Everything about Monosaccharide totally explained
Monosaccharides (from
Greek : single,
sacchar: sugar) are the simplest
carbohydrates. They can't be
hydrolyzed into simpler sugars. They consist of one sugar and are usually
colorless,
water-
soluble,
crystalline solids. Some monosaccharides have a
sweet taste. Examples of monosaccharides include
glucose (dextrose),
fructose,
galactose,
xylose and
ribose. Monosaccharides are the building blocks of
disaccharides like
sucrose (common sugar) and
polysaccharides (such as
cellulose and
starch). Further, each carbon atom that supports a
hydroxyl group (except for the first and last) is
chiral, giving rise to a number of
isomeric forms all with the same chemical formula. For instance,
galactose and
glucose are both
aldohexoses, but they've different chemical and physical properties.
Structure
With few exceptions (
for example,
deoxyribose), monosaccharides have the
chemical formula (CH
2O)
n + m with the chemical structure H(CHOH)
nC=O(CHOH)
mH. If n or m is zero, it's an aldehyde and is termed an
aldose, otherwise it's a ketone and is termed a
ketose. Monosaccharides contain either a
ketone or
aldehyde functional group, and
hydroxyl groups on most or all of the non-
carbonyl carbon
atoms.
Cyclic structure
Most monosaccharides form cyclic structures, which predominate in aqueous solution, by forming
hemiacetals or
hemiketals (depending on whether they're aldoses or ketoses) between an alcohol and the carbonyl group of the same sugar.
Glucose, for example, readily forms a hemiacetal linkage between its carbon-1 and the hydroxyl group of its carbon-5. Since such a reaction introduces an additional
stereogenic center, two
anomers are formed (α-isomer and β-isomer) from each distinct straight-chain monosaccharide. The interconversion between these two forms is called
mutarotation.
A common way of representing the cyclic structure of monosaccharides is the
Haworth projection.
In Haworth projection, the α-isomer has the OH- of the
anomeric carbon under the ring structure, and the β-isomer, has the OH- of the anomeric carbon on top of the ring structure.
In chair conformation, the α-isomer has the OH- of the anomeric carbon in an axial position, whereas the β-isomer has the OH- of the anomeric carbon in equatorial position.
Isomerism
The total number of possible
stereoisomers of one compound (n) is dependent on the number of
stereogenic centers (c) in the molecule. The upper limit for the number of possible stereoisomers is n = 2
c.
The only carbohydrate without an isomer is
dihydroxyacetone or DHA.
Monosaccharide nomenclature
Monosaccharides are classified by the number of
carbon atoms they contain:
Monosaccharides are classified the type of
carbonyl group they contain:
Aldose, -CHO (aldehyde)
Ketose, C=O (ketone)
Monosaccharides are classified according to their molecular configuration at the chiral carbon furthest removed from the aldehyde or ketone group. The chirality at this carbon is compared to the chirality of carbon 2 on glyceraldehyde. If it's equivalent to D-glyceraldehyde's C2, the sugar is D; if it's equivalent to L-glyceraldehyde's C2, the sugar is L. Due to the chirality of the sugar molecules, an aqueous solution of a D or L saccharides will rotate light. D-glyceraldehyde causes polarized light to rotate clockwise (dextrorotary); L-glyceraldehyde causes polarized light to rotate counterclockwise (levorotary). Unlike glyceraldehyde, D/L designation on more complex sugars isn't associated with their direction of light rotation. Since more complex sugars contain multiple chiral carbons, the direction of light rotation can't be predicted by the chirality of the carbon that defines D/L nomenclature.
D, configuration as in D-glyceraldehyde
L, configuration as in L-glyceraldehyde
All these classifications can be combined, resulting in names like D-aldohexose or ketotriose.
List of monosaccharides
This is a list of some common monosaccharides, not all are found in nature—some have been synthesized:
Trioses:
Tetroses:
Pentoses:
Hexoses:
Heptoses:
Octoses: octolose, 2-keto-3-deoxy-manno-octonate
Nonoses: sialose
Reactions
Formation of acetals.
Formation of hemiacetals and hemiketals.
Formation of ketals.Further Information
Get more info on 'Monosaccharide'.
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