the racemic X-ray crystal structure (Figure ). Similarly, the tetrameric nature of magainin 2 was suggested to be critical for the activity of this amphibian host defense peptide (Figure ). Heterochiral interactions between L-and D-protein isomers observed during structure elucidation may also lead to fruitful development in binder creation. In the studies of the transmembrane helix of the influenza M2 ion channel protein (TM-M2), a heterochiral coiled-coil association was observed between the two peptide enantiomers in the presence of detergent octyl-glucoside (DL-OG) or within the monoolein lipid cubic phase (LCP) (Figure ). The LCP structure shows an 11residue helical repeat (hendecad, 3,4,4 spacing) in the coiled-coil, which differs from the homochiral coiled-coils that adopts a 7residue helical repeat (heptad; 3,4 spacing). The crystals grown in DL-OG do not form a hendecad repeat, as steric clashes involving Ile39 and Ile42 prevent proper 3,4,4 interaction. Substitution of these residues with alanine or glutamate produced the hendecad repeat coiled-coil in the racemic DL-OG structure, thus reinforcing the argument that hendecad repeat is a feature of heterochiral coiled coils. Such heterochiral interactions can be used to design D-proteins drugs, which are generally nonproteolytic and non-immunogenic (see section 2.3). The resulting drug-target complexes can also be resolved using racemic protein crystallography to aid in rational optimization (Figure ).