In addition to main chain interactions across the length of the peptide, S598 is anchored to the MHC via the side chains of Gln-3, Phe-5 and Ile-8. Gln-3 forms hydrogen bonds with the Ser-99 and Gln-114 of the HC (Figure 5B ), while Phe-5 and Ile-8 are buried within hydrophobic pockets. In addition, the side chains of Phe-5 and Gln-3 pack against each other and between the aromatic rings of HC residues Tyr-159 and Phe-74, constraining the peptide's backbone conformation at those positions.