The twenty naturally occurring amino acids that comprise proteins are (almost) ALL of the L- form. The L- (Laevorotatory) form is the stereoisomer that rotates plane polarized light to the left (I won't elaborate on this).

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.

The CORN Law

Imagine looking along the Hydrogen - alpha Carbon bond of an amino acid (this starting point is VERY IMPORTANT). The image of alanine has been oriented this way, and you can see that you are viewing along the Hydrogen - alpha Carbon bond which is located at the centre of the image.

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 ( Strictly speaking the L- and D- notation has been superceded by the more modern R- and S- notation for referring to stereo-isomers.)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.

Only ONE of the twenty amino acids is not in the L- form, and that is glycine (click here for image). The reason for this is that the side chain group is a hydrogen atom. Therefore, what we had called the amino acid's alpha carbon is bonded to two hydrogens, which renders the molecule as achiral or non-chiral (in other words, the terms L- or D- forms are not applicable).
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