Ecballium elaterium trypsin inhibitor II (EETI-II) variants explored in the design and folding of heterochiral proteins. Ramachandran plots, solution nuclear magnetic resonance structure ensembles, and primary sequences with disulfide connectivity (yellow) for (A) wild-type EETI-II (Protein Data Bank entry 2IT7), (B) integrin binding EETI-II 2.5F, and (C) a heterochiral EETI-II 2.5F analogue in which the chirality of the amino acids in the protein loop is inverted relative to the chirality of the rest of the scaffold. Uppercase letters denote l-amino acids or achiral amino acids, and lowercase and underlined letters denote d-amino acids; β denotes β-alanine.