Analdose is amonosaccharide (a simple sugar) with a carbon backbone chain with acarbonyl group on the endmost carbon atom, making it analdehyde, andhydroxyl groups connected to all the other carbon atoms. Aldoses can be distinguished fromketoses, which have the carbonyl group away from the end of the molecule, and are thereforeketones.
Like most carbohydrates, simple aldoses have the generalchemical formula Cn(H2O)n. Becauseformaldehyde (n=1) andglycolaldehyde (n=2) are not generally considered to be carbohydrates,[1] the simplest possible aldose is thetrioseglyceraldehyde, which only contains threecarbonatoms.[2]
Because they have at least one asymmetric carbon center, all aldoses exhibitstereoisomerism. Aldoses can exist in either aD- form orL- form. The determination is made based on the chirality of the asymmetric carbon furthest from the aldehyde end, namely the second-last carbon in the chain. Aldoses with alcohol groups on the right of theFischer projection areD-aldoses, and those with alcohols on the left areL-aldoses.D-aldoses are more common thanL-aldoses in nature.[1]
Examples of aldoses includeglyceraldehyde,erythrose,ribose,glucose andgalactose. Ketoses and aldoses can be chemically differentiated throughSeliwanoff's test, where the sample is heated with acid andresorcinol.[3] The test relies on thedehydration reaction which occurs more quickly in ketoses, so that while aldoses react slowly, producing a light pink color, ketoses react more quickly and strongly to produce a dark red color.
Aldoses canisomerize to ketoses through theLobry-de Bruyn-van Ekenstein transformation.
Aldoses are differentiated by the number of carbon atoms in the main chain. The minimum number of carbon atoms in a backbone needed to form a molecule that is still considered a carbohydrate is 3, and carbohydrates with three carbon atoms are called trioses. The only aldotriose isglyceraldehyde, which has one chiral stereocenter with 2 possible enantiomers,D- andL-glyceraldehyde.
Some common aldoses are:
The most commonly discussed category of aldoses are those with six carbon atoms,aldohexoses. Some aldohexoses that are widely called by common names are:[4]
Aldoses are commonly referred to by names specific to one stereoisomer of the compound. This distinction is especially vital in biochemistry, as many systems can only use one enantiomer of the carbohydrate and not the other. However, aldoses are not locked into any one conformation: they can and do fluctuate between different forms.
Aldoses cantautomerize to ketoses in a dynamic process with anenol intermediate (more specifically, an enediol).[1] This process is reversible, so aldoses and ketoses can be thought of as being in equilibrium with each other. However, aldehydes and ketones are almost always more stable than the corresponding enol forms, so the aldo- and keto- forms normally predominate. This process, with its enol intermediate, also allows stereoisomerization. Basic solutions accelerate the interconversion of isomers.
Carbohydrates with more than four carbon atoms exist in an equilibrium between the closed ring, or cyclic form, and the open-chain form. Cyclic aldoses are usually drawn asHaworth projections, and open chain forms are commonly drawn asFischer projections, both of which represent important stereochemical information about the forms they depict.[1]
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