Glyceraldehyde

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Glyceraldehyde

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Template:Longitem	Template:Chembox Elements/molecular formula
Molar mass	Template:Chem molar mass
Density	1.455 g/cm3
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Glyceraldehyde (glyceral) is a triose monosaccharide with chemical formula C₃H₆O₃. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism. The word comes from combining glycerol and aldehyde, as glyceraldehyde is glycerol with one alcohol group oxidized to an aldehyde.

Structure

Glyceraldehyde has one chiral center and therefore exists as two different enantiomers with opposite optical rotation:

• In the [[Absolute configuration#By relative configuration: D- and L-|Template:Smallcaps all nomenclature]], either Template:Smallcaps all from Latin Dexter meaning "right", or Template:Smallcaps all from Latin Laevo meaning "left"
• In the R/S nomenclature, either R from Latin Rectus meaning "right", or S from Latin Sinister meaning "left"

	Template:Smallcaps all-glyceraldehyde (R)-glyceraldehyde (+)-glyceraldehyde	Template:Smallcaps all-glyceraldehyde (S)-glyceraldehyde (−)-glyceraldehyde
Fischer projection	D-glyceraldehyde	L-glyceraldehyde
Skeletal formula	D-glyceraldehyde	L-glyceraldehyde

While the optical rotation of glyceraldehyde is (+) for R and (−) for S, this is not true for all monosaccharides. The stereochemical configuration can only be determined from the chemical structure, whereas the optical rotation can only be determined empirically (by experiment).

It was by a lucky guess that the molecular Template:Smallcaps all geometry was assigned to (+)-glyceraldehyde in the late 19th century, as confirmed by X-ray crystallography in 1951.

Aqueous and concentrated solutions of glyceraldehyde

The description above focuses on classification of isomers, but the glyceraldehyde is subject to a further complications: the tendency of hydroxy-aldehydes to exist as hydrates. NMR measurements indicate that in aqueous solution, glyceraldehyde exists in a hydrate owing to this reaction:

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The same study indicates that concentrated ("syrupy") forms of glyceraldehyde exist as dimers, indicating hemiacetal formation.

Nomenclature

In the [[Absolute configuration#By relative configuration: D- and L-|Template:Smallcaps all system]], glyceraldehyde is used as the configurational standard for carbohydrates. Monosaccharides with an absolute configuration identical to (R)-glyceraldehyde at the last stereocentre, for example C5 in glucose, are assigned the stereo-descriptor Template:Smallcaps all. Those similar to (S)-glyceraldehyde are assigned an Template:Smallcaps all.

Synthesis and reactions

Glyceraldehyde can be prepared from acetals of acrolein (CH₂=CHCHO) in two steps, oxidation followed by hydrolysis of the acetal. Its cyclohexylidene acetal can also be produced by oxidative cleavage of the bis(acetal) of mannitol. Glyceraldehyde can also be synthesized through the partial oxidation of Glycerol, for example with a fenton type reagent.

Glyceraldehyde is a precursor to four-carbon sugars (tetroses) via cyanation followed by hydrolysis of the cyanohydrin:

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Biochemistry

The enzyme glycerol dehydrogenase (NADP⁺) has two substrates, glycerol and NADP⁺, and 3 products, D-glyceraldehyde, NADPH and H+.

The interconversion of the phosphates of glyceraldehyde (glyceraldehyde 3-phosphate) and dihydroxyacetone (dihydroxyacetone phosphate), catalyzed by the enzyme triosephosphate isomerase, is an intermediate step in glycolysis.

See also

• Stereoisomerism