Which of the following amino acids is most likely to be located in the interior of a soluble protein molecule, not in contact with water molecules of the solvent?

As a general rule, amino acids with nonpolar side chains tend to be located in the interior of soluble protein molecules. These nonpolar side chains can paicipate in hydrophobic interactions. Of the amino acids listed, valine is the only one with a nonpolar side chain. Aspaic acid has an acidic side chain, arginine and lysine have basic side chains, and serine has an uncharged polar side chain.