Which of the following amino acids is classified as nonpolar?

Leucine is classified as a nonpolar amino acid due to its structure and the nature of its side chain. It possesses a branched-chain aliphatic side chain that consists of a hydrophobic cluster of carbon atoms, which makes it nonpolar. This hydrophobic characteristic allows leucine to play a significant role in the folding of proteins as it tends to be buried within the interior of protein structures, away from the aqueous environment.

In contrast, the other amino acids listed—serine, arginine, and threonine—contain hydroxyl (-OH) or guanidinium groups that are polar in nature, allowing them to form hydrogen bonds and interact with water. These properties classify them as polar or charged amino acids, which are generally more soluble in water than nonpolar amino acids. This is an essential distinction in understanding how different amino acids contribute to protein structure and function.