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?
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The first part of your question illustrates a common confusion of beginners between the physiochemical properties of free amino acids in solution, and the properties of that part of an amino acid that remains after it has participated in a condensation reaction to form part of a polypeptide. They are different. (It was for that reason that I modified the question to make it clear that the ‘consideration’ of glycine as nonpolar applies to it in its protein context.)
I have prepared a diagram to clarify.
The second question was “is the backbone of the macromolecule nonpolar?”.
No. The backbone of a protein has partial charges on the oxygen and nitrogen atoms of the peptide bond, as shown below. This is because it has a partial double-bonded character. These partially charged atoms can (and frequently do) form hydgrogen bonds, either with another part of the backbone (secondary structure such as α-helix or β-sheet) or with polar side-chains.
I'm just converting @alwaysconfused's comment into answer (with a bit of more details).
Yes, glycine does have a dipole moment (15.7 D), but when it comes to biochemistry, then the dipole moment on the side chain is considered. Since the structure of glycine is:
H2N - CH2 - COOH <---> H3N+ - CH2 - COO-
So the side chain is:
which has, obviously, 0 dipole moment. Thus glycine, though polar, is considered non-polar in protein structures.
Glycine has no side chains. In a neutral solution; not forming a peptide bond, glycine has the following the following structure
H3N+ - CH2 - COO-
which is polar because of the -ve and +ve charge. Once it forms a peptide bond with another aminoacids (aa), it becomes in the following form:
aa-----H2N - CH2 - CO ------aa
As you can see, it doesn't have any charges on it since the amino group and the carboxylic acid group formed bonds with another 2 amino acids.