We previously prepared the de novo designed peptide. [YGG(IEKKIEA)(4)], which forms a parallel triple-stranded coiled coil. To prepare an AAB-type heterotrimeric alpha-helical bundle, two variants, where the Ile(15) residue in the hydrophobic position was replaced with either an Ala or Trp residue, were designed and named IZ-2A and IZ-2W, respectively. Circular dichroism spectroscopy, peptide titration, sedimentation equilibrium, and gel filtration analyses revealed the formation of an (IZ-2A)(2)/IZ-2W complex. The NOESY spectra analyses indicated the presence of interstrand interactions between the two Ala residues and the Trp residue in the hydrophobic core. The (IZ-2A)(2)/IZ-2W complex has the structural uniqueness of native proteins.