes per year. Both enantiomers may be separated by chiral resolution of the racemic mixture, and the (S)- enantiomer may be specifically obtained by fermentation of fumaric acid.[9]

Self-condensation of malic acid with fuming sulfuric acid gives the pyrone coumalic acid:[10]

Malic acid was important in the discovery of the Walden inversion and the Walden cycle, in which (-)-malic acid first is converted into (+)-chlorosuccinic acid by action of phosphorus pentachloride. Wet silver oxide then converts the chlorine compound to (+)-malic acid, which then reacts with PCl5 to the (-)-chlorosuccinic acid. The cycle is completed when silver oxide takes this compound back to (-)-malic acid.

Interactive pathway map

Click on genes, proteins and metabolites below to link to respective articles. [§ 1]
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See also

References

External links

  • Calculator: Water and solute activities in aqueous malic acid


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