Malic acid

Not to be confused with maleic acid or malonic acid.

"Malate" redirects here. For the district in Manila, see Malate, Manila.

Malic acid
IUPAC name hydroxybutanedioic acid
Other names L-Malic acid D-Malic acid (-)-Malic acid (+)-Malic acid (S)-Hydroxybutanedioic acid (R)-Hydroxybutanedioic acid
Identifiers
CAS number	6915-15-7 Y
PubChem	525
ChemSpider	510 Y, 83793 D-(+)-malic acid Y, 193317 L-(-)-malic acid Y
UNII	817L1N4CKP Y
EC number	230-022-8
KEGG	D04843 Y
ChEBI	CHEBI:6650 Y
Jmol-3D images	Image 1
SMILES O=C(O)CC(O)C(=O)O
InChI InChI=1S/C4H6O5/c5-2(4(8)9)1-3(6)7/h2,5H,1H2,(H,6,7)(H,8,9) Y Key: BJEPYKJPYRNKOW-UHFFFAOYSA-N Y InChI=1/C4H6O5/c5-2(4(8)9)1-3(6)7/h2,5H,1H2,(H,6,7)(H,8,9) Key: BJEPYKJPYRNKOW-UHFFFAOYAM
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
Y acid (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

Malic acid is an organic compound with the formula HO₂CCH₂CHOHCO₂H. It is a dicarboxylic acid which 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.

• 

  L-Malic acid
• 

  D-Malic acid

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 process of malolactic fermentation converts malic acid to much milder lactic acid.

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,^[5] USA^[6] and Australia and New Zealand^[7] (where it is listed by its INS number 296).

Production and main reactions

Malic acid is produced industrially by the double hydration of maleic anhydride.^[8]

Self-condensation of malic acid with fuming sulfuric acid gives the pyrone coumalic acid:^[9]

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.^[10]

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Citric_acid_cycle edit

See also

• Acids in wine
• Crassulacean acid metabolism
• Malate-aspartate shuttle

References

1. ^ chemBlink Online Database of Chemicals from Around the World
2. ^ Dawson, R. M. C. et al., Data for Biochemical Research, Oxford, Clarendon Press, 1959.
3. ^ The Origin of the Names Malic, Maleic, and Malonic Acid Jensen, William B. J. Chem. Educ. 2007, 84, 924. Abstract
4. ^ "Methods For Analysis of Musts and Wines", Ough and Amerine, John Wiley and Sons, 2nd Edition, 1988, page 67
5. ^ UK Food Standards Agency: "Current EU approved additives and their E Numbers". http://www.food.gov.uk/safereating/chemsafe/additivesbranch/enumberlist. Retrieved 2011-10-27. 
6. ^ US Food and Drug Administration: "Listing of Food Additives Status Part II". http://www.fda.gov/Food/FoodIngredientsPackaging/FoodAdditives/ucm191033.htm#ftnT. Retrieved 2011-10-27. 
7. ^ Australia New Zealand Food Standards Code"Standard 1.2.4 - Labelling of ingredients". http://www.comlaw.gov.au/Details/F2011C00827. Retrieved 2011-10-27. 
8. ^ Karlheinz Miltenberger "Hydroxycarboxylic Acids, Aliphatic" in Ullmann's Encyclopedia of Industrial Chemistry,2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a13 507
9. ^ Richard H. Wiley and Newton R. Smith (1963), "Coumalic acid", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv4p0201 ; Coll. Vol. 4: 201 
10. ^ The interactive pathway map can be edited at WikiPathways: "TCACycle_WP78". http://www.wikipathways.org/index.php/Pathway:WP78. 

External links

• Calculator: Water and solute activities in aqueous malic acid