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2.3.4. Hexoses

Hexoses

They are monosaccharides made up of chains of six carbon atoms. The aldohexoses have four asiméticos carbons and therefore there are sixteen (24 = 16) different possible molecular structures of interest in biology having the D-(+)-glucose, the (+)-mannose and D-(+)-galactose. Among the ketohexosesD-(-)-fructose stands out.

Since the open structure of hexoses is not linear, due to the angles between the carbon bonds, the first and last atoms are relatively close. For this reason, in solution, the linear structure generally closes on itself, forming a hexagon, similar to that of a molecule called pyran, or a pentagon, similar to that of a molecule called furan.

Glucose

It is the most abundant carbohydrate, also called grape sugar. In the blood it is found in concentrations of one gram per liter.

Glucose polymers form important polysaccharides:

  • with an energy reserve function, such as starch  in vegetables or glycogen  in animals.
  • with structural function, such as cellulose from plants.

In nature, D- (+) glucose is found, also called dextrose (dextrorotatory carbohydrate).

Glucosa: forma lineal y ciclada.

When glucose dissolves in water, the carbonyl group takes up a water molecule. The hydrate thus formed is unstable. The carbonyl group (-CHO) is close to the fifth carbon, and their radicals react between them, releasing a water molecule and both being joined by an oxygen atom. The molecule thus acquires the shape of a hexagonal cycle, like that of pyran, which is why the molecule is called glucopyranose. An intramolecular hemiacetal bond (which is what the bonding of an aldehyde with an alcohol is called) has formed intramolecularly. The -OH group that now has carbon 1 is called hydroxyl hemiacetalic. This carbon is now asymmetric and is called anomeric carbonAccording to the position of its -OH group on one side or the other of the plane, two new structures called anomers are distinguished: the α anomer and the β anomer.

  • The anomer is α when the alcoholic group (-OH) of C1 is in the trans position, that is, on the other side of the plane where -CH2OH is located, attached to C5 (downwards).
  • The anomer is β when these two radicals are in the cis position, that is, on the same side of the plane (upward).

Glucosa: de Fisher a Haworth

By Wikimuzg (Made by me with xdrawchem and gimp.) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons

These cyclic structures of glucose are not flat, but can adopt two conformations in space, since the bonds are oriented in space and not in plane:

  • The conformation of ship.
  • Chair conformation.

Cyclization can also adopt the structure of furan, with a pentagonal shape, as is the case of ribose and deoxyribose, which are therefore called ribofuranose and deoxyribofuranose.
 

Mannose

Aldohexose that is found in the form of D-mannose in certain plant tissues and polymerized to form mannosans, in bacteria, yeasts and higher plants.

Fructose

It is a ketohexose found in the form of β-D-fructofuranose. It is strongly left-handed, which is why it is also called levulose. It is found free in fruit and, together with glucose, forms the disaccharide sucrose.

Forma D y L de fructosa

By NEUROtiker (Own work) [Public domain], via Wikimedia Commons

Beta D fructosa

By NEUROtiker (talk · contribs) (Own work) [Public domain], via Wikimedia Commons