Monosaccharides (from Greek monos: single, sacchar: sugar), also called simple sugar, are the simplest form of sugar and the most basic units of carbohydrates. They cannot be further hydrolyzed to simpler chemical compounds. The general formula is C
nH
2nO
n. They are usually colorless, water-soluble, and crystalline solids. Some monosaccharides have a sweet taste. But all the compounds which fit into this general formula may not be classified as carbohydrates. For example, Acetic Acid which fits in the formula is not a carbohydrate.
nH
2nO
n. They are usually colorless, water-soluble, and crystalline solids. Some monosaccharides have a sweet taste. But all the compounds which fit into this general formula may not be classified as carbohydrates. For example, Acetic Acid which fits in the formula is not a carbohydrate.
Examples of monosaccharides include glucose (dextrose), fructose (levulose), and galactose. Monosaccharides are the building blocks of disaccharides (such as sucrose and lactose) and polysaccharides (such as cellulose and starch). Each carbon atom that supports a hydroxyl group is chiral, except those at the end of the chain. This gives rise to a number of isomeric forms, all with the same chemical formula. For instance, galactose and glucose are both aldohexoses, but have different physical structures and chemical properties.
The monosaccharide glucose plays a pivotal role in metabolism, where the chemical energy is extracted through glycolysis and the citric acid cycle to provide energy to living organisms. Some other monosaccharides can be converted in the living organism to glucose.
Structure and nomenclature
With few exceptions (e.g., deoxyribose), monosaccharides have this chemical formula: (CH2O)x, where conventionally x ≥ 3. Monosaccharides can be classified by the number x of carbon atoms they contain: triose (3), tetrose (4), pentose (5), hexose (6), heptose (7), and so on.
Glucose, used as an energy source and for the synthesis of starch, glycogen and cellulose, is a hexose. Ribose and deoxyribose (in RNA and DNA respectively) are pentose sugars. Examples of heptoses include the ketoses, mannoheptulose and sedoheptulose. Monosaccharides with eight or more carbons are rarely observed as they are quite unstable. In aqueous solutions monosaccharides exist as rings if they have more than four carbons.
Linear-chain monosaccharides
Simple monosaccharides have a linear and unbranched carbon skeleton with one carbonyl (C=O) functional group, and one hydroxyl (OH) group on each of the remaining carbon atoms. Therefore, the molecular structure of a simple monosaccharide can be written as H(CHOH)n(C=O)(CHOH)mH, where n + 1 + m = x; so that its elemental formula is CxH2xOx.
By convention, the carbon atoms are numbered from 1 to x
along the backbone, starting from the end that is closest to the C=O
group. Monosaccharides are the simplest units of carbohydrates and the
simplest form of sugar.
If the carbonyl is at position 1 (that is, n or m is zero), the molecule begins with a formyl group H(C=O)− and is technically an aldehyde. In that case, the compound is termed an aldose. Otherwise, the molecule has a keto group, a carbonyl −(C=O)− between two carbons; then it is formally a ketone, and is termed a ketose. Ketoses of biological interest usually have the carbonyl at position 2.
The various classifications above can be combined, resulting in names such as "aldohexose" and "ketotriose".
A more general nomenclature for open-chain monosaccharides
combines a Greek prefix to indicate the number of carbons (tri-, tetr-,
pent-, hex-, etc.) with the suffixes "-ose" for aldoses and "-ulose" for
ketoses.
In the latter case, if the carbonyl is not at position 2, its position
is then indicated by a numeric infix. So, for example, H(C=O)(CHOH)4H is pentose, H(CHOH)(C=O)(CHOH)3H is pentulose, and H(CHOH)2(C=O)(CHOH)2H is pent-3-ulose.
Open-chain stereoisomers
Two monosaccharides with equivalent molecular graphs (same chain length and same carbonyl position) may still be distinct stereoisomers, whose molecules differ in spatial orientation. This happens only if the molecule contains a stereogenic center, specifically a carbon atom that is chiral
(connected to four distinct molecular sub-structures). Those four
bonds can have any of two configurations in space distinguished by their
handedness.
In a simple open-chain monosaccharide, every carbon is chiral except
the first and the last atoms of the chain, and (in ketoses) the carbon
with the keto group.
For example, the triketose H(CHOH)(C=O)(CHOH)H (glycerone, dihydroxyacetone) has no stereogenic center, and therefore exists as a single stereoisomer. The other triose, the aldose H(C=O)(CHOH)2H (glyceraldehyde), has one chiral carbon — the central one, number 2 — which is bonded to groups −H, −OH, −C(OH)H2,
and −(C=O)H. Therefore, it exists as two stereoisomers whose molecules
are mirror images of each other (like a left and a right glove).
Monosaccharides with four or more carbons may contain multiple chiral
carbons, so they typically have more than two stereoisomers. The number
of distinct stereoisomers with the same diagram is bounded by 2c, where c is the total number of chiral carbons.
The Fischer projection is a systematic way of drawing the skeletal formula
of an acyclic monosaccharide so that the handedness of each chiral
carbon is well specified. Each stereoisomer of a simple open-chain
monosaccharide can be identified by the positions (right or left) in the
Fischer diagram of the chiral hydroxyls (the hydroxyls attached to the
chiral carbons).
Most stereoisomers are themselves chiral (distinct from their
mirror images). In the Fischer projection, two mirror-image isomers
differ by having the positions of all chiral hydroxyls reversed
right-to-left. Mirror-image isomers are chemically identical in
non-chiral environments, but usually have very different biochemical
properties and occurrences in nature.
While most stereoisomers can be arranged in pairs of mirror-image
forms, there are some non-chiral stereoisomers that are identical to
their mirror images, in spite of having chiral centers. This happens
whenever the molecular graph is symmetrical, as in the 3-ketopentoses
H(CHOH)2(CO)(CHOH)2H, and the two halves are
mirror images of each other. In that case, mirroring is equivalent to a
half-turn rotation. For this reason, there are only three distinct
3-ketopentose stereoisomers, even though the molecule has two chiral
carbons.
Distinct stereoisomers that are not mirror-images of each other
usually have different chemical properties, even in non-chiral
environments. Therefore, each mirror pair and each non-chiral
stereoisomer may be given a specific monosaccharide name.
For example, there are 16 distinct aldohexose stereoisomers, but the
name "glucose" means a specific pair of mirror-image aldohexoses. In the
Fischer projection, one of the two glucose isomers has the hydroxyl at
left on C3, and at right on C4 and C5; while the other isomer has the
reversed pattern. These specific monosaccharide names have conventional
three-letter abbreviations, like "Glu" for glucose and "Thr" for threose.
Generally, a monosaccharide with n asymmetrical carbons has 2n
stereoisomers. The number of open chain stereoisomers for an aldose
monosaccharide is larger by one than that of a ketose monosaccharide of
the same length. Every ketose will have 2(n−3) stereoisomers where n > 2 is the number of carbons. Every aldose will have 2(n−2) stereoisomers where n
> 2 is the number of carbons.
These are also referred to as epimers which have the different
arrangement of −OH and −H groups at the asymmetric or chiral carbon
atoms (this does not apply to those carbons having the carbonyl
functional group).
Configuration of monosaccharides
Like many chiral molecules, the two stereoisomers of glyceraldehyde will gradually rotate the polarization direction of linearly polarized light as it passes through it, even in solution. The two stereoisomers are identified with the prefixes D- and L-, according to the sense of rotation: D-glyceraldehyde is dextrorotatory (rotates the polarization axis clockwise), while L-glyceraldehyde is levorotatory (rotates it counterclockwise).
D- and L-glucose
The D- and L-
prefixes are also used with other monosaccharides, to distinguish two
particular stereoisomers that are mirror-images of each other. For this
purpose, one considers the chiral carbon that is furthest removed from
the C=O group. Its four bonds must connect to −H, −OH, −C(OH)H, and the
rest of the molecule. If the molecule can be rotated in space so that
the directions of those four groups match those of the analog groups in D-glyceraldehyde's C2, then the isomer receives the D- prefix. Otherwise, it receives the L- prefix.
In the Fischer projection, the D- and L- prefixes specifies the configuration at the carbon atom that is second from bottom: D- if the hydroxyl is on the right side, and L- if it is on the left side.
Note that the D- and L-
prefixes do not indicate the direction of rotation of polarized light,
which is a combined effect of the arrangement at all chiral centers.
However, the two enantiomers will always rotate the light in opposite
directions, by the same amount. See also D/L system.
Cyclisation of monosaccharides
A monosaccharide often switches from the acyclic (open-chain) form to a cyclic form, through a nucleophilic addition
reaction between the carbonyl group and one of the hydroxyls of the
same molecule. The reaction creates a ring of carbon atoms closed by
one bridging oxygen atom. The resulting molecule has a hemiacetal or hemiketal
group, depending on whether the linear form was an aldose or a ketose.
The reaction is easily reversed, yielding the original open-chain form.
In these cyclic forms, the ring usually has five or six atoms. These forms are called furanoses and pyranoses, respectively — by analogy with furan and pyran,
the simplest compounds with the same carbon-oxygen ring (although they
lack the double bonds of these two molecules). For example, the
aldohexose glucose
may form a hemiacetal linkage between the hydroxyl on carbon 1 and the
oxygen on carbon 4, yielding a molecule with a 5-membered ring, called glucofuranose. The same reaction can take place between carbons 1 and 5 to form a molecule with a 6-membered ring, called glucopyranose. Cyclic forms with a seven-atom ring (the same of oxepane), rarely encountered, are called heptoses.
Conversion between the furanose, acyclic, and pyranose forms of D-glucose
Pyranose forms of some pentose sugars
Pyranose forms of some hexose sugars
For many monosaccharides (including glucose), the cyclic forms
predominate, in the solid state and in solutions, and therefore the same
name commonly is used for the open- and closed-chain isomers. Thus,
for example, the term "glucose" may signify glucofuranose,
glucopyranose, the open-chain form, or a mixture of the three.
Cyclization creates a new stereogenic
center at the carbonyl-bearing carbon. The −OH group that replaces the
carbonyl's oxygen may end up in two distinct positions relative to the
ring's midplane. Thus each open-chain monosaccharide yields two cyclic
isomers (anomers), denoted by the prefixes α- and β-. The molecule can change between these two forms by a process called mutarotation, that consists in a reversal of the ring-forming reaction followed by another ring formation.
Haworth projection
The stereochemical structure of a cyclic monosaccharide can be represented in a Haworth projection. In this diagram, the α-isomer for the pyranose form of a D-aldohexose has the −OH of the anomeric carbon
below the plane of the carbon atoms, while the β-isomer has the −OH of
the anomeric carbon above the plane. Pyranoses typically adopt a chair
conformation, similar to that of cyclohexane.
In this 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 (considering D-aldohexose sugars).
α-D-Glucopyranose
β-D-Glucopyranose
Derivatives
A large number of biologically important modified monosaccharides exist:
- Amino sugars such as:
- Sulfosugars such as:
- Others such as:
