Structure and nomenclature
With few exceptions (e.g., deoxyribose), monosaccharide have this chemical formula: Cx(H2O)y, where x is at least 3. Monosaccharide can be classified by the number x of carbon atoms they contain: diose (2) triose (3) tetrose (4), pentose (5), hexose (6), heptose (7), and so on.
The most important monosaccharide, glucose, is a hexose. Examples of heptoses include the ketoses mannoheptulose and sedoheptulose. Monosaccharides with eight or more carbons are rarely observed as they are quite unstable.
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.
Monosaccharides are classified according to three different characteristics:
Placement of its carbonyl group
Number of carbon atoms it contains
Placement of its carbonyl group
If the carbonyl group is an aldehyde and attached to C-1, the monosaccharide is an Aldose; if the carbonyl group is a ketone attached to C-2, the monosaccharide is a Ketose. Therefore monosaccharides are called aldoses and ketoses derivatives of polyhydric alcohols. Glucose is an example of an aldose and fructose is an example of ketose.
According to the number of carbon atoms:
triose: 3 carbons tetrose: 4 carbons pentose: 5 carbons hexose: 6 carbons ketopentose: ketose with 5 carbons
a)Formula of Fisher
Simple carbohydrates:
The open formula of monosaccharide’s named formula of Fisher.
Aldohexose has 4 asymmetric carbon atoms and its number of stereoisomers is equal N = 2n = 24 = 16 (8 pairs of enantiomers) of D and α - 20K.For the ketohexose number of asymmetric carbon atom is equal N = 23 = 8 (4 pairs of enantiomers)
In order to determine which of these epimerswas glucose, Fischer made use of the inherent C2 symmetry in the four-carbon dissymmetric core of one epimer (B). This is shown in the following diagram by a red dot where the symmetry axis passes through the projection formula. Because of this symmetry, if the aldehyde and 1º-alcohol functions at the ends of the chain are exchanged,epimer B would be unchanged; whereas A would be converted to a different compound.
Fischer looked for and discovered a second aldohexose that represented the end group exchange for the epimer lacking the latent C2 symmetry (A). This compound was L-(+)-gulose, and its exchange relationship to D-(+)-glucose was demonstrated by oxidation to a common aldaric acid product. The remaining epimer is therefore mannose.
b) Cyclic formula of M.S. divided into the halfacetalic formula of Colley - Tollence and cyclic formula of Cheourse.
1) Half-acetalic formula of Colley - Tollence: D - glucose cyclized with the formation with the two halfacetals as -α and - β - glucose:
If 1 and 5 on group combine one plane α form, if it occupies different plane, it would be β - form.
The preferred structural form of many monosaccharides may be that of a cyclic hemiacetal. Five and six-membered rings are favored over other ring sizes because of their low angle and eclipsing strain. Cyclic structures of this kind are termed furanose (five-membered) or pyranose (six-membered), reflecting the ring size relationship to the common heterocyclic compounds furan and pyran shown on the right. Ribose, an important aldo-pentose, commonly adopts a furanose structure, as shown in the following illustration. By convention for the D-family, the five-membered furanose ring is drawn in an edgewise projection with the ring oxygen positioned away from the viewer. The anomeric carbon atom (colored red here) is placed on the right. The upper bond to this carbon is defined as beta, the lower bond then is alpha.
2) Cyclic formula of Cheourse
The piranose cycle forms six cycle compound, the furanose cycle forms fifth cycle compound.
