Images of the WT and W513S mutant S510-Aba peptides bound to H-2D^b. A, Cartoon representation of the refined model of H-2D^b/S510-Aba showing the Ag binding cleft from the peptide’s N-terminus. All residues of S510-Aba (in green), as well as selected residues of the H-2D^b H chain (in cyan), are shown in stick format. Dashed lines represent key interactions between S510-Aba and H-2D^b. Also shown are residues of a crystallographically related molecule (in gray) involved in crystal contacts with the Ag binding cleft, as well as an ordered sulfate ion observed at the interface between symmetry-related peptide chains. Peptide residues are labeled in italics and residues from the symmetry-related molecule are labeled with an inverted comma. B, The H-2D^b/S510-Aba Ag binding cleft viewed from the peptide’s C-terminal. A section of the α2 helix of the H chain has been removed to reveal the bound peptide. C and D, Equivalencies to A and B for the W513S H-2D^b/S_510–518 structure. In these images the H-2D^b H chain is drawn in slate and the W513S mutant S510-Aba peptide is shown in yellow.