4.7 Optical Rotation

Enantiomers have an interesting property that if polarized light be shone through a single enantiomer, the plane of polarization will rotate. The molecules act like tiny pieces of helix and the light, being a weird quantum physics entity, acts like two opposite spirals. The little helix molecules prefer one spiral over the other, and the light rotates a bit. Enantiomers are optically active.

A racemic mixture won't do this. Racemates are not optically active because the two enantiomers cancel each other out.

The isomer that rotates light counterclockwise is called levorotary, from the Latin laevo, left, and is indicated with an l- or (-)- (minus) prefix, while the clockwise isomer is called dextrorotary, and gets a d- or (+)- prefix.

The amino acids that make up our proteins are all one isomer by molecular shape, though some are laevorotary and some are dextrorotary. By convention, they are all called L- amino acids, with a small caps prefix, for their structural resemblance to l-glyceraldehyde HOCH₂CH(OH)CHO.

Like with E/Z isomers, IUPAC has a system for classifying enantiomers in a straightforward way. The four groups are prioritized with the same rules as for E/Z isomers, i.e. based on atomic number. Then, holding the molecular model so that the group with the lowest priority is facing away from you, observe which direction the 3 remaining groups are counted in order of decreasing priority. If the priorities decrease clockwise, then it is the (R)- isomer (this is also from a German word that I don't recall), and if the priorities decrease counterclockwise, it's the (S)- isomer (ditto).

Using the example of L-alanine, the lowest priority is the hydrogen (element 1) on its alpha carbon. Two of the four groups are joined to the alpha carbon by carbon atoms (element 6), so there is a tie; we look at what atoms are joined to those and find that one of them has oxygen atoms (element 8) while the other has only hydrogen. Therefore a carboxyl group has priority over a methyl group. Lastly, the remaining group is the amino group, and its nitrogen atom (element 7) has priority over all the others. Therefore, the decreasing priority is amino > carboxyl > methyl, and we see that sequence is arranged counterclockwise, so it is (S)-alanine.

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