Which of the following amino acids is considered hydrophobic due to its long alkyl side chain?

Valine is considered a hydrophobic amino acid primarily because of its side chain structure, which consists of a branched alkyl group. Specifically, valine has an isopropyl side chain (–CH(CH3)2) that makes it nonpolar. Nonpolar amino acids like valine do not interact favorably with water due to the absence of polar or charged functional groups, leading them to be classified as hydrophobic.

In contrast, asparagine contains a polar side chain that can form hydrogen bonds, making it hydrophilic. Cysteine, while it has a relatively nonpolar side chain, contains a thiol group which can participate in polar interactions. Histidine has an imidazole side chain that can be positively charged at physiological pH, giving it polar characteristics and allowing it to engage in ionizable interactions.

Therefore, valine stands out as the hydrophobic amino acid due to its long, branched alkyl side chain, enhancing its lack of affinity for water.