Binding of the linear γC and RGD chimera peptides and comparison with cyclic peptides. Ligand, α subunit, and β subunit are shown in green, light blue, and beige, respectively, with O, N, and S atoms in red, blue, and yellow, respectively. Water molecules are red spheres. Ca²⁺ ions are gold spheres, and Mg²⁺ ions (a–c) or Mn²⁺ ions (d) are silver spheres. Fc-Fo omit maps after refinement in the absence of ligands and the displayed water molecules are contoured at 3σ. The complexes are α_IIbβ₃ (a–c) with γC dodecamer (a) or chimera decamer (b) peptides, disulfide-cyclized eptifibatide peptide (c), or α_vβ₃ with backbone-cyclized cilengitide peptide (d; Xiong et al., 2002). One-letter code is used for residues except for homoarginine (Har). Residue C7 in eptifibatide (c) is amidated.