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== Optical isomerism of amino acids ==
== Optical isomerism of amino acids ==
All α-amino acids except glycine CH<sub>2</sub>(NH<sub>2</sub>)COOH and proline are complex enough to have the two [[optical isomerism|optical isomers]] that are mirror images of each other (L and R). This is because they have four different groups around the alpha carbon, forming a [[chiral centre]] and allowing to occur.
All α-amino acids except glycine CH<sub>2</sub>(NH<sub>2</sub>)COOH and proline are complex enough to have the two [[optical isomerism|optical isomers]] that are mirror images of each other (L and R). This is because they have four different groups around the alpha carbon, forming a [[chiral centre]].
All living organisms use exclusively L optical isomers of amino acids for they proteins. The reasons of this are unknown and difficult to explain from the evolutionary point of view, as a naturally occurring chemical reaction normally produces L and R isomers in equal proportions, and most of the physical properties of the L and R forms are identical.
All living organisms use exclusively L optical isomers of amino acids for they proteins. The reasons of this are unknown and difficult to explain from the evolutionary point of view, as a naturally occurring chemical reaction normally produces L and R isomers in equal proportions, and most of the physical properties of the L and R forms are identical.