Which of the following amino acids would most likely be found on the surface of a protein molecule?

This question requires two logical steps: first, you need to appreciate that the hydrophilic amino acids are more likely to appear on the surface of a protein molecule, while hydrophobic amino acids are most likely be found in its interior. Next, you need to figure out which of the amino acids listed is hydrophilic. If you recall that arginine is a basic amino acid that is positively charged at physiologic pH, you should be able to answer this question right away. All of the other choices have neutral side chains and are uncharged at physiological pH. They would most likely be found in the hydrophobic core of the protein structure. Alanine, isoleucine, and leucine all have aliphatic side chains; phenylalanine and tryptophan have aromatic side chains. Ref: Rodwell V.W. (2011). Chapter 30. Conversion of Amino Acids to Specialized Products. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.