Effects of Na2SO4 on hydrophobic and electrostatic interactions between amphipathic α‐helices

Abstract: The effects of 2 molal Na₂SO₄ at neutral pH on hydrophobic and electrostatic interactions between amphipathic α‐helices were investigated by circular dichroism spectroscopy. The amphipathic peptides that were studied included LEK (acetyl‐ L EE L KKK L EE L KKK L EE L ‐NH₂) and LEE (acetyl‐ L EE L EEE L EE L EEE L EE L ‐NH₂). In phosphate buffer at neutral pH, only LEK adopted a predominantly α‐helical conformation, attributable to glu^––lys⁺ interactions where a major contribution is evidently a hydrogen bond (Biochemistry 32 : 9668–9676). Despite the presence of lys⁺ in the e and g′ positions of the abcdefg heptad repeat, LEK exhibited mean‐residue ellipticities at 222 nm ([θ]₂₂₂) which were dependent on peptide concentration, indicating the presence of a coiled coil. In the presence of 2 molal Na₂SO₄ at 25–75°C, the helical content of LEK increased, with the greatest increase observed at 75°C. The value of the ellipticity ratio R ([θ]₂₂₂/[θ]₂₀₈) of LEK in 2 molal Na₂SO₄ also increased, indicating a stronger interhelical association. At 50°C and 75°C, LEK remained predominantly α‐helical. In phosphate buffer at neutral pH, LEE was mainly random coil. In the presence of 2 molal Na₂SO₄, however, the peptide formed α‐helices that associated to form a coiled coil. At 50°C and 75°C, LEE became predominantly random coil but the remaining α‐helices were still associating. These results are consistent with the strengthening of interhelical hydrophobic interactions and the absence of screening of helix‐stabilizing and helix‐destabilizing electrostatic interactions in amphipathic α‐helices by Na₂SO₄.