Staying with the image of alanine (which you downloaded earlier), we can examine how to identify the L- form of an amino acid. This method of L- form identification is commonly used mnemonic CORN Law.
CORN is an acronym for -COOH (the main chain caboxylic acid group), the -R group (representation of side chain) and -NH2 (the nitrogen of the main chain amine group).
Starting at the carboxylic acid group, if you move your eyes clockwise and see the mentioned -COOH group then the -R group then the -NH2 group. From this you will be able to make the acronym CORN. (NB In our image we have conveyed the amino acid to be in solution so that the carboxylic acid becomes a cation, and the amine group becomes an anion BUT this will not affect the test).
(And now the VERY IMPORTANT deduction) - Since the acronym CORN is produced with a clockwise eye movement then the molecule is the L- form. Therefore, we have L-alanine.
The L- comes from the fact that the amino group is on the L-eft side in its Fisher projection of the compound. ( Strictly speaking the L- and D- notation has been superceded by the more modern R- and S- notation for referring to stereo-isomers.)
However, if we could only make the word CORN moving our eyes in an anticlockwise direction, then we have the D- form. This form would not be a unit of a protein in the natural world.
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