Specific coiled-coil heterotrimers result from steric matching of hydrophobic core side chains. A 2:1 heterotrimer is formed by peptides containing alanine or cyclohexylalanine, respectively, at a central core residue. Detailed thermodynamic analysis reveals that the designed complex is considerably more stable than the corresponding alanine homotrimer (DeltaT(m) = 25 degreesC, DeltaDeltaG(unf) = 4.5 kcal/mol), while control complexes with naphthylalanine or cyclopropylalanine peptides are much less stable. However, the cyclohexylalanine homotrimer is of comparable stability to the 2:1 complex, prompting an investigation of multiply substituted peptides. A specific 1:11 heterotrimer is formed from three independent peptide strands, each bearing one large (cyclohexylalanine) and two small (alanine) side chains at the same three core positions but in different order. The combined impact of three substitutions improves specificity to the point where each pure peptide and all pairwise equimolar mixtures form significantly less stable complexes (DeltaT(m) = 22-24 degreesC). The capacity for specific complex formation governed by multiple unnatural core side chains should facilitate design of numerous new peptide assemblies.