Dynamics and aggregation of the peptide ion channel alamethicin. Measurements using spin-labeled peptides

Two spin-labeled derivatives of the ion conductive peptide alamethicin were synthesized and used to examine its binding and state of aggregation. One derivative was spin labeled at the C-terminus and the other, a leucine analogue, was spin labeled at the N-terminus. In methanol, both the C and N terminal labeled peptides were monomeric. In aqueous solution, the C-terminal derivative was monomeric at low concentrations, but aggregated at higher concentrations with a critical concentration of 23 microM. In the membrane, the C-terminal label was localized to the membrane-aqueous interface using 13C-NMR, and could assume more than one orientation. The membrane binding of the C-terminal derivative was examined using EPR, and it exhibited a cooperativity seen previously for native alamethicin. However, this cooperativity was not the result of an aggregation of the peptide in the membrane. When the spectra of either the C or N-terminal labeled peptide were examined over a wide range of membrane lipid to peptide ratios, no evidence for aggregation could be found and the peptides remained monomeric under all conditions examined. Because electrical measurements on this peptide provide strong evidence for an ion-conductive aggregate, the ion-conductive form of alamethicin likely represents a minor fraction of the total membrane bound peptide.