Which amino acid is NOT part of the 7 amino acids with nonpolar, nonaromatic side chains?

The amino acid serine is classified as polar due to its side chain containing a hydroxyl group (-OH), which can participate in hydrogen bonding. This polar characteristic is significant because it allows serine to interact favorably with water and other polar molecules, differentiating it from the nonpolar, nonaromatic amino acids that contain hydrophobic side chains that do not engage in such interactions.

In contrast, isoleucine, proline, and leucine all have side chains that are hydrophobic, meaning they are more nonpolar and do not readily interact with water. Isoleucine has a branched aliphatic chain, proline contains a cyclic structure that contributes to its unique properties but still remains nonpolar, and leucine also has a branched aliphatic side chain. Collectively, these properties characterize them as nonpolar, nonaromatic amino acids.

Thus, serine's status as a polar amino acid makes it the correct choice as the one that does not belong in the category of nonpolar, nonaromatic amino acids.