Final answer:
Some amino acids not commonly found in an alpha helix include tryptophan and lysine.
Step-by-step explanation:
In an alpha helix, the most commonly found amino acids are those with small side chains, such as glycine, alanine, serine, and threonine. However, there are some amino acids that are not commonly found in an alpha helix due to their larger or bulkier side chains. Examples of such amino acids include tryptophan and lysine. Tryptophan has a large aromatic side chain, while lysine has a long and positively charged side chain. These amino acids are less favorable in the compact structure of an alpha helix.