Stabilization of amphipathic alpha-helical and beta-helical conformations in synthetic peptides in the presence and absence of ionic interactions.

Abstract

The role of ionic interactions in stabilizing amphipathic alpha-helices was studied in the synthetic peptide Ac-NLEELKKKLEELKG-NH2 (NLEKG14), potentially stabilized by attraction between complementary ions in successive turns of the helix, and in the peptide Ac-NLEELEEELEELEG-NH2 (NLEG14), in which no side-chain ionic attractions are possible. At a pH below the pKa of glutamate, NLEG14 had a higher helix content than NLEKG14. At pH 3 to pH 10, the helicity of NLEKG14 did not change, whereas NLEG14 was converted to random coil at pH 7. The role of ionic interactions in stabilizing the conformation of beta-structures was studied in the synthetic peptides Ac-KLKLKLELELELG-NH2 (KLEG13) and Ac-ELELELELELELG-NH2 (ELG13). At a pH below the pKa of glutamate, ELG13 had a higher beta-content than KLEG13, as judged by their dichroic spectra, but at higher pH, ELG13 was converted to random coil, whereas KLEG13 retained a predominantly beta-conformation. At pH 7, high NaCl concentration produced a significant increase in the alpha-helix content of NLEKG14, converted NLEG14 from random coil to alpha-helix and converted ELG13 from random coil to beta-conformation. Overall, the results demonstrate that ionic attraction between side-chains plays a lesser role than hydrogen bonding and hydrophobic effects in stabilizing the alpha- and beta-conformations exhibited by these amphipathic peptides.