Effects of cation binding to hydrophobic helical peptides on orientation, aggregation, and ion-channel activity in phospholipid bilayer membranes

Abstract

Effects of cation binding to the C-terminal of α-helical peptides on conformation, orientation, aggregation, and ion-channel activity in a phospholipid bilayer membrane have been studied. The hydrophobic helical peptides having a crown ether unit at the C-terminal region, Boc-[Ala-Aib]_n-Ala-Cr (Cr represents a benzo-18-crown-6 unit, n= 4,8), and an anthryl group at the N-terminal region as well, Boc-Ser(Ant)-[Ala-Aib]_n-Ala-Cr (Ant represents an anthrylmethyl group, n= 4,8), have been synthesized. The helix content of the peptides increased upon complexation with K⁺ because of interaction of the negative pole of the helic macrodipole with the cation. The peptides were incorporated into a phospholipid bilayer membrane, and aggregated taking a transmembrane orientation. Boc-[Ala-Aib]₄-Ala-Cr showed an ion-channel-like activity in a bilayer membrane. The activity was higher than that of Boc-[Ala-Aib]₄-OMe, since the crown ether unit functions as a cation-binding site of the channel. The aggregation of the crown-peptides was promoted especially in the presence of Rb⁺ and Cs⁺ due to formation of a sandwich-type crown/cation complex. For this reason. Boc-[Ala-Aib]₄-Ala-Cr showed a higher channel-like activity in the presence of Cs⁺ than K⁺ On the other hand, Boc-[Ala-Aib]₄-Ala-Cr aggregated upon incorporation into a bilayer membrane, and showed an ion-channel-like activity. In any one of the peptides, the connection of a crown ether unit to the hydrophobic helical peptide augmented its channel-forming ability by facilitating aggregation with a transmembrane orientation.