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Monosaccharide

Monosaccharides are the simplest form of carbohydrates. 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, 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 have different chemical and physical properties.

1. Structure

With few exceptions (e.g., deoxyribose), monosaccharides have the chemical formula (CH2O)n + m with the chemical structure H(CHOH)nC=O(CHOH)mH. If n or m is zero, it is an aldehyde and is termed an aldose, otherwise it is 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.

Fructose, a monosaccharide, as a Haworth projection

1.1. Cyclic structure

Most monosaccharides form cyclic structures, which predominate in aqueous solution, by forming hemiacetals or hemiketals (depending on whether they are 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.

Glucose cyclic structure

1.2. 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=2c. The only carbohydrate without an isomer is dihydroxyacetone or DHA. The carbohydrate is also formed inside the emodicalatism.

2. Monosaccharide Nomenclature

Monosaccharides are classified by the number of carbon atoms they contain:

• Triose, 3 carbon atoms
• Tetrose, 4 carbon atoms
• Pentose, 5 carbon atoms
• Hexose, 6 carbon atoms
• Heptose, 7 carbon atoms
• Octose, 8 carbon atoms
• Nonose, 9 carbon atoms
• Decose, 10 carbon atoms

Monosaccharides are classified the type of keto group they contain:

• Aldose, -CHO (aldehyde)
• Ketose, C=O (ketone)

Monosaccharides are classified according to their molecular configuration at carbon 2:

• 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.

3. List of monosaccharides

This is a list of some common monosaccharides, not all are found in nature - some have been synthesized:

• Trioses:
• Aldotriose: glyceraldehyde
• Ketotriose: dihydroxyacetone
• Tetroses:
• Aldotetrose: erythrose and threose
• Ketotetrose: erythrulose
• Pentoses:
• Aldopentoses: arabinose, lyxose, ribose and xylose
• Ketopentoses: ribulose and xylulose
• Hexoses:
• Aldohexoses: allose, altrose, galactose, glucose, gulose, idose, mannose and talose
• Ketohexoses: fructose, psicose, sorbose and tagatose
• Heptoses:
• Keto-heptoses: mannoheptulose, sedoheptulose
• Octoses: octolose, 2-keto-3-deoxy-manno-octonate
• Nonoses: sialose

Disaccharide

A disaccharide is a sugar (a carbohydrate) composed of two monosaccharides. It is formed when two sugars are joined together and a molecule of water is removed. For example, milk sugar (lactose) is made from glucose and galactose whereas cane sugar (sucrose) is made from glucose and fructose.

1. Chemistry

The two monosaccharides are bonded via a dehydration reaction that leads to the loss of a molecule of water. The glycosidic bond can be formed between any hydroxyl groups on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations (regiochemistry) and stereochemistry (alpha- or beta-) result in disaccharides that are diastereoisomers with different chemical and physical properties.

Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting. 'Disaccharide' is one of the four chemical groupings of carbohydrates (monosaccharide, disaccharide, oligosaccharide, and polysaccharide).

Sucrose, a common disaccharide

2. Common disaccharides

Maltose and cellobiose are hydrolysis products of the polysaccharides, starch and cellulose, respectively.

Polysaccharide

Polysaccharides (sometimes called glycans) are relatively complex carbohydrates.

They are polymers made up of many monosaccharides joined together by glycosidic linkages. They are therefore very large, often branched, molecules. They tend to be amorphous, insoluble in water, and have no sweet taste.

When all the constituent monosaccharides are of the same type they are termed homopolysaccharides; when more than one type of monosaccharide is present they are termed heteropolysaccharides.

Examples include storage polysaccharides such as starch and glycogen and structural polysaccharides such as cellulose and chitin.

Polysaccharides have a general formula of Cn(H2O)n-1 where n is usually a large number between 200 and 2500. The general formula can also be represented as (C6H10O5)n where n=40-3000.

1. Starches

Starches are glucose polymers in which glucopyranose units are bonded by alpha-linkages. It is made up of a mixture of Amylose and Amylopectin. Amylose consists of a linear chain of several hundred glucose molecules and Amylopectin is a branched molecule made of several thousand glucose units.
Starches are insoluble in water. They can be digested by hydrolysis, catalyzed by enzymes called amylases, which can break the alpha-linkages. Humans and other animals have amylases, so they can digest starches. Potato, rice, wheat, and maize are major sources of starch in the human diet.

2. Cellulose

The structural components of plants are formed primarily from cellulose. Wood is largely cellulose and lignin, while paper and cotton are nearly pure cellulose. Cellulose is a polymer made with repeated glucose units bonded together by beta-linkages. Humans and many other animals lack an enzyme to break the beta-linkages, so they do not digest cellulose. Certain animals can digest cellulose, because bacteria possessing the enzyme are present in their gut. The classic example is the termite.

3. Acidic polysaccharides

Acidic polysaccharides are polysaccharides that contain carboxyl groups, phosphate groups and/or sulfuric ester groups.

4. Bacterial capsule polysaccharides

Pathogenic bacteria commonly produce a thick, mucous-like, layer of polysaccharide. This "capsule" cloaks antigenic proteins on the bacterial surface that would otherwise provoke an immune response and thereby lead to the destruction of the bacteria. Capsular polysaccharides are water soluble, commonly acidic, and have molecular weights on the order of 100-1000 kDa. They are linear and consist of regularly repeating subunits of one ~ six monosaccharides. There is enormous structural diversity; nearly two hundred different polysaccharides are produced by E. coli alone. Mixtures of capsular polysaccharides, either conjugated or native are used as vaccines.

Bacteria and many other microbes, including fungi and algae, often secrete polysaccharides as an evolutionary adaptation to help them adhere to surfaces and to prevent them from drying out. Humans have developed some of these polysaccharides into useful products, including xanthan gum, dextran, gellan gum, and pullulan.

5. External links

• Polysaccharide Structure