Why is glycine considered a nonpolar amino acid but a polar molecule?

Question

Glycine has a dipole moment, so why is it considered a nonpolar amino acid when discussing its occurrence in proteins?Also, is the backbone of a protein nonpolar?

Answer 1

Great question!

Glycine is a very unique amino acid, and its absence of an R group make it difficult to characterize.

The amino acid backbones are made by an amino group and a carboxylic acid group attached to a chiral carbon in the center known as the alpha carbon.

This backbone itself is capable of hydrogen bonding and is very polar thanks to the electronegative nitrogen and oxygen present, but when determining the behavior of a specific amino acid it is normally the R group that is taken into account.

Since glycine lacks an R group it is placed into the "nonpolar" category for the lack of better placement. Glycine is also the only achiral amino acid as it has 2 hydrogens bonded to its alpha carbon.

In my opinion, glycine should be labeled as a "structural" amino acid as its R group does not participate in any form of catalysis, but instead it is very flexible and can rotate freely around its alpha carbon.