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| >> Basics |
| Aldoses and Ketoses |
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Monosaccharides are made up of carbon, hydrogen and oxygen in the proportion
CnH2nOn. The carbon skeleton is unbranched,
with a hydroxyl group at each carbon except the one containing carbonyl oxygen.
The two families of monosaccharides are Aldoses, with the carbonyl at one end
of chain, and ketoses, with the carbonyl anywhere else in the chain.
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Ketoses are further classified as 2-ketoses, 3-ketoses, etc. according to the position
of the carbonyl. 2-ketoses are the most common: if unspecified, a ketose is a 2-ketose.
Depending on number of carbons, monosaccharides are called trioses, tetroses, pentoses,
and hexoses etc. The smallest aldose is glyceraldehyde, containing one asymmetric carbon.
Changing the configuration of the substituents around that carbon forms two isomers,
called the L- and D- isomers of the sugar.
The two series of aldoses, the L- and D-
aldoses can be thought of as
derived
from these isomers of glyceraldehyde.
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The ketose erythrulose is used as the reference model for the
derivation.
D-Sugars
occur more commonly in nature, excepting L-Arabinose,
L-Rhamnose, L-Fucose, and
L-Galactose
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| Fischer Projection Formulae |
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Fischer Projection Formulae are used to identify the configuration at the chiral centers of a
sugar. The sugar is oriented so that the carbon skeleton is vertically aligned with the aldehyde
at the top and the alchohol at the bottom, so that vertical substituents are shown below the
plane of the page, while horizontal substituents are shown above. The projection of the vertical
and horizontal substituents are shown as plain vertical and horizontal lines, giving the Fischer
Projection of the chiral center. The position of the hydroxyl group -- right or left --
determines whether the carbon is in D- or L-
configuration, respectively.
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| Cahn-Ingold-Prelog System |
The standard notation for chiral centers in other areas of organic chemistry; it is not preferred
in carbohydrate chemistry due to confusion caused in some monosaccharide derivatives. The four
nonequivalent substituents are assigned priorities based on atomic number. For example, in
glyceraldehyde, priority decreases -OH -CHO
-CH2OH -H.
The center is oriented with the lowest priority substituent pointing away from eye. If the remaining
three substituents in decreasing order are placed clockwise, the center is in R (rectus) configuration, in
the anticlockwise ordering, the center is in S (sinister) configuration. In carbohydrates the
D and L system is used more commonly than R and S.
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| Anomeric Forms |
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Monosaccharides are found in solution predominantly in cyclic form. A cyclic hemiacetal is
formed by an addition reaction between the carbonyl group and the alchohol group. Stable
cyclic monosaccharides have a ring with five or more atoms (a ring oxygen and four or more carbons).
Carbohydrates with a five-atom ring are called Furanoses, while those with a six-atom ring are called
Pyranoses. The ratio of the ring forms in solution was studied by Angyl (1984).
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The cyclic forms of monosaccharides contain an additional chiral center, referred to as the
anomeric carbon. The two isomers differing at the anomeric carbon are designated as a-
and b-anomers. The optical activity of the anomers of monosaccharides was studied by
Hudson (1909). According to his findings, in the D-series
the more dextrorotory anomer is designated a, and the less dextrorotory anomer is
designated b. It should be noted that further studies have shown that the D
anomer is not always the more dextrorotary, and that this system of notation refers only to configuration, not
optical activity.
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References:
Hudson, C.S. (1909) The significance of certain numerical relations in the sugar group. J. Am. Chem. Soc.,31,66-86.
Angyal, S.J. (1984) The Composition of reducing sugars in solution. Adv. Carbohydr. Chem. Biochem. 42, 15-68.
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