The design of an antiparallel coiled-coil 1:1:1 heterotrimer is described. Control of strand orientation results from proper alignment of sterically matched hydrophobic core side chains. Matched core layers position one cyclohexylalanine side chain against two alanine ones. Substitution of three consecutive heptad a positions with all permutations of two alanines and one cyclohexylalanine (AAX, AXA, AAX, where A = alanine, X = cyclohexylalanine) affords a parallel 1:1:1 heterotrimer, as previously reported. Here, we report that moving the substitution sites in one strand to d rather than a positions affords a new peptide that can form an antiparallel complex with the other original components. The new assembly is characterized by circular dichroism spectroscopy ([theta](222) = -30 317 deg cm(2) dmol(-1), T-m = 77 degreesC, DeltaG(unf) = 17.1 kcal/mol), and its stoichiometry and aggregation number are confirmed by nickel tag affinity analysis and analytical ultracentrifugation. Disulfide exchange data support the preference for an antiparallel arrangement. Examination of the functionally identical parallel complex demonstrates that the antiparallel structure is comparably stable, as confirmed by a direct competition assay that established an equilibrium 55:45 ratio of each assembly.