malic acid

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
EC number	230-022-8
ChemSpider ID	510
Properties
Molecular formula	C4H6O5
Molar mass	134.09 g mol−1
Density	1.609 g/cm³
Melting point	130 °C
Solubility in water	558 g/l (at 20 °C)[1]
Acidity (pKa)	pKa1 = 3.4, pKa2 = 5.13
Related compounds
Other anions	malate
Related carboxylic acids	propionic acid butyric acid succinic acid tartaric acid crotonic acid fumaric acid pentanoic acid
Related compounds	butanol butyraldehyde crotonaldehyde sodium malate
Y (what is this?)  (verify) 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. This dicarboxylic acid is the active ingredient in many sour or tart foods. Malic acid is found mostly in unripe fruits.

The salts and esters of malic acid are known as malates. The malate anion is an intermediate in the citric acid cycle.

History

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.^[2]

Chemical structure

In biological sources, malic acid is homochiral. Malic acid has two stereoisomers, a left-handed L-enantiomer and a right-handed D-enantiomer. L-Malic acid is the naturally occurring form, whereas D-malic acid is produced synthetically.

L-Malic acid

D-Malic acid

Biochemistry

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 produced from starch in guard cells of plant leaves. A build up of malate leads to a low water potential. Water then flows into the guard cells causing the stoma to open. However, this process does not always induce the opening of stomata.

Malic acid in food

Malic acid contributes to the sourness of green apples. Malic acid is present in grapes. 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 so-called "extreme candy", for example Mega Warheads or Sour Punch candies. It is also used with or in place of the less sour citric acid in sour sweets such as Jolly Ranchers, Sweet Tarts and Salt & Vinegar flavor potato chips. These sweets are sometimes labeled with a warning that excessive consumption can cause irritation of the mouth.

Chemistry

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.

Self-condensation of malic acid with fuming sulfuric acid gives the pyrone coumalic acid:^[3]

See also

• Malate-aspartate shuttle
• Two other dicarboxylic acids have similar names and should not be confused with malic acid:
  • Maleic acid
  • Malonic acid
• Acids in wine

External links

• Calculator: Water and solute activities in aqueous malic acid

References

v • d • e Citric Acid Cycle Metabolic Pathway
			Oxaloacetate			Malate			Fumarate			Succinate			Succinyl-CoA
Acetyl-CoA				NADH + H+	NAD+			H2O		FADH2	FAD		CoA + ATP(GTP)	Pi + ADP(GDP)
	+	H2O														NADH + H+ + CO2
		CoA														NAD+
					H2O		H2O			NAD(P)+	NAD(P)H + H+			CO2
			Citrate			cis-Aconitate			Isocitrate			Oxalosuccinate			α-Ketoglutarate