Contribution of Aromatic Interactions to α-Helix Stability

Abstract

The influence of natural and unnatural i, i + 4 aromatic side chain−side chain interactions on α-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−f₅F) 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−f₅F interaction energies are equivalent at internal positions in the sequence (ΔG_F-F = ΔG_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 α-helix. Positioning the interactions at the C-terminus led to much stronger interactions (ΔG_F-F = −0.8 kcal/mol; ΔG_F-f5F = −0.55 kcal/mol) likely because of favorable χ₁ rotameric preferences for aromatic residues at C-capping regions of α-helices, suggesting that aromatic side chain−side chain interactions are an effective α-helix C-capping method.