Sickling occurs in deoxyhemoglobin S but not in oxyhemoglobin S. Oxyhemoglobin has a small, hydrophobic pocket in a β chain region located in the interior of the protein. In deoxyhemoglobin, however, this pocket is located on the surface of the protein. In deoxyhemoglobin S, Val 6 6 interacts with this surface pocket, leading to aggregation of HbS. Choose two amino acids that would be reasonable candidates for the pocket–Val 6 6 interaction.