Electrostatic effects on ion selectivity and rectification in designed ion channel peptides

To help determine how amino acid sequence can influence ionic conduction properties in alpha-helical structures, we have synthesized and studied three closely related, channel-forming peptides. The sequences are based on a 21-residue amphiphilic Leu-Ser-Ser-Leu-Leu-Ser-Leu heptad repeat motif and differ in having either neutral, negatively, or positively charged N-termini. The channels formed by the neutral peptide are modestly cation selective and exhibit asymmetric current-voltage curves arising from the partial charges at the ends of the alpha-helix. Addition of a negatively charged Glu residue converted the channel to a completely cation-selective structure and essentially eliminated its rectification. Addition of a positively charged Arg residue near the N-terminus of the peptide reduced this channel's cation selectivity and increased the extent of rectification, These effects on channel ionic conductance can be explained by a theoretical electrostatic model and provide insights into the workings of more complex channel proteins.