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Carbohydrates And Nucleic Acids Notes

Pharmacology Notes > BIOL10212 Biochemistry Notes

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Biochemistry - Lecture 7 (19/02/2018)

Carbohydrates, Nucleic Acids and Lipids
Carbohydrates

Carbohydrates are the "hydrates of carbon" and have the empirical formula of (CH2O)n ,
where n is 3 or larger.

Trioses (C3) are the smallest and simplest Carbohydrates, then there are Tetroses (C4),
Pentoses (C5), Hexoses (C6) and Heptoses (C7).




Monosaccharides are made up of one monomeric unit.
Oligosaccharides are made up of 2-20 Monosaccharides.
Polysaccharides are made up of more than 20 Monosaccharides.
Glycoconjugates are linked to proteins or lipids.

Monosaccharides

There are two families based on position of the most oxidised carbon:

Stereochemistry of Monosaccharides

There are 2 different stereoisomers of monosaccharides: D and L.

The 3 carbon atoms are arranged in a straight line with a Hydroxide and
Hydrogen atom on either side of the central carbon, and the asymmetric carbon in the centre points towards you, this is the orientation needed to decide whether it is D or L.

The horizontal bonds are out of the plane, and the vertical bonds are into the plane.

If there is more than one asymmetric carbon atom then deciding whether it is D or L is determined by the chiral carbon furthest away from the aldehyde
(or ketone) group.
D Isomers are much more common in nature.


Diastereoisomers are a pair of stereoisomeric compounds that are not mirror images of one another. Pentoses and Hexoses
Pentoses and Hexoses are Cyclic.

Fischer projections are useful for understanding the stereochemistry of molecules but many common monosaccharides are actually cyclic.
This occurs as the aldehydes and ketone groups can react with an alcohol group and produce hemiacetals or hemiketals.

Reaction
(Alcohol Added)

Aldehyde

Ketone

Hemiacetal formation occurs when the molecule forms a ring within itself, this occurs when an alcohol combines with an aldehyde or a ketone.

Hemiacetal formation can generate two different isomeric forms (Anomers) at the C1 carbon atom*:

The Hemiacetal is Alpha if the hydroxyl group points downwards.
It is Beta if the hydroxyl group points upwards/diagonally.

This hydroxyl group is what forms structures with other monosaccharides, and so the direction that it is pointing in is important for the disaccharide structure.

This ring formation is reversible and so these structures can alternate.

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