The influence of natural and unnatural i, i + 4 aromatic side chain-side chain interactions on a-helix stability was determined in Ala-Lys host peptides by circular dichroism (CD). All interactions investigated provided some stability to the helix; however, phenylalanine-phenylalanine (F-F) and phenylalanine-pentafluorophenylalanine (F-f(5)F) interactions resulted in the greatest enhancement in helicity, doubling the helical content over i, i + 5 control peptides at internal positions. Quantification of these interactions using AGADIR multistate helix-coil algorithm revealed that the F-F and F-f(5)F interaction energies are equivalent at internal positions in the sequence (DeltaG(F-F) = DeltaG(F-f5F) = -0.27 kcal/mol), despite the differences in their expected geometries. As the strength of a face-to-face stacked phenyl-pentafluorophenyl interaction should surpass an edge-to-face or offset-stacked phenyl-phenyl interaction, we believe this result reflects the inability of the side chains in F-f5F to attain a fully stacked geometry within the context of an a-helix. Positioning the interactions at the C-terminus led to much stronger interactions (DeltaG(F-F) = -0.8 kcal/mol; Delta(GF-f5F) = -0.55 kcal/mol) likely because of favorable chi(1) rotameric preferences for aromatic residues at C-capping regions of a-helices, suggesting that aromatic side chain-side chain interactions are an effective a-helix C-capping method.