Computer models constructed using MODELLER or Robetta of the energetically most-favored conformations of the αIIb loop between amino acids 842 and 873. (A) αIIb with the loop (arrow) in an energetically favored conformation derived from MODELLER. The loop colors indicate an αIIb cleavage site (R859) that separates the heavy (orange) and light (gray) chains. The PMI-1 epitope is immediately proximal to the cleavage site, as is at least a portion of the anti-αIIb (H-160) epitope. The B1B5 epitope is on the light chain, indicated by the gray and green ribbons. (B) The 200 most energetically favorable conformations of the loop were derived using MODELLER. The most favorable conformations are in red, less favorable ones in green, and least favorable ones in blue. (C) Five loop conformations were derived using Robetta.