What is the largest of the twenty amino acids?

Tryptophan is considered the largest of the twenty amino acids due to its structure. It contains an indole side chain, which is a larger and more complex functional group compared to the side chains of other amino acids. The size of an amino acid can be assessed based on the number of carbon, nitrogen, and oxygen atoms it contains in its side chain, as well as the overall molecular weight.

Glycine is the simplest amino acid with a single hydrogen atom as its side chain, making it the smallest. Alanine has a methyl group as its side chain, which is larger than glycine but still small compared to tryptophan. Cysteine has a thiol group in its side chain and is more complex than glycine and alanine, but it cannot match the size of tryptophan's indole structure.

Thus, tryptophan stands out as the largest amino acid due to its bulky aromatic side chain, which contributes to its size and complexity.