Discuss the stereochemistry of monosaccharides, nucleotides and amino acids. How does the stereochemistry of these building blocks affect the structures of the polymers that they form?
Stereoisomers have the same displayed formula but a different arrangement of atoms in space. Enantiomers rotate plane polarised light in opposite ways – each is known as either the D or L form (although this naming originally indicated which way the light was rotated, the fact that this cannot be predicted from atomic structure means that we instead compare the molecule to a similar known one – for example all monosaccharides and amino acids are named based on the structure of glyceraldehyde). The two enantiomers are non-superimposable mirror images of each other - they have a chiral centre (often a carbon atom with four different groups of atoms covalently bonded to it).
Monosaccharides (sugars) are the monomers of polysaccharides (carbohydrates), linked by O-glycosidic bonds which are formed from condensation reactions between the monosaccharides.
Monosaccharides have a chiral centre – the carbon one from the end of the chain furthest from the carbonyl group. By convention, the arrangement of atoms in space around this carbon is used to dictate whether the monosaccharide is in its D or L form – although many of the other carbons in the chain are also chiral. Enantiomers are categorised into D and L forms according to their similarities with the corresponding forms of glyceraldehyde, the two enantiomers of which are shown below:
It is clear that the D enantiomer has the hydroxyl group on the right hand side of the chiral carbon, whereas the L form has it on the left. The D and L forms of monosaccharides are therefore mirror images of each other.
In biological systems, sugars are almost always in the D form – for example the high specificity of enzymes means that they only recognise the D form and so will only catalyse polypeptide synthesis/ hydrolysis when D enantiomers are used.
Pentose and hexose monosaccharides can cyclise to form a ring. There are two forms of each cyclic monosaccharide – α and β. These are shown for glucose below: