Final answer:
Leucine is a hydrophobic amino acid that is insoluble in water due to its nonpolar side chain.
Step-by-step explanation:
Leucine is a hydrophobic amino acid, meaning it is insoluble in water. This is because leucine has a nonpolar side chain, which cannot engage in hydrogen bonding with water molecules. Nonpolar amino acids like leucine are often found in the interior part of protein structures, where they are buried and away from contact with water.